[PATCH] libstdc++: Fix constexpr exceptions for -fno-exceptions

[Jonathan Wakely]

The if-consteval branches in std::make_exception_ptr and
std::exception_ptr_cast use a try-catch block, which gives an error for
-fno-exceptions. Just make them return a null pointer at compile-time
when -fno-exceptions is used, because there's no way to get an active
exception with -fno-exceptions.

For std::exception_ptr_cast the consteval branch doesn't depend on RTTI
being enabled, so we can move the check for __cpp_rtti into the runtime
branch. We can also remove the #else group and just fall through to the
return nullptr statement if there was no return from whichever branch of
the if-consteval was taken.

Also adjust some formatting and whitespace.

libstdc++-v3/ChangeLog:

        * libsupc++/exception_ptr.h (make_exception_ptr): Return null
        for consteval when -fno-exceptions is used.
        (exception_ptr_cast): Likewise. Allow consteval path to work
        with -fno-rtti.
---

The 17_intro and 18_support tests all pass, and the compiler's
g++.dg/cpp26/constexpr-eh* tests. The full testsuite is running now for
x86_64-linux.

 libstdc++-v3/libsupc++/exception_ptr.h | 28 ++++++++++++++++----------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/libsupc++/exception_ptr.h 
b/libstdc++-v3/libsupc++/exception_ptr.h
index ee009155a39c..709d5d1740ca 100644
--- a/libstdc++-v3/libsupc++/exception_ptr.h
+++ b/libstdc++-v3/libsupc++/exception_ptr.h
@@ -83,9 +83,9 @@ namespace std _GLIBCXX_VISIBILITY(default)
 
 #if __cpp_lib_exception_ptr_cast >= 202506L
   template<typename _Ex>
-  constexpr const _Ex* exception_ptr_cast(const exception_ptr&) noexcept;
+    constexpr const _Ex* exception_ptr_cast(const exception_ptr&) noexcept;
   template<typename _Ex>
-  void exception_ptr_cast(const exception_ptr&&) = delete;
+    void exception_ptr_cast(const exception_ptr&&) = delete;
 #endif
 
   namespace __exception_ptr
@@ -138,8 +138,8 @@ namespace std _GLIBCXX_VISIBILITY(default)
        _GLIBCXX_USE_NOEXCEPT;
 #if __cpp_lib_exception_ptr_cast >= 202506L
       template<typename _Ex>
-      friend constexpr const _Ex* std::exception_ptr_cast(const exception_ptr&)
-       noexcept;
+       friend constexpr const _Ex*
+       std::exception_ptr_cast(const exception_ptr&) noexcept;
 #endif
 
       const void* _M_exception_ptr_cast(const type_info&) const
@@ -301,8 +301,9 @@ namespace std _GLIBCXX_VISIBILITY(default)
     _GLIBCXX26_CONSTEXPR exception_ptr
     make_exception_ptr(_Ex __ex) _GLIBCXX_USE_NOEXCEPT
     {
-#if __cplusplus >= 202400L
+#if __cpp_lib_exception_ptr_cast >= 202506L
       if consteval {
+#ifdef __cpp_exceptions
        try
          {
            throw __ex;
@@ -311,6 +312,9 @@ namespace std _GLIBCXX_VISIBILITY(default)
          {
            return current_exception();
          }
+#else
+       return exception_ptr();
+#endif
       }
 #endif
 #if __cplusplus >= 201103L && __cpp_rtti
@@ -353,7 +357,8 @@ namespace std _GLIBCXX_VISIBILITY(default)
 #if __cpp_lib_exception_ptr_cast >= 202506L
   template<typename _Ex>
     [[__gnu__::__always_inline__]]
-    constexpr const _Ex* exception_ptr_cast(const exception_ptr& __p) noexcept
+    constexpr const _Ex*
+    exception_ptr_cast(const exception_ptr& __p) noexcept
     {
       static_assert(!std::is_const_v<_Ex>);
       static_assert(!std::is_reference_v<_Ex>);
@@ -361,8 +366,9 @@ namespace std _GLIBCXX_VISIBILITY(default)
       static_assert(!std::is_array_v<_Ex>);
       static_assert(!std::is_pointer_v<_Ex>);
       static_assert(!std::is_member_pointer_v<_Ex>);
-#ifdef __cpp_rtti
+
       if consteval {
+#ifdef __cpp_exceptions
        if (__p._M_exception_object)
          try
            {
@@ -375,14 +381,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
          catch (...)
            {
            }
-       return nullptr;
+#endif
       } else {
+#ifdef __cpp_rtti
        const type_info &__id = typeid(const _Ex&);
        return static_cast<const _Ex*>(__p._M_exception_ptr_cast(__id));
-      }
-#else
-      return nullptr;
 #endif
+      }
+      return nullptr;
     }
 #endif
 
-- 
2.50.0