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

[Jonathan Wakely <jwakely@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 2415 bytes --]

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.

[-- Attachment #2: patch.txt --]
[-- Type: text/plain, Size: 4984 bytes --]

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 }