[libc++] <type_traits>: Avoid instantiating a pointer type.

[bsdjhb]

Summary

GCC expands the pointer type in this conditional expression even for
template types _Up that are not arrays. This raises an error when
std::decay<> is used with reference types (as is done in LLVM's
sources). Using add_pointer<> causes GCC to only instantiate a
pointer type for array types.

[bsdjhb]

I ran into this error in a perhaps unusual environment which is using GCC 12.0 to compile LLVM 14 (included in FreeBSD) using libc++ as the C++ runtime (rather than using libstdc++). The exact error from GCC is included in the review I opened for FreeBSD at https://reviews.freebsd.org/D36898. I forward ported the patch from LLVM 14 but do not have an easy way to test the setup I used to reproduce the GCC error on LLVM 14. I did not use __add_pointer_t because line 49 uses add_pointer<>, but if __add_pointer_t is preferred I'm happy to switch it to that instead.

[philnik]

Please add a regression test in libcxx/test/std/meta/type.traits/meta.trans/meta.trans.other.

libcxx/include/__type_traits/decay.h

45

[bsdjhb]

• Add tests.

[ldionne]

Thanks for fixing! This LGTM but I'll let @philnik give the libc++ approval since he had requested changes.

[ldionne]

Also, please rebase onto main and reupload the diff. It should fix the failing CI.

[bsdjhb]

Rebase

[philnik]

LGTM with green CI.

libcxx/include/__type_traits/decay.h

45

[bsdjhb]

Adjust formatting.

libcxx/include/__type_traits/decay.h

 };
 
 template <class _Up>
 struct __decay<_Up, true> {
 public:
     typedef _LIBCPP_NODEBUG typename conditional
                      <
                          is_array<_Up>::value,
-                         __remove_extent_t<_Up>*,
+                         __add_pointer_t<__remove_extent_t<_Up> >,
                          typename conditional
                          <
                               is_function<_Up>::value,
                               typename add_pointer<_Up>::type,
                               __remove_cv_t<_Up>
                          >::type
                      >::type type;
 };

libcxx/test/std/utilities/meta/meta.trans/meta.trans.other/decay.pass.cpp

 }
 
 int main(int, char**)
 {
     test_decay<void, void>();
     test_decay<int, int>();
     test_decay<const volatile int, int>();
     test_decay<int*, int*>();
+    test_decay<int&, int>();
+    test_decay<const volatile int&, int>();
     test_decay<int[3], int*>();
     test_decay<const int[3], const int*>();
     test_decay<void(), void (*)()>();
 #if TEST_STD_VER > 11
+    test_decay<int&&, int>();
+    test_decay<const volatile int&&, int>();
     test_decay<int(int) const, int(int) const>();
     test_decay<int(int) volatile, int(int) volatile>();
     test_decay<int(int)  &, int(int)  &>();
     test_decay<int(int) &&, int(int) &&>();
 #endif
 
   return 0;
 }