checker: fix return error with multi_return optional (#16250)

vlib/v/checker/return.v

@@ -94,6 +94,21 @@ pub fn (mut c Checker) return_stmt(mut node ast.Return) {
 		return
 	}
 	if expected_types.len > 0 && expected_types.len != got_types.len {
+		// `fn foo() !(int, string) { return Err{} }`
+		if (exp_is_optional || exp_is_result) && node.exprs.len == 1 {
+			got_typ := c.expr(node.exprs[0])
+			got_typ_sym := c.table.sym(got_typ)
+			if got_typ_sym.kind == .struct_ && c.type_implements(got_typ, ast.error_type, node.pos) {
+				node.exprs[0] = ast.CastExpr{
+					expr: node.exprs[0]
+					typname: 'IError'
+					typ: ast.error_type
+					expr_type: got_typ
+				}
+				node.types[0] = ast.error_type
+				return
+			}
+		}
 		arg := if expected_types.len == 1 { 'argument' } else { 'arguments' }
 		midx := imax(0, imin(expected_types.len, expr_idxs.len - 1))
 		mismatch_pos := node.exprs[expr_idxs[midx]].pos()
@@ -138,6 +153,7 @@ pub fn (mut c Checker) return_stmt(mut node ast.Return) {
 					}
 					continue
 				}
+				// `fn foo() !int { return Err{} }`
 				if got_typ_sym.kind == .struct_
 					&& c.type_implements(got_typ, ast.error_type, node.pos) {
 					node.exprs[expr_idxs[i]] = ast.CastExpr{

vlib/v/tests/results_multi_return_test.v

@@ -16,9 +16,9 @@ fn foo() ?string {
 }
 
 fn bar() !(string, int) {
-	a := foo() or { return IError(Err{
+	a := foo() or { return Err{
 		msg: 'error test'
-	}) }
+	} }
 	return a, 1
 }
 

vlib/v/tests/string_optional_none_test.v

@@ -12,7 +12,7 @@ fn (err MyError) code() int {
 }
 
 fn foo() int|none|IError {
-	return IError(MyError{})
+	return MyError{}
 }
 
 fn test_string_optional_none() {

vlib/vweb/parse.v

@@ -24,7 +24,7 @@ fn parse_attrs(name string, attrs []string) !([]http.Method, string) {
 		}
 		if attr.starts_with('/') {
 			if path != '' {
-				return IError(http.MultiplePathAttributesError{})
+				return http.MultiplePathAttributesError{}
 			}
 			path = attr
 			x.delete(i)
@@ -33,9 +33,9 @@ fn parse_attrs(name string, attrs []string) !([]http.Method, string) {
 		i++
 	}
 	if x.len > 0 {
-		return IError(http.UnexpectedExtraAttributeError{
+		return http.UnexpectedExtraAttributeError{
 			attributes: x
-		})
+		}
 	}
 	if methods.len == 0 {
 		methods = [http.Method.get]