resolver: Reenable VS validation for returning position

Fixed: tint:730
Change-Id: I7d5ebeffe2d522182d07f554a671bebab1cce2e6
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/54420
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>

src/resolver/entry_point_validation_test.cc

@@ -530,9 +530,7 @@ TEST_F(ResolverEntryPointValidationTest, Parameter_Struct_DuplicateLocation) {
 12:34 note: while analysing entry point main)");
 }
 
-// TODO(bclayton): Reenable after CTS is updated
-TEST_F(ResolverEntryPointValidationTest,
-       DISABLED_VertexShaderMustReturnPosition) {
+TEST_F(ResolverEntryPointValidationTest, VertexShaderMustReturnPosition) {
   // [[stage(vertex)]]
   // fn main() {}
   Func(Source{{12, 34}}, "main", {}, ty.void_(), {},

src/resolver/resolver.cc

@@ -1119,8 +1119,7 @@ bool Resolver::ValidateEntryPoint(const ast::Function* func,
         }
       }
     }
-    // TODO(bclayton): Reenable after CTS is updated
-    if (((false)) && !found) {
+    if (!found) {
       diagnostics_.add_error(
           "a vertex shader must include the 'position' builtin in its return "
           "type",

src/transform/single_entry_point_test.cc

@@ -103,7 +103,8 @@ fn main() {
 TEST_F(SingleEntryPointTest, MultipleEntryPoints) {
   auto* src = R"(
 [[stage(vertex)]]
-fn vert_main() {
+fn vert_main() -> [[builtin(position)]] vec4<f32> {
+  return vec4<f32>();
 }
 
 [[stage(fragment)]]
@@ -145,8 +146,9 @@ var<private> c : f32;
 var<private> d : f32;
 
 [[stage(vertex)]]
-fn vert_main() {
+fn vert_main() -> [[builtin(position)]] vec4<f32> {
   a = 0.0;
+  return vec4<f32>();
 }
 
 [[stage(fragment)]]
@@ -194,8 +196,9 @@ let c : f32 = 1.0;
 let d : f32 = 1.0;
 
 [[stage(vertex)]]
-fn vert_main() {
+fn vert_main() -> [[builtin(position)]] vec4<f32> {
   let local_a : f32 = a;
+  return vec4<f32>();
 }
 
 [[stage(fragment)]]