parser: check fn decl multi return types without parentheses (#14508)

vlib/v/parser/fn.v

@@ -400,6 +400,11 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		p.inside_fn_return = false
 		return_type_pos = return_type_pos.extend(p.prev_tok.pos())
 	}
+	if p.tok.kind == .comma {
+		mr_pos := return_type_pos.extend(p.peek_tok.pos())
+		p.error_with_pos('multiple return types in function declaration must use parentheses, .e.g (int, string)',
+			mr_pos)
+	}
 	mut type_sym_method_idx := 0
 	no_body := p.tok.kind != .lcbr
 	end_pos := p.prev_tok.pos()

vlib/v/parser/tests/fn_decl_multi_return_types_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/fn_decl_multi_return_types_err.vv:4:10: error: multiple return types in function declaration must use parentheses, .e.g (int, string)
+    2 | }
+    3 |
+    4 | fn foo() int, int {
+      |          ~~~~~~~~
+    5 |     return 0, 0
+    6 | }

vlib/v/parser/tests/fn_decl_multi_return_types_err.vv

@@ -0,0 +1,6 @@
+fn main() {
+}
+
+fn foo() int, int {
+	return 0, 0
+}