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

/*
PERMUTE_ARGS: -preview=in
TEST_OUTPUT:
---
fail_compilation/diagin.d(14): Error: function `diagin.foo(in int)` is not callable using argument types `()`
fail_compilation/diagin.d(14):        too few arguments, expected `1`, got `0`
fail_compilation/diagin.d(16): Error: none of the overloads of template `diagin.foo1` are callable using argument types `!()(bool[])`
fail_compilation/diagin.d(20):        Candidate is: `foo1(T)(in T v, string)`
---
 */

void main ()
{
    foo();
    bool[] lvalue;
    foo1(lvalue);
}

void foo(in int) {}
void foo1(T)(in T v, string) {}

// Ensure that `in` has a unique mangling
static assert(foo.mangleof       == `_D6diagin3fooFIiZv`);
static assert(foo1!int.mangleof  == `_D6diagin__T4foo1TiZQiFNaNbNiNfIiAyaZv`);
static assert(foo1!char.mangleof == `_D6diagin__T4foo1TaZQiFNaNbNiNfIaAyaZv`);