2.059 HEAD

If you change std.utf.toUTFz to

const(wchar)* toUTF16z(C)(const(C)[] str) @system
    if(isSomeChar!C)
{
    return toUTFz!(const(wchar)*)(str);
}

unittest
{
    toUTF16z("hello world");
}

then compilation fails with this error:

std/utf.d(1373): Error: pure function 'toUTF16z' cannot call impure function 'toUTFz'
std/utf.d(1378): Error: template instance std.utf.toUTF16z!(char) error instantiating
make[1]: *** [generated/linux/debug/64/unittest/std/utf] Error 1
make: *** [unittest] Error 2

toUTFz is _not_ marked as pure, and neither is toUTF16z. I don't believe that toUTFz _can_ be pure (certainly, it _shouldn't_ be given what it does), so it should always infer as impure, which would cause toUTF16z be inferred as impure if it's changed to call toUTFz as is done above.

However, if you move toUTF16z to after the definition of toUTFz, then it compiles. So, it appears to be a forward declaration issue.