gcc/testsuite/gdc.test/fail_compilation/diaginref.d

/*
REQUIRED_ARGS: -preview=in
TEST_OUTPUT:
---
fail_compilation/diaginref.d(11): Error: attribute `ref` is redundant with previously-applied `in`
fail_compilation/diaginref.d(13): Error: attribute `in` cannot be added after `ref`: remove `ref`
---
 */

void foo(in string) {}
void foo1(in ref string) {}
void foo2(T)(in T v, string) {}
void foo3(T)(ref in T v, string) {}