checker: fix returning error on reference results (#14313)

vlib/v/checker/return.v

@@ -24,6 +24,7 @@ pub fn (mut c Checker) return_stmt(mut node ast.Return) {
 		return
 	}
 	exp_is_optional := expected_type.has_flag(.optional)
+	exp_is_result := expected_type.has_flag(.result)
 	mut expected_types := [expected_type]
 	if expected_type_sym.info is ast.MultiReturn {
 		expected_types = expected_type_sym.info.types
@@ -72,11 +73,13 @@ pub fn (mut c Checker) return_stmt(mut node ast.Return) {
 			}
 		}
 	}
-	// allow `none` & `error` return types for function that returns optional
+	// allow `none` & `error` return types for function that returns option or result
 	option_type_idx := c.table.type_idxs['Option']
+	result_type_idx := c.table.type_idxs['_result']
 	got_types_0_idx := got_types[0].idx()
-	if exp_is_optional
+	if (exp_is_optional
-		&& got_types_0_idx in [ast.none_type_idx, ast.error_type_idx, option_type_idx] {
+		&& got_types_0_idx in [ast.none_type_idx, ast.error_type_idx, option_type_idx])
+		|| (exp_is_result && got_types_0_idx in [ast.error_type_idx, result_type_idx]) {
 		if got_types_0_idx == ast.none_type_idx && expected_type == ast.ovoid_type {
 			c.error('returning `none` in functions, that have a `?` result type is not allowed anymore, either `return error(message)` or just `return` instead',
 				node.pos)

vlib/v/tests/results_test.v

@@ -73,3 +73,11 @@ fn unsafe_return_error() !int {
 fn test_unsafe_return_error() {
 	unsafe_return_error() or { assert err.msg() == 'abc' }
 }
+
+fn return_reference_type(path string) !&string {
+	if path.len == 0 {
+		return error('vfopen called with ""')
+	}
+	str := ''
+	return &str
+}