Improve error on unsigned implicit conversion to signed.

Author: lerno

releasenotes.md

@@ -13,6 +13,7 @@
 - Improve error message when doing a rethrow in a function that doesn't return an optional.
 - Add `--list-asm` to view all supported `asm` instructions.
 - Formatting option "%h" now supports pointers.
+- Improve error on unsigned implicit conversion to signed.
 
 ### Fixes
 - mkdir/rmdir would not work properly with substring paths on non-windows platforms.

src/compiler/sema_expr.c

@@ -7134,6 +7134,22 @@ static bool sema_replace_with_overload(SemaContext *context, Expr *expr, Expr *l
 	return true;
 }
 
+static inline bool sema_check_untyped_promotion(SemaContext *context, Expr *expr, bool is_left, CanonicalType *max_flat, Type *max)
+{
+	Type *flat = type_flatten(expr->type);
+	if (!type_is_unsigned(flat) || type_size(max_flat) != type_size(flat)) return true;
+	if (sema_cast_const(expr) && expr_is_const_int(expr) && expr_const_will_overflow(&expr->const_expr, max_flat->type_kind))
+	{
+		RETURN_SEMA_ERROR(expr,
+			"This expression (%s) will be implicitly converted to a signed type due to the %s-hand side being signed, but the value does not fit %s. "
+			"To fix this, either cast the value explicitly, or make the %s-hand side an unsigned type.",
+			expr_const_to_error_string(&expr->const_expr), is_left ? "right" : "left", type_quoted_error_string(max),
+			is_left ? "right" : "left");
+	}
+
+	return true;
+
+}
 static bool sema_binary_arithmetic_promotion(SemaContext *context, Expr *left, Expr *right, Type *left_type, Type *right_type,
 											 Expr *parent, const char *error_message, bool allow_bool_vec,
 											 OperatorOverload *operator_overload_ref,
@@ -7155,6 +7171,12 @@ static bool sema_binary_arithmetic_promotion(SemaContext *context, Expr *left, E
 		}
 		RETURN_SEMA_ERROR(parent, error_message, type_quoted_error_string(left->type), type_quoted_error_string(right->type));
 	}
+	Type *flat_max = type_flatten(max);
+	if (type_is_signed(flat_max))
+	{
+		if (!sema_check_untyped_promotion(context, left, true, flat_max, max)) return false;
+		if (!sema_check_untyped_promotion(context, right, false, flat_max, max)) return false;
+	}
 	return cast_implicit_binary(context, left, max, failed_ref) &&
 		   cast_implicit_binary(context, right, max, failed_ref);
 }

test/test_suite/cast/out_of_range_unlong.c3

@@ -0,0 +1,6 @@
+fn int main()
+{
+	ulong x = ulong.max - 1; // #error: This expression (18446744073709551615) will be implicitly converted to a signed type due to the right-hand side being signed
+	ulong x1 = -1 + ulong.max; // #error: converted to a signed type due to the left-hand side being signed, but the value does
+	return 0;
+}