constexpr: treat comparisons between types as integer constexpr

The expression parsing code builds an EXPR_COMPARE expression around two
EXPR_TYPE expressions for __builtin_types_compatible_p().

The EXPR_TYPE expressions are tagged as being integer constant expressions
in order to trick the generic comparison evaluation code into flagging the
result as an integer constant expression again.

Avoid this trickery by making evaluate_compare() unconditionally tag a
comparison between types as an integer constant expression.

Signed-off-by: Nicolai Stange <[email protected]>
Signed-off-by: Luc Van Oostenryck <[email protected]>

Author: nicstange

evaluate.c

@@ -1016,16 +1016,21 @@ static struct symbol *evaluate_compare(struct expression *expr)
 	struct symbol *ctype;
 	const char *typediff;
 
-	expr->flags = left->flags & right->flags & ~CEF_CONST_MASK;
-	expr->flags &= ~CEF_ADDR;
-
 	/* Type types? */
-	if (is_type_type(ltype) && is_type_type(rtype))
+	if (is_type_type(ltype) && is_type_type(rtype)) {
+		/*
+		 * __builtin_types_compatible_p() yields an integer
+		 * constant expression
+		 */
+		expr->flags = CEF_SET_ICE;
 		goto OK;
+	}
 
 	if (is_safe_type(left->ctype) || is_safe_type(right->ctype))
 		warning(expr->pos, "testing a 'safe expression'");
 
+	expr->flags = left->flags & right->flags & ~CEF_CONST_MASK & ~CEF_ADDR;
+
 	/* number on number */
 	if (lclass & rclass & TYPE_NUM) {
 		ctype = usual_conversions(expr->op, expr->left, expr->right,

expression.c

@@ -131,7 +131,6 @@ static struct token *parse_type(struct token *token, struct expression **tree)
 {
 	struct symbol *sym;
 	*tree = alloc_expression(token->pos, EXPR_TYPE);
-	(*tree)->flags = CEF_SET_ICE; /* sic */
 	token = typename(token, &sym, NULL);
 	if (sym->ident)
 		sparse_error(token->pos,
@@ -459,8 +458,6 @@ struct token *primary_expression(struct token *token, struct expression **tree)
 		}
 		if (token->special == '[' && lookup_type(token->next)) {
 			expr = alloc_expression(token->pos, EXPR_TYPE);
-			/* sic */
-			expr->flags = CEF_SET_ICE;
 			token = typename(token->next, &expr->symbol, NULL);
 			token = expect(token, ']', "in type expression");
 			break;

validation/constexpr-types-compatible-p.c

@@ -0,0 +1,8 @@
+static int a[] = {[__builtin_types_compatible_p(int, int)] = 0};
+
+/*
+ * check-name: __builtin_types_compatible_p() constness verification.
+ *
+ * check-error-start
+ * check-error-end
+ */