resolver: Replace ICEs with proper errors

If you attempted to:
 * Use a function as a type
 * Use a variable as a type
 * Use a variable as a call target

You would get an ICE. Replace these with proper error diagnostics.

Change-Id: Ibbe4cd1b59b1aadd451aa0445ad137859aa071eb
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/79765
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/resolver/call_validation_test.cc

@@ -252,6 +252,19 @@ TEST_F(ResolverCallValidationTest, LetPointerPrivate) {
             "identifier expression or a function parameter");
 }
 
+TEST_F(ResolverCallValidationTest, CallVariable) {
+  // var v : i32;
+  // fn f() {
+  //   v();
+  // }
+  Global("v", ty.i32(), ast::StorageClass::kPrivate);
+  Func("f", {}, ty.void_(), {CallStmt(Call(Source{{12, 34}}, "v"))});
+
+  EXPECT_FALSE(r()->Resolve());
+  EXPECT_EQ(r()->error(), R"(error: cannot call variable 'v'
+note: 'v' declared here)");
+}
+
 }  // namespace
 }  // namespace resolver
 }  // namespace tint

src/resolver/resolver.cc

@@ -266,12 +266,26 @@ sem::Type* Resolver::Type(const ast::Type* ty) {
     if (ty->As<ast::ExternalTexture>()) {
       return builder_->create<sem::ExternalTexture>();
     }
-    if (auto* type = ResolvedSymbol<sem::Type>(ty)) {
-      return type;
-    }
+    return Switch(
+        ResolvedSymbol(ty),  //
+        [&](sem::Type* type) { return type; },
+        [&](sem::Variable* var) {
+          auto name = builder_->Symbols().NameFor(var->Declaration()->symbol);
+          AddError("cannot use variable '" + name + "' as type", ty->source);
+          AddNote("'" + name + "' declared here", var->Declaration()->source);
+          return nullptr;
+        },
+        [&](sem::Function* func) {
+          auto name = builder_->Symbols().NameFor(func->Declaration()->symbol);
+          AddError("cannot use function '" + name + "' as type", ty->source);
+          AddNote("'" + name + "' declared here", func->Declaration()->source);
+          return nullptr;
+        },
+        [&](Default) {
           TINT_UNREACHABLE(Resolver, diagnostics_)
               << "Unhandled ast::Type: " << ty->TypeInfo().name;
           return nullptr;
+        });
   }();
 
   if (s) {
@@ -1347,18 +1361,24 @@ sem::Call* Resolver::Call(const ast::CallExpression* expr) {
   Mark(ident);
 
   auto* resolved = ResolvedSymbol(ident);
-  if (auto* ty = As<sem::Type>(resolved)) {
-    return type_ctor_or_conv(ty);
-  }
-
-  if (auto* fn = As<sem::Function>(resolved)) {
-    return FunctionCall(expr, fn, std::move(args), arg_behaviors);
-  }
-
+  return Switch(
+      resolved,  //
+      [&](sem::Type* type) { return type_ctor_or_conv(type); },
+      [&](sem::Function* func) {
+        return FunctionCall(expr, func, std::move(args), arg_behaviors);
+      },
+      [&](sem::Variable* var) {
+        auto name = builder_->Symbols().NameFor(var->Declaration()->symbol);
+        AddError("cannot call variable '" + name + "'", ident->source);
+        AddNote("'" + name + "' declared here", var->Declaration()->source);
+        return nullptr;
+      },
+      [&](Default) -> sem::Call* {
         auto name = builder_->Symbols().NameFor(ident->symbol);
         auto builtin_type = sem::ParseBuiltinType(name);
         if (builtin_type != sem::BuiltinType::kNone) {
-    return BuiltinCall(expr, builtin_type, std::move(args), std::move(arg_tys));
+          return BuiltinCall(expr, builtin_type, std::move(args),
+                             std::move(arg_tys));
         }
 
         TINT_ICE(Resolver, diagnostics_)
@@ -1367,6 +1387,7 @@ sem::Call* Resolver::Call(const ast::CallExpression* expr) {
             << "\n"
             << "name: " << builder_->Symbols().NameFor(ident->symbol);
         return nullptr;
+      });
 }
 
 sem::Call* Resolver::BuiltinCall(const ast::CallExpression* expr,

src/resolver/type_validation_test.cc

@@ -695,6 +695,30 @@ TEST_F(ResolverTypeValidationTest, ArrayOfNonStorableType) {
             "an array");
 }
 
+TEST_F(ResolverTypeValidationTest, VariableAsType) {
+  // var<private> a : i32;
+  // var<private> b : a;
+  Global("a", ty.i32(), ast::StorageClass::kPrivate);
+  Global("b", ty.type_name("a"), ast::StorageClass::kPrivate);
+
+  EXPECT_FALSE(r()->Resolve());
+  EXPECT_EQ(r()->error(),
+            R"(error: cannot use variable 'a' as type
+note: 'a' declared here)");
+}
+
+TEST_F(ResolverTypeValidationTest, FunctionAsType) {
+  // fn f() {}
+  // var<private> v : f;
+  Func("f", {}, ty.void_(), {});
+  Global("v", ty.type_name("f"), ast::StorageClass::kPrivate);
+
+  EXPECT_FALSE(r()->Resolve());
+  EXPECT_EQ(r()->error(),
+            R"(error: cannot use function 'f' as type
+note: 'f' declared here)");
+}
+
 namespace GetCanonicalTests {
 struct Params {
   builder::ast_type_func_ptr create_ast_type;