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

  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.

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(-)
  

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
> 
>
  

On Mon, Oct 10, 2022 at 8:09 AM Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> 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?

I'm also concerned about the inability to name make_signed<X> in a
context that doesn't require its instantiation (e.g.,
conditional_t<is_integral_v<T>, make_signed<T>,
type_identity<T>>::type). That seems a plausible use case, and
breaking it doesn't seem great to me (conformance aside).
  

On Mon, 10 Oct 2022 at 14:50, Tim Song via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Mon, Oct 10, 2022 at 8:09 AM Patrick Palka via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > 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?
>
> I'm also concerned about the inability to name make_signed<X> in a
> context that doesn't require its instantiation (e.g.,
> conditional_t<is_integral_v<T>, make_signed<T>,
> type_identity<T>>::type). That seems a plausible use case, and
> breaking it doesn't seem great to me (conformance aside).

Ah yes, that's a problem.

We could fix it like this:

  template<typename _Tp>
    struct make_unsigned;

  template<typename _Tp>
#if __cpp_concepts
    requires is_integral<_Tp>::value || __is_enum(_Tp)
    struct make_unsigned<_Tp>
#else
    struct make_unsigned
#endif
    { typedef typename __make_unsigned_selector<_Tp>::__type type; };

  But that doesn't really improve the diagnostic much. It's simpler
just to revert the addition of the constraints.
  

On Mon, 10 Oct 2022 at 15:05, Jonathan Wakely wrote:
>
> On Mon, 10 Oct 2022 at 14:50, Tim Song via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > On Mon, Oct 10, 2022 at 8:09 AM Patrick Palka via Libstdc++
> > <libstdc++@gcc.gnu.org> wrote:
> > >
> > > 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?
> >
> > I'm also concerned about the inability to name make_signed<X> in a
> > context that doesn't require its instantiation (e.g.,
> > conditional_t<is_integral_v<T>, make_signed<T>,
> > type_identity<T>>::type). That seems a plausible use case, and
> > breaking it doesn't seem great to me (conformance aside).
>
> Ah yes, that's a problem.
>
> We could fix it like this:
>
>   template<typename _Tp>
>     struct make_unsigned;
>
>   template<typename _Tp>
> #if __cpp_concepts
>     requires is_integral<_Tp>::value || __is_enum(_Tp)
>     struct make_unsigned<_Tp>
> #else
>     struct make_unsigned
> #endif
>     { typedef typename __make_unsigned_selector<_Tp>::__type type; };
>
>   But that doesn't really improve the diagnostic much. It's simpler
> just to revert the addition of the constraints.

Reverted like so. Tested powerpc64le-linux, pushed to trunk.
commit 1d2f07ed4ce028a7c1f9b18f5d959f30213545ca
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Oct 10 15:06:53 2022

    libstdc++: Revert addition of constraints to make_signed/make_unsigned
    
    Constraining the primary template makes it unusable in uninstantiated
    contexts.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/type_traits (make_signed, make_unsigned): Remove
            constraints on primary template.
            * testsuite/20_util/make_signed/requirements/typedefs_neg.cc:
            Undo changes to expected error in C++20 mode.
            * testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc:
            Likewise.
            * testsuite/24_iterators/range_access/range_access_cpp20_neg.cc:
            Likewise.
            * testsuite/20_util/make_signed/requirements/uninstantiated.cc:
            New test.
            * testsuite/20_util/make_unsigned/requirements/uninstantiated.cc:
            New test.

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 6108b98aa6a..1d7c3b04a0e 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -1802,9 +1802,6 @@ _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; };
 
@@ -1937,9 +1934,6 @@ _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; };
 
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 88b8ae887ef..451eeb8c92d 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
@@ -43,5 +43,4 @@ void test01()
   using T8 = make_signed<float>::type; // { dg-error "here" }
 }
 
-// { dg-error "invalid use of incomplete type" "" { target c++17_down } 0 }
-// { dg-error "constraint failure" "" { target c++20 } 0 }
+// { dg-error "invalid use of incomplete type" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/20_util/make_signed/requirements/uninstantiated.cc b/libstdc++-v3/testsuite/20_util/make_signed/requirements/uninstantiated.cc
new file mode 100644
index 00000000000..3facf0c595a
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/make_signed/requirements/uninstantiated.cc
@@ -0,0 +1,8 @@
+// { dg-do compile { target c++11 } }
+#include <type_traits>
+
+// Check that we can name invalid specializations, just don't instantiate them.
+
+using X = std::make_signed<float>;
+using Y = std::make_signed<bool>;
+using Z = std::make_signed<void>;
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 50f15e7037c..49d520936a3 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
@@ -43,5 +43,4 @@ void test01()
   using T8 = make_unsigned<float>::type; // { dg-error "here" }
 }
 
-// { dg-error "invalid use of incomplete type" "" { target c++17_down } 0 }
-// { dg-error "constraint failure" "" { target c++20 } 0 }
+// { dg-error "invalid use of incomplete type" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/20_util/make_unsigned/requirements/uninstantiated.cc b/libstdc++-v3/testsuite/20_util/make_unsigned/requirements/uninstantiated.cc
new file mode 100644
index 00000000000..9b860967379
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/make_unsigned/requirements/uninstantiated.cc
@@ -0,0 +1,8 @@
+// { dg-do compile { target c++11 } }
+#include <type_traits>
+
+// Check that we can name invalid specializations, just don't instantiate them.
+
+using X = std::make_unsigned<float>;
+using Y = std::make_unsigned<bool>;
+using Z = std::make_unsigned<void>;
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 26c8ae0ee1e..c0825a58587 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
@@ -46,5 +46,4 @@ test03()
   C c;
   std::ssize(c);  // { dg-error "no matching function" }
 }
-// { dg-error "incomplete type .*make_signed.*S" "" { target c++17_down } 0 }
-// { dg-error "constraint failure" "" { target c++20 } 0 }
+// { dg-error "incomplete type .*make_signed.*S" "" { target *-*-* } 0 }