Re: [committed] libstdc++: std::make_signed_t<cv bool> should be ill-formed

[Patrick Palka <ppalka@redhat.com>]

On Mon, 10 Oct 2022, Jonathan Wakely via Libstdc++ wrote:

> Tested powerpc64le-linux. Pushed to trunk.
> 
> -- >8 --
> 
> Currently we only reject std::make_signed_t<bool> but not cv bool.
> Similarly for std::make_unsigned_t<cv bool>.
> 
> As well as making those ill-formed, this adds a requires-clause to the
> make_signed and make_unsigned primary templates. This makes
> non-integral, non-enum cases fail immediately with a clear error, rather
> than giving an error about __make_signed_selector<T, false, false> being
> incomplete.

IIUC the requires-clause turns what was once a hard error into a SFINAE
error, so e.g. for

  template<class T> typename make_signed<T>::type f(int);
  template<class T> void f(...);
  int main() { f<void>(0); }

the call to f would previously be rejected due to an error outside the
immediate context about incomplete __make_signed_selector, and now with
the requires-clause resolves to the second overload.  I wonder if this
new behavior is conforming -- the examples in [structure.specifications]
of how to implement 'Mandates' suggest that a failed 'Mandates' should
yield a hard error?

> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/std/type_traits (make_signed, make_unsigned): Add
> 	specializations for cv bool. Add requires-clause for C++20 to
> 	improve diagnostics for non-integral, non-enum cases.
> 	* testsuite/20_util/make_signed/requirements/typedefs_neg.cc:
> 	Check cv bool.
> 	* testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc:
> 	Likewise.
> 	* testsuite/24_iterators/range_access/range_access_cpp20_neg.cc:
> 	Adjust expected errors for C++20 and later.
> 	* testsuite/lib/prune.exp: Prune "in requirements  [with ...]"
> 	lines from diagnostics.
> ---
>  libstdc++-v3/include/std/type_traits          | 18 ++++++++++---
>  .../make_signed/requirements/typedefs_neg.cc  | 27 +++++++++----------
>  .../requirements/typedefs_neg.cc              | 25 ++++++++---------
>  .../range_access/range_access_cpp20_neg.cc    |  3 ++-
>  libstdc++-v3/testsuite/lib/prune.exp          |  1 +
>  5 files changed, 40 insertions(+), 34 deletions(-)
> 
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index b74565eb521..6108b98aa6a 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -1802,12 +1802,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    // Primary template.
>    /// make_unsigned
>    template<typename _Tp>
> +#if __cpp_concepts
> +    requires is_integral<_Tp>::value || __is_enum(_Tp)
> +#endif
>      struct make_unsigned
>      { typedef typename __make_unsigned_selector<_Tp>::__type type; };
>  
>    // Integral, but don't define.
> -  template<>
> -    struct make_unsigned<bool>;
> +  template<> struct make_unsigned<bool>;
> +  template<> struct make_unsigned<bool const>;
> +  template<> struct make_unsigned<bool volatile>;
> +  template<> struct make_unsigned<bool const volatile>;
>  
>    /// @cond undocumented
>  
> @@ -1932,12 +1937,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    // Primary template.
>    /// make_signed
>    template<typename _Tp>
> +#if __cpp_concepts
> +    requires is_integral<_Tp>::value || __is_enum(_Tp)
> +#endif
>      struct make_signed
>      { typedef typename __make_signed_selector<_Tp>::__type type; };
>  
>    // Integral, but don't define.
> -  template<>
> -    struct make_signed<bool>;
> +  template<> struct make_signed<bool>;
> +  template<> struct make_signed<bool const>;
> +  template<> struct make_signed<bool volatile>;
> +  template<> struct make_signed<bool const volatile>;
>  
>  #if __cplusplus > 201103L
>    /// Alias template for make_signed
> diff --git a/libstdc++-v3/testsuite/20_util/make_signed/requirements/typedefs_neg.cc b/libstdc++-v3/testsuite/20_util/make_signed/requirements/typedefs_neg.cc
> index 051bb64c710..88b8ae887ef 100644
> --- a/libstdc++-v3/testsuite/20_util/make_signed/requirements/typedefs_neg.cc
> +++ b/libstdc++-v3/testsuite/20_util/make_signed/requirements/typedefs_neg.cc
> @@ -20,31 +20,28 @@
>  // <http://www.gnu.org/licenses/>.
>  
>  #include <type_traits>
> -#include <testsuite_character.h>
>  
> -enum test_enum { first_selection };
> +struct pod_class { };
>  
>  void test01()
>  {
>    using std::make_signed;
>  
>    // Negative tests.
> -  typedef make_signed<bool>::type     	test1_type;
> +  using T1 = make_signed<bool>::type; // { dg-error "incomplete" }
> +  using T2 = make_signed<const bool>::type; // { dg-error "incomplete" }
> +  using T3 = make_signed<volatile bool>::type; // { dg-error "incomplete" }
> +  using T4 = make_signed<const volatile bool>::type; // { dg-error "incomplete" }
>  
> -  typedef make_signed<__gnu_test::pod_uint>::type     	test2_type;
> +  using T5 = make_signed<pod_class>::type; // { dg-error "here" }
>  
> -  typedef make_signed<int[4]>::type     test3_type;
> +  using T6 = make_signed<int[4]>::type; // { dg-error "here" }
>  
> -  typedef void (fn_type) ();
> -  typedef make_signed<fn_type>::type  	test4_type;
> +  using fn_type = void ();
> +  using T7 = make_signed<fn_type>::type; // { dg-error "here" }
>  
> -  typedef make_signed<float>::type  	test5_type;
> +  using T8 = make_signed<float>::type; // { dg-error "here" }
>  }
>  
> -// { dg-error "invalid use of incomplete type" "" { target *-*-* } 32 }
> -// { dg-error "required from here" "" { target *-*-* } 34 }
> -// { dg-error "required from here" "" { target *-*-* } 36 }
> -// { dg-error "required from here" "" { target *-*-* } 39 }
> -// { dg-error "required from here" "" { target *-*-* } 41 }
> -
> -// { dg-error "invalid use of incomplete type" "" { target *-*-* } 0 }
> +// { dg-error "invalid use of incomplete type" "" { target c++17_down } 0 }
> +// { dg-error "constraint failure" "" { target c++20 } 0 }
> diff --git a/libstdc++-v3/testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc b/libstdc++-v3/testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc
> index ff98cc42ef7..50f15e7037c 100644
> --- a/libstdc++-v3/testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc
> +++ b/libstdc++-v3/testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc
> @@ -21,7 +21,6 @@
>  
>  #include <type_traits>
>  
> -enum test_enum { first_selection };
>  struct pod_class { };
>  
>  void test01()
> @@ -29,22 +28,20 @@ void test01()
>    using std::make_unsigned;
>  
>    // Negative tests.
> -  typedef make_unsigned<bool>::type     	test1_type;
> +  using T1 = make_unsigned<bool>::type; // { dg-error "incomplete" }
> +  using T2 = make_unsigned<const bool>::type; // { dg-error "incomplete" }
> +  using T3 = make_unsigned<volatile bool>::type; // { dg-error "incomplete" }
> +  using T4 = make_unsigned<const volatile bool>::type; // { dg-error "incomplete" }
>  
> -  typedef make_unsigned<pod_class>::type     	test2_type;
> +  using T5 = make_unsigned<pod_class>::type; // { dg-error "here" }
>  
> -  typedef make_unsigned<int[4]>::type     test3_type;
> +  using T6 = make_unsigned<int[4]>::type; // { dg-error "here" }
>  
> -  typedef void (fn_type) ();
> -  typedef make_unsigned<fn_type>::type  	test4_type;
> +  using fn_type = void ();
> +  using T7 = make_unsigned<fn_type>::type; // { dg-error "here" }
>  
> -  typedef make_unsigned<float>::type  		test5_type;
> +  using T8 = make_unsigned<float>::type; // { dg-error "here" }
>  }
>  
> -// { dg-error "invalid use of incomplete type" "" { target *-*-* } 32 }
> -// { dg-error "required from here" "" { target *-*-* } 34 }
> -// { dg-error "required from here" "" { target *-*-* } 36 }
> -// { dg-error "required from here" "" { target *-*-* } 39 }
> -// { dg-error "required from here" "" { target *-*-* } 41 }
> -
> -// { dg-error "invalid use of incomplete type" "" { target *-*-* } 0 }
> +// { dg-error "invalid use of incomplete type" "" { target c++17_down } 0 }
> +// { dg-error "constraint failure" "" { target c++20 } 0 }
> diff --git a/libstdc++-v3/testsuite/24_iterators/range_access/range_access_cpp20_neg.cc b/libstdc++-v3/testsuite/24_iterators/range_access/range_access_cpp20_neg.cc
> index c0825a58587..26c8ae0ee1e 100644
> --- a/libstdc++-v3/testsuite/24_iterators/range_access/range_access_cpp20_neg.cc
> +++ b/libstdc++-v3/testsuite/24_iterators/range_access/range_access_cpp20_neg.cc
> @@ -46,4 +46,5 @@ test03()
>    C c;
>    std::ssize(c);  // { dg-error "no matching function" }
>  }
> -// { dg-error "incomplete type .*make_signed.*S" "" { target *-*-* } 0 }
> +// { dg-error "incomplete type .*make_signed.*S" "" { target c++17_down } 0 }
> +// { dg-error "constraint failure" "" { target c++20 } 0 }
> diff --git a/libstdc++-v3/testsuite/lib/prune.exp b/libstdc++-v3/testsuite/lib/prune.exp
> index d457e975218..6d0b77a8ccd 100644
> --- a/libstdc++-v3/testsuite/lib/prune.exp
> +++ b/libstdc++-v3/testsuite/lib/prune.exp
> @@ -51,6 +51,7 @@ proc libstdc++-dg-prune { system text } {
>      regsub -all "(^|\n)\[^\n\]*:   (recursively )?required \[^\n\]*" $text "" text
>      regsub -all "(^|\n)\[^\n\]*:   . skipping \[0-9\]* instantiation contexts \[^\n\]*" $text "" text
>      regsub -all "(^|\n)\[^\n\]*:   in .constexpr. expansion \[^\n\]*" $text "" text
> +    regsub -all "(^|\n)\[^\n\]*:   in requirements  .with\[^\n\]*" $text "" text
>      regsub -all "(^|\n)    inlined from \[^\n\]*" $text "" text
>      # Why doesn't GCC need these to strip header context?
>      regsub -all "(^|\n)In file included from \[^\n\]*" $text "" text
> -- 
> 2.37.3
> 
>