commit 43ab1dc24abd6fded8d5bf6547f0de6851beb200
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Apr 8 10:50:57 2021

    libstdc++: Make std::is_scoped_enum work with incomplete types
    
    Tim Song pointed out that using __underlying_type is ill-formed for
    incomplete enumeration types, and is_scoped_enum doesn't require a
    complete type. This changes the trait to check for conversion to int
    instead of to the underlying type.
    
    In order to give the correct result when the trait is used in the
    enumerator-list of an incomplete type the partial specialization for
    enums has an additional check that fails for incomplete types. This
    assumes that an incompelte enumeration type must be an unscoped
    enumeration, and so the primary template (with a std::false_type base
    characteristic) can be used. This isn't necessarily true, but it is not
    currently possible to refer to a scoped enumeration type before its type
    is complete (PR c++/89025).
    
    It should be possible to use requires(remove_cv_t<_Tp> __t) in the
    partial specialization's assignablility check, but that currently gives
    an ICE (PR c++/99968) so there is an extra partial specialization of
    is_scoped_enum<const _Tp> to handle const types.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/type_traits (is_scoped_enum<T>): Constrain partial
            specialization to not match incomplete enum types. Use a
            requires-expression instead of instantiating is_convertible.
            (is_scoped_enum<const T>): Add as workaround for PR c++/99968.
            * testsuite/20_util/is_scoped_enum/value.cc: Check with
            incomplete types and opaque-enum-declarations.

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index eeab1ca65fc..1f8b57b04a0 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -3287,9 +3287,20 @@ template <typename _From, typename _To>
     : false_type
     { };
 
-  template<typename _Tp> requires __is_enum(_Tp)
+  template<typename _Tp>
+    requires __is_enum(_Tp)
+    && requires(_Tp __t) { __t = __t; } // fails if incomplete
     struct is_scoped_enum<_Tp>
-    : __not_<is_convertible<_Tp, __underlying_type(_Tp)>>::type
+    : bool_constant<!requires(_Tp __t, void(*__f)(int)) { __f(__t); }>
+    { };
+
+  // FIXME remove this partial specialization and use remove_cv_t<_Tp> above
+  // when PR c++/99968 is fixed.
+  template<typename _Tp>
+    requires __is_enum(_Tp)
+    && requires(_Tp __t) { __t = __t; } // fails if incomplete
+    struct is_scoped_enum<const _Tp>
+    : bool_constant<!requires(_Tp __t, void(*__f)(int)) { __f(__t); }>
     { };
 
   template<typename _Tp>
diff --git a/libstdc++-v3/testsuite/20_util/is_scoped_enum/value.cc b/libstdc++-v3/testsuite/20_util/is_scoped_enum/value.cc
index bab7263ae4a..2cef857a042 100644
--- a/libstdc++-v3/testsuite/20_util/is_scoped_enum/value.cc
+++ b/libstdc++-v3/testsuite/20_util/is_scoped_enum/value.cc
@@ -32,6 +32,8 @@ template<typename T>
   concept Is_scoped_enum
     = __gnu_test::test_category<std::is_scoped_enum, T>(true);
 
+struct Incomplete_struct;
+
 void
 test01()
 {
@@ -45,6 +47,9 @@ test01()
   static_assert( ! Is_scoped_enum<U> );
   enum F : int { f1, f2 };
   static_assert( ! Is_scoped_enum<F> );
+  static_assert( ! Is_scoped_enum<Incomplete_struct> );
+  struct S;
+  static_assert( ! Is_scoped_enum<S> );
   struct S { };
   static_assert( ! Is_scoped_enum<S> );
 
@@ -60,3 +65,36 @@ test01()
   static_assert( ! Is_scoped_enum<int(*)()> );
   static_assert( ! Is_scoped_enum<int(&)()> );
 }
+
+enum opaque_unscoped : short;
+enum class opaque_scoped;
+enum class opaque_scoped_with_base : long;
+
+static_assert( ! Is_scoped_enum<opaque_unscoped> );
+static_assert( Is_scoped_enum<opaque_scoped> );
+static_assert( Is_scoped_enum<opaque_scoped_with_base> );
+
+void
+test02()
+{
+  enum unscoped {
+    u_is_enum = std::is_enum_v<unscoped>,
+    u_is_scoped = std::is_scoped_enum_v<unscoped>,
+  };
+  static_assert( unscoped::u_is_enum );
+  static_assert( ! unscoped::u_is_scoped );
+
+  enum unscoped_fixed : char {
+    uf_is_enum = std::is_enum_v<unscoped_fixed>,
+    uf_is_scoped = std::is_scoped_enum_v<unscoped_fixed>,
+  };
+  static_assert( unscoped_fixed::uf_is_enum);
+  static_assert( ! unscoped_fixed::uf_is_scoped );
+
+  enum class scoped {
+    is_enum = std::is_enum_v<scoped>,
+    is_scoped = std::is_scoped_enum_v<scoped>,
+  };
+  static_assert( (bool) scoped::is_enum );
+  static_assert( (bool) scoped::is_scoped );
+}