diff --git a/src/tint/resolver/resolver.cc b/src/tint/resolver/resolver.cc
index a7e917d..ded0afa 100644
--- a/src/tint/resolver/resolver.cc
+++ b/src/tint/resolver/resolver.cc
@@ -167,7 +167,7 @@
 
     if (!enabled_extensions_.Contains(ast::Extension::kChromiumDisableUniformityAnalysis)) {
         if (!AnalyzeUniformity(builder_, dependencies_)) {
-            // TODO(jrprice): Reject programs that fail uniformity analysis.
+            return false;
         }
     }
 
diff --git a/src/tint/resolver/uniformity.cc b/src/tint/resolver/uniformity.cc
index e4c31c9..b5e5687 100644
--- a/src/tint/resolver/uniformity.cc
+++ b/src/tint/resolver/uniformity.cc
@@ -1548,9 +1548,8 @@
         // Helper to produce a diagnostic message with the severity required by this invocation of
         // the `MakeError` function.
         auto report = [&](Source source, std::string msg) {
-            // TODO(jrprice): Switch to error instead of warning when feedback has settled.
             diag::Diagnostic error{};
-            error.severity = note ? diag::Severity::Note : diag::Severity::Warning;
+            error.severity = note ? diag::Severity::Note : diag::Severity::Error;
             error.system = diag::System::Resolver;
             error.source = source;
             error.message = msg;
diff --git a/src/tint/resolver/uniformity_test.cc b/src/tint/resolver/uniformity_test.cc
index e2e2796..18e6a16 100644
--- a/src/tint/resolver/uniformity_test.cc
+++ b/src/tint/resolver/uniformity_test.cc
@@ -51,8 +51,7 @@
                 EXPECT_EQ(program.Diagnostics().count(), 0u) << error_;
             }
         } else {
-            // TODO(jrprice): expect false when uniformity issues become errors.
-            EXPECT_TRUE(valid) << error_;
+            EXPECT_FALSE(valid);
         }
     }
 
@@ -318,7 +317,7 @@
     bool should_pass = !(MayBeNonUniform(condition) && RequiredToBeUniform(function));
     RunTest(src, should_pass);
     if (!should_pass) {
-        EXPECT_THAT(error_, ::testing::StartsWith("test:31:5 warning: "));
+        EXPECT_THAT(error_, ::testing::StartsWith("test:31:5 error: "));
         EXPECT_THAT(error_, ::testing::HasSubstr("must only be called from uniform control flow"));
     }
 }
@@ -391,7 +390,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:17:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:17:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -440,7 +439,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:21:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:21:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -528,7 +527,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:11:7 warning: parameter 'i' of 'foo' must be uniform
+              R"(test:11:7 error: parameter 'i' of 'foo' must be uniform
   foo(rw);
       ^^
 
@@ -581,7 +580,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:10:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:10:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -646,7 +645,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:17:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:17:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -697,7 +696,7 @@
     if (!should_pass) {
         EXPECT_EQ(
             error_,
-            R"(test:5:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+            R"(test:5:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -732,7 +731,7 @@
     if (!should_pass) {
         EXPECT_EQ(
             error_,
-            R"(test:9:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+            R"(test:9:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -777,7 +776,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:10:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:10:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -808,7 +807,7 @@
     RunTest(src, should_pass);
     if (!should_pass) {
         EXPECT_EQ(error_,
-                  R"(test:5:5 warning: 'dpdx' must only be called from uniform control flow
+                  R"(test:5:5 error: 'dpdx' must only be called from uniform control flow
     dpdx(0.5);
     ^^^^
 
@@ -842,7 +841,7 @@
     RunTest(src, should_pass);
     if (!should_pass) {
         EXPECT_EQ(error_,
-                  R"(test:9:5 warning: 'dpdx' must only be called from uniform control flow
+                  R"(test:9:5 error: 'dpdx' must only be called from uniform control flow
     dpdx(0.5);
     ^^^^
 
@@ -879,7 +878,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:5:5 warning: 'dpdx' must only be called from uniform control flow
+              R"(test:5:5 error: 'dpdx' must only be called from uniform control flow
     dpdx(0.5);
     ^^^^
 
@@ -909,7 +908,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:5 warning: 'dpdx' must only be called from uniform control flow
+              R"(test:9:5 error: 'dpdx' must only be called from uniform control flow
     dpdx(0.5);
     ^^^^
 
@@ -1025,7 +1024,7 @@
         EXPECT_THAT(
             error_,
             ::testing::StartsWith(
-                R"(test:13:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+                R"(test:13:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();)"));
         EXPECT_THAT(error_,
                     ::testing::HasSubstr("test:14:9 note: reading from read_write storage buffer "
@@ -1065,7 +1064,7 @@
         EXPECT_THAT(
             error_,
             ::testing::StartsWith(
-                R"(test:14:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+                R"(test:14:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();)"));
         EXPECT_THAT(error_,
                     ::testing::HasSubstr("test:13:9 note: reading from read_write storage buffer "
@@ -1106,7 +1105,7 @@
         EXPECT_THAT(
             error_,
             ::testing::StartsWith(
-                R"(test:15:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+                R"(test:15:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();)"));
         EXPECT_THAT(error_,
                     ::testing::HasSubstr("test:13:9 note: reading from read_write storage buffer "
@@ -1157,7 +1156,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:7:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:7:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -1212,7 +1211,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -1282,7 +1281,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -1366,7 +1365,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:14:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:14:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -1404,7 +1403,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -1447,7 +1446,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:20:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:20:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -1521,7 +1520,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:20:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:20:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -1562,7 +1561,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -1608,7 +1607,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:16:9 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:16:9 error: 'workgroupBarrier' must only be called from uniform control flow
         workgroupBarrier();
         ^^^^^^^^^^^^^^^^
 
@@ -1649,7 +1648,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -1690,7 +1689,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -1826,7 +1825,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -1871,7 +1870,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:6:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:6:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -1906,7 +1905,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:14:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:14:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -1945,7 +1944,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:14:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:14:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -1982,7 +1981,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -2037,7 +2036,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:10:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:10:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2092,7 +2091,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -2153,7 +2152,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2198,7 +2197,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:21:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:21:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2240,7 +2239,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -2281,7 +2280,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -2342,7 +2341,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:7:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:7:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2379,7 +2378,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -2444,7 +2443,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:17:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:17:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2491,7 +2490,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:23:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:23:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2534,7 +2533,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -2577,7 +2576,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -2672,7 +2671,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:6:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:6:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2700,7 +2699,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:7:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:7:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2729,7 +2728,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2758,7 +2757,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2787,7 +2786,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2816,7 +2815,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2844,7 +2843,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:7:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:7:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2875,7 +2874,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -2952,7 +2951,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -3035,7 +3034,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:12:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:12:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -3072,7 +3071,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:13:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:13:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -3229,7 +3228,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -3259,7 +3258,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -3296,7 +3295,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:7:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:7:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -3326,7 +3325,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:7:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:7:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -3361,7 +3360,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:11:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:11:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -3428,7 +3427,7 @@
       fallthrough;
       ^^^^^^^^^^^
 
-test:14:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+test:14:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -3498,7 +3497,7 @@
       fallthrough;
       ^^^^^^^^^^^
 
-test:14:9 warning: 'workgroupBarrier' must only be called from uniform control flow
+test:14:9 error: 'workgroupBarrier' must only be called from uniform control flow
         workgroupBarrier();
         ^^^^^^^^^^^^^^^^
 
@@ -3537,7 +3536,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:14:9 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:14:9 error: 'workgroupBarrier' must only be called from uniform control flow
         workgroupBarrier();
         ^^^^^^^^^^^^^^^^
 
@@ -3577,7 +3576,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:19:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:19:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -3643,7 +3642,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:18:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:18:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -3712,7 +3711,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:21:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:21:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -3754,7 +3753,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -3892,7 +3891,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:17:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:17:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -3929,7 +3928,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -3959,7 +3958,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4024,7 +4023,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:11:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:11:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4052,7 +4051,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:7:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:7:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4081,7 +4080,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4113,7 +4112,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:12:7 warning: parameter 'p' of 'bar' must be uniform
+              R"(test:12:7 error: parameter 'p' of 'bar' must be uniform
   bar(&v);
       ^
 
@@ -4187,7 +4186,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4235,7 +4234,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:10:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:10:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4265,7 +4264,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4297,7 +4296,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:11:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:11:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4348,7 +4347,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:11:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:11:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4398,7 +4397,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:12:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:12:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4451,7 +4450,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:12:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:12:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4485,7 +4484,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:13:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:13:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4568,7 +4567,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:21:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:21:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4662,7 +4661,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:18:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:18:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4699,7 +4698,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:16:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:16:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4736,7 +4735,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:16:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:16:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4781,7 +4780,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:24:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:24:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4820,7 +4819,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:18:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:18:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4856,7 +4855,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:7:7 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:7:7 error: 'workgroupBarrier' must only be called from uniform control flow
       workgroupBarrier();
       ^^^^^^^^^^^^^^^^
 
@@ -4892,7 +4891,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4929,7 +4928,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:16:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:16:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -4966,7 +4965,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:16:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:16:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5007,7 +5006,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:20:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:20:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5066,7 +5065,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:14:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:14:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5123,7 +5122,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:14:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:14:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5178,7 +5177,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:12:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:12:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5216,7 +5215,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:14:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:14:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5256,7 +5255,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:16:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:16:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5324,10 +5323,9 @@
     main_body.Push(b.If(b.Equal("v254", 0_i), b.Block(b.CallStmt(b.Call("workgroupBarrier")))));
     b.Func("main", utils::Empty, ty.void_(), main_body);
 
-    // TODO(jrprice): Expect false when uniformity issues become errors.
-    EXPECT_TRUE(RunTest(std::move(b))) << error_;
+    EXPECT_FALSE(RunTest(std::move(b)));
     EXPECT_EQ(error_,
-              R"(warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(error: 'workgroupBarrier' must only be called from uniform control flow
 note: control flow depends on non-uniform value
 note: reading from module-scope private variable 'non_uniform_global' may result in a non-uniform value)");
 }
@@ -5363,7 +5361,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:6:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:6:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5392,7 +5390,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5437,7 +5435,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5470,7 +5468,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5503,7 +5501,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5532,7 +5530,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5581,7 +5579,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:10:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:10:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5614,7 +5612,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:12:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:12:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5667,7 +5665,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:12:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:12:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5704,7 +5702,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:13:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:13:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5741,7 +5739,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:13:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:13:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5782,7 +5780,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:6:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:6:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5811,7 +5809,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5856,7 +5854,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5886,7 +5884,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5919,7 +5917,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5952,7 +5950,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:9:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:9:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -5986,7 +5984,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:10:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:10:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6040,7 +6038,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:6:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:6:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6067,7 +6065,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:6:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:6:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6094,7 +6092,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:6:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:6:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6124,7 +6122,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6154,7 +6152,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:8:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:8:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6188,7 +6186,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:14:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:14:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -6227,7 +6225,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -6266,7 +6264,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -6305,7 +6303,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:3 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:3 error: 'workgroupBarrier' must only be called from uniform control flow
   workgroupBarrier();
   ^^^^^^^^^^^^^^^^
 
@@ -6448,7 +6446,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:6:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:6:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6475,7 +6473,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:6:5 warning: 'storageBarrier' must only be called from uniform control flow
+              R"(test:6:5 error: 'storageBarrier' must only be called from uniform control flow
     storageBarrier();
     ^^^^^^^^^^^^^^
 
@@ -6532,10 +6530,9 @@
     foo_body.Push(b.If(b.Equal(v_last, 0_i), b.Block(b.CallStmt(b.Call("workgroupBarrier")))));
     b.Func("foo", utils::Empty, ty.void_(), foo_body);
 
-    // TODO(jrprice): Expect false when uniformity issues become errors.
-    EXPECT_TRUE(RunTest(std::move(b))) << error_;
+    EXPECT_FALSE(RunTest(std::move(b)));
     EXPECT_EQ(error_,
-              R"(warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(error: 'workgroupBarrier' must only be called from uniform control flow
 note: control flow depends on non-uniform value
 note: reading from module-scope private variable 'v0' may result in a non-uniform value)");
 }
@@ -6561,7 +6558,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:10:5 warning: 'foo' must only be called from uniform control flow
+              R"(test:10:5 error: 'foo' must only be called from uniform control flow
     foo();
     ^^^
 
@@ -6604,7 +6601,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:18:5 warning: 'foo' must only be called from uniform control flow
+              R"(test:18:5 error: 'foo' must only be called from uniform control flow
     foo();
     ^^^
 
@@ -6647,7 +6644,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:19:7 warning: parameter 'c' of 'foo' must be uniform
+              R"(test:19:7 error: parameter 'c' of 'foo' must be uniform
   foo(non_uniform);
       ^^^^^^^^^^^
 
@@ -6694,7 +6691,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:18:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:18:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6732,7 +6729,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6774,7 +6771,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^
 
@@ -6820,7 +6817,7 @@
 
     RunTest(src, false);
     EXPECT_EQ(error_,
-              R"(test:15:5 warning: 'workgroupBarrier' must only be called from uniform control flow
+              R"(test:15:5 error: 'workgroupBarrier' must only be called from uniform control flow
     workgroupBarrier();
     ^^^^^^^^^^^^^^^^