From 76cf7c1d431db822e5e7aea05ec621432889d3ad Mon Sep 17 00:00:00 2001
From: Jens Maurer <Jens.Maurer@gmx.net>
Date: Wed, 1 Apr 2026 23:48:12 +0200
Subject: [PATCH] P3842R2 A conservative fix for constexpr
 uncaught_exceptions() and current_exception()

Fixes NB US 67-118, FI-121, DE-120, GB 03-119, PL-012 (C++26 CD).
---
 source/support.tex | 48 +++++++++++++++++++++-------------------------
 1 file changed, 22 insertions(+), 26 deletions(-)

diff --git a/source/support.tex b/source/support.tex
index 203a1bf45f..1b5f2cd892 100644
--- a/source/support.tex
+++ b/source/support.tex
@@ -3837,18 +3837,19 @@
   terminate_handler set_terminate(terminate_handler f) noexcept;
   [[noreturn]] void terminate() noexcept;
 
-  constexpr int uncaught_exceptions() noexcept;
+  int uncaught_exceptions() noexcept;
 
   using exception_ptr = @\unspec@;
 
-  constexpr exception_ptr current_exception() noexcept;
+  constexpr exception_ptr @\exposid{current-exception}@() noexcept;    // \expos
+  exception_ptr current_exception() noexcept;
   [[noreturn]] constexpr void rethrow_exception(exception_ptr p);
   template<class E> constexpr exception_ptr make_exception_ptr(E e) noexcept;
   template<class E> constexpr const E* exception_ptr_cast(const exception_ptr& p) noexcept;
   template<class E> void exception_ptr_cast(const exception_ptr&&) = delete;
 
-  template<class T> [[noreturn]] constexpr void throw_with_nested(T&& t);
-  template<class E> constexpr void rethrow_if_nested(const E& e);
+  template<class T> [[noreturn]] void throw_with_nested(T&& t);
+  template<class E> void rethrow_if_nested(const E& e);
 }
 \end{codeblock}
 
@@ -4073,7 +4074,7 @@
 
 \indexlibraryglobal{uncaught_exceptions}%
 \begin{itemdecl}
-constexpr int uncaught_exceptions() noexcept;
+int uncaught_exceptions() noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4142,7 +4143,8 @@
 
 \indexlibraryglobal{current_exception}%
 \begin{itemdecl}
-constexpr exception_ptr current_exception() noexcept;
+constexpr exception_ptr @\exposid{current-exception}@() noexcept;
+exception_ptr current_exception() noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4212,15 +4214,9 @@
 try {
   throw e;
 } catch(...) {
-  return current_exception();
+  return @\exposid{current-exception}@();
 }
 \end{codeblock}
-
-\pnum
-\begin{note}
-This function is provided for convenience and
-efficiency reasons.
-\end{note}
 \end{itemdescr}
 
 \indexlibraryglobal{exception_ptr_cast}%
@@ -4256,18 +4252,18 @@
 namespace std {
   class nested_exception {
   public:
-    constexpr nested_exception() noexcept;
-    constexpr nested_exception(const nested_exception&) noexcept = default;
-    constexpr nested_exception& operator=(const nested_exception&) noexcept = default;
-    constexpr virtual ~nested_exception() = default;
+    nested_exception() noexcept;
+    nested_exception(const nested_exception&) noexcept = default;
+    nested_exception& operator=(const nested_exception&) noexcept = default;
+    virtual ~nested_exception() = default;
 
     // access functions
-    [[noreturn]] constexpr void rethrow_nested() const;
-    constexpr exception_ptr nested_ptr() const noexcept;
+    [[noreturn]] void rethrow_nested() const;
+    exception_ptr nested_ptr() const noexcept;
   };
 
-  template<class T> [[noreturn]] constexpr void throw_with_nested(T&& t);
-  template<class E> constexpr void rethrow_if_nested(const E& e);
+  template<class T> [[noreturn]] void throw_with_nested(T&& t);
+  template<class E> void rethrow_if_nested(const E& e);
 }
 \end{codeblock}
 
@@ -4284,7 +4280,7 @@
 
 \indexlibraryctor{nested_exception}%
 \begin{itemdecl}
-constexpr nested_exception() noexcept;
+nested_exception() noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4295,7 +4291,7 @@
 
 \indexlibrarymember{rethrow_nested}{nested_exception}%
 \begin{itemdecl}
-[[noreturn]] constexpr void rethrow_nested() const;
+[[noreturn]] void rethrow_nested() const;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4307,7 +4303,7 @@
 
 \indexlibrarymember{nested_ptr}{nested_exception}%
 \begin{itemdecl}
-constexpr exception_ptr nested_ptr() const noexcept;
+exception_ptr nested_ptr() const noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4318,7 +4314,7 @@
 
 \indexlibrarymember{throw_with_nested}{nested_exception}%
 \begin{itemdecl}
-template<class T> [[noreturn]] constexpr void throw_with_nested(T&& t);
+template<class T> [[noreturn]] void throw_with_nested(T&& t);
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4341,7 +4337,7 @@
 
 \indexlibrarymember{rethrow_if_nested}{nested_exception}%
 \begin{itemdecl}
-template<class E> constexpr void rethrow_if_nested(const E& e);
+template<class E> void rethrow_if_nested(const E& e);
 \end{itemdecl}
 
 \begin{itemdescr}