parser: add a more precise and helpful error message for int! and int? result type syntax (#20061)

Author: felipensp

vlib/v/checker/tests/wrong_option_type.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/wrong_option_type.vv:1:10: error: wrong syntax, it must be ?int, not int?
+    1 | fn foo() int?{
+      |          ~~~
+    2 |     return 0
+    3 | }

vlib/v/checker/tests/wrong_option_type.vv

@@ -0,0 +1,3 @@
+fn foo() int?{
+	return 0
+}

vlib/v/checker/tests/wrong_result_type.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/wrong_result_type.vv:1:10: error: wrong syntax, it must be !int, not int!
+    1 | fn foo() int!{
+      |          ~~~
+    2 |     return 0
+    3 | }

vlib/v/checker/tests/wrong_result_type.vv

@@ -0,0 +1,3 @@
+fn foo() int!{
+	return 0
+}

vlib/v/parser/fn.v

@@ -440,7 +440,14 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		return_type = p.parse_type()
 		p.inside_fn_return = false
 		return_type_pos = return_type_pos.extend(p.prev_tok.pos())
+
+		if p.tok.kind in [.question, .not] {
+			ret_type_sym := p.table.sym(return_type)
+			p.error_with_pos('wrong syntax, it must be ${p.tok.kind}${ret_type_sym.name}, not ${ret_type_sym.name}${p.tok.kind}',
+				return_type_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)',