Resolver: validate else condition is boolean

Bug: tint:786
Change-Id: I5e4987e4eeb0608c640fc5c8ea035c8dca0ef187
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/50461
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

src/resolver/resolver.cc

@@ -1196,6 +1196,15 @@ bool Resolver::IfStatement(ast::IfStatement* stmt) {
       if (!Expression(cond)) {
         return false;
       }
+
+      auto* else_cond_type = TypeOf(cond)->UnwrapAll();
+      if (else_cond_type != builder_->ty.bool_()) {
+        diagnostics_.add_error(
+            "else statement condition must be bool, got " +
+                else_cond_type->FriendlyName(builder_->Symbols()),
+            cond->source());
+        return false;
+      }
     }
     Mark(else_stmt->body());
     if (!BlockStatement(else_stmt->body())) {

src/resolver/resolver_test.cc

@@ -125,7 +125,7 @@ TEST_F(ResolverTest, Stmt_If) {
 
   auto* else_body = Block(Assign(else_lhs, else_rhs));
 
-  auto* else_cond = Expr(3);
+  auto* else_cond = Expr(true);
   auto* else_stmt = create<ast::ElseStatement>(else_cond, else_body);
 
   auto* lhs = Expr("v");

src/resolver/validation_test.cc

@@ -147,6 +147,20 @@ TEST_F(ResolverValidationTest, Stmt_If_NonBool) {
             "12:34 error: if statement condition must be bool, got f32");
 }
 
+TEST_F(ResolverValidationTest, Stmt_Else_NonBool) {
+  // else (1.23f) {}
+
+  WrapInFunction(If(Expr(true), Block(),
+                    Else(create<ast::ScalarConstructorExpression>(
+                             Source{{12, 34}}, Literal(1.23f)),
+                         Block())));
+
+  EXPECT_FALSE(r()->Resolve());
+
+  EXPECT_EQ(r()->error(),
+            "12:34 error: else statement condition must be bool, got f32");
+}
+
 TEST_F(ResolverValidationTest,
        Stmt_VariableDecl_MismatchedTypeScalarConstructor) {
   u32 unsigned_value = 2u;  // Type does not match variable type