tint/uniformity: Fix struct member partial pointers

This assertion was mistakenly changed to only allow index accessors
when the ast::AccessorExpression base class was added.

Bug: chromium:1420257
Change-Id: Ic2b695dd7605c8852ab6d822602dca5d28ecf2d4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/122103
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/tint/resolver/uniformity.cc

@@ -1353,14 +1353,14 @@ class UniformityGraph {
         // To determine if we're dereferencing a partial pointer, unwrap *&
         // chains; if the final expression is an identifier, see if it's a
         // partial pointer. If it's not an identifier, then it must be an
-        // index/accessor expression, and thus a partial pointer.
+        // index/member accessor expression, and thus a partial pointer.
         auto* e = UnwrapIndirectAndAddressOfChain(u);
         if (auto* var_user = sem_.Get<sem::VariableUser>(e)) {
             if (current_function_->partial_ptrs.Contains(var_user->Variable())) {
                 return true;
             }
         } else {
-            TINT_ASSERT(Resolver, e->Is<ast::IndexAccessorExpression>());
+            TINT_ASSERT(Resolver, e->Is<ast::AccessorExpression>());
             return true;
         }
         return false;

src/tint/resolver/uniformity_test.cc

@@ -6375,6 +6375,43 @@ test:11:9 note: reading from read_write storage buffer 'rw' may result in a non-
 )");
 }
 
+TEST_F(UniformityAnalysisTest, StructMember_MemberBecomesUniformThroughPartialPointer) {
+    // For aggregate types, we conservatively consider them to be non-uniform once they
+    // become non-uniform. Test that after assigning a uniform value to a member, that member is
+    // still considered to be non-uniform.
+    std::string src = R"(
+struct S {
+  a : i32,
+  b : i32,
+}
+@group(0) @binding(0) var<storage, read_write> rw : i32;
+
+fn foo() {
+  var s : S;
+  s.a = rw;
+  *&s.a = 0;
+  if (s.a == 0) {
+    workgroupBarrier();
+  }
+}
+)";
+
+    RunTest(src, false);
+    EXPECT_EQ(error_,
+              R"(test:13:5 error: 'workgroupBarrier' must only be called from uniform control flow
+    workgroupBarrier();
+    ^^^^^^^^^^^^^^^^
+
+test:12:3 note: control flow depends on possibly non-uniform value
+  if (s.a == 0) {
+  ^^
+
+test:10:9 note: reading from read_write storage buffer 'rw' may result in a non-uniform value
+  s.a = rw;
+        ^^
+)");
+}
+
 TEST_F(UniformityAnalysisTest, StructMember_MemberBecomesUniformThroughCapturedPartialPointer) {
     // For aggregate types, we conservatively consider them to be non-uniform once they
     // become non-uniform. Test that after assigning a uniform value to a member, that member is