cgen: fix mutable ptr sumtype (#24021)

vlib/v/gen/c/cgen.v

@@ -2757,7 +2757,7 @@ fn (mut g Gen) call_cfn_for_casting_expr(fname string, expr ast.Expr, exp ast.Ty
 	mut mutable_idx := 0
 
 	is_not_ptr_and_fn := !got_is_ptr && !got_is_fn
-	is_sumtype_cast := is_not_ptr_and_fn && fname.contains('_to_sumtype_')
+	is_sumtype_cast := !got_is_fn && fname.contains('_to_sumtype_')
 	is_comptime_variant := is_not_ptr_and_fn && expr is ast.Ident
 		&& g.comptime.is_comptime_variant_var(expr)
 

vlib/v/tests/sumtypes/sumtype_mutable_test.v

@@ -0,0 +1,30 @@
+module main
+
+type MySumType = MyStructA | MyStructB
+
+struct MyStructA {
+mut:
+	test bool
+}
+
+struct MyStructB {
+}
+
+fn test_main() {
+	mut my_struct := &MyStructA{
+		test: false
+	}
+	if $d('mutable_sumtype', false) {
+		assert my_struct.test == false
+		my_struct.test = true
+		assert my_struct.test == true
+		but_why(mut my_struct)
+		assert my_struct.test == false
+	}
+}
+
+fn but_why(mut passed MySumType) {
+	if mut passed is MyStructA {
+		passed.test = false
+	}
+}