cgen, checker: fix for with mut generic value (fix #24360) (#24426)

Author: felipensp

vlib/v/checker/for.v

@@ -200,8 +200,12 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 				}
 			}
 
-			value_type := c.table.value_type(unwrapped_typ)
+			mut value_type := c.table.value_type(unwrapped_typ)
+			if node.val_is_mut {
+				value_type = value_type.ref()
+			}
 			node.scope.update_var_type(node.val_var, value_type)
+			node.val_type = value_type
 
 			if is_comptime {
 				c.type_resolver.update_ct_type(node.val_var, value_type)

vlib/v/gen/c/for.v

@@ -154,6 +154,9 @@ fn (mut g Gen) for_in_stmt(node_ ast.ForInStmt) {
 
 		node.cond_type = unwrapped_typ
 		node.val_type = g.table.value_type(unwrapped_typ)
+		if node.val_is_mut {
+			node.val_type = node.val_type.ref()
+		}
 		node.scope.update_var_type(node.val_var, node.val_type)
 		node.kind = unwrapped_sym.kind
 

vlib/v/tests/generics/generic_for_mut_val_test.v

@@ -0,0 +1,42 @@
+struct ValueMenu {
+	name string
+}
+
+fn (mut v ValueMenu) resp(resp []u8) {
+	println('menu ${v.name} ${resp}')
+}
+
+fn value_resp[T](mut t T, r []u8) {
+	$if T is $array {
+		value_array_resp(mut t, r)
+	}
+}
+
+fn value_array_resp[T](mut t T, r []u8) {
+	$if T is []ValueMenu {
+		for _, mut i in t { // mut i -> C error
+			i.resp(r)
+		}
+		for i := 0; i < t.len; i++ { // WORKAROUND
+			t[i].resp(r)
+		}
+	}
+}
+
+struct Colors {
+mut:
+	normal []ValueMenu = [ValueMenu{'blink'}, ValueMenu{'hue'}]
+}
+
+fn (c &Colors) resp(resp []u8) {
+	$for f in c.fields {
+		mut m := c.$(f.name)
+		value_resp(mut m, resp)
+	}
+}
+
+fn test_main() {
+	colors := &Colors{}
+	colors.resp([u8(0), 1, 2])
+	assert true
+}