parser: disallow defining methods on option receivers - fn (x ?Type) method() { (#17351)

vlib/v/checker/fn.v

@@ -146,6 +146,9 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 		}
 	}
 	if node.is_method {
+		if node.receiver.typ.has_flag(.option) {
+			c.error('option types cannot have methods', node.receiver_pos)
+		}
 		mut sym := c.table.sym(node.receiver.typ)
 		if sym.kind == .array && !c.is_builtin_mod && node.name == 'map' {
 			// TODO `node.map in array_builtin_methods`

vlib/v/checker/tests/option_in_receiver_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/option_in_receiver_err.vv:10:9: error: option types cannot have methods
+    8 | }
+    9 | 
+   10 | fn (mut f ?Foo) t1(arr ?[]int) ?string {
+      |         ~~~
+   11 |     return arr?.len.str()
+   12 | }

vlib/v/checker/tests/option_in_receiver_err.vv

@@ -0,0 +1,12 @@
+struct Foo {
+mut:
+	name string
+}
+
+fn (mut f Foo) t0(arr ?[]int) ?string {
+	return arr?.len.str()
+}
+
+fn (mut f ?Foo) t1(arr ?[]int) ?string {
+	return arr?.len.str()
+}