P3068R6 Allowing exception throwing in constant-evaluation

Author: jensmaurer

source/expressions.tex

@@ -7690,6 +7690,13 @@
 unless it deallocates a region of storage
 allocated within the evaluation of $E$;
 
+\item
+a construction of an exception object,
+unless the exception object and
+all of its implicit copies created by invocations of
+\tcode{std::current_exception} or \tcode{std::rethrow_exception}\iref{propagation}
+are destroyed within the evaluation of $E$;
+
 \item
 an \grammarterm{await-expression}\iref{expr.await};
 
@@ -7701,15 +7708,18 @@
 relational\iref{expr.rel}, or equality\iref{expr.eq}
 operator where the result is unspecified;
 
-\item
-a \grammarterm{throw-expression}\iref{expr.throw};
-
 \item
 a \keyword{dynamic_cast}\iref{expr.dynamic.cast} or
 \keyword{typeid}\iref{expr.typeid} expression
 on a glvalue that refers to an object
-whose dynamic type is constexpr-unknown or
-that would throw an exception;
+whose dynamic type is constexpr-unknown;
+
+\item
+a \tcode{dynamic_cast}\iref{expr.dynamic.cast} expression,
+\tcode{typeid}\iref{expr.typeid} expression, or
+\tcode{new-expression}\iref{expr.new}
+that would throw an exception
+where no definition of the exception type is reachable;
 
 \item
 an \grammarterm{asm-declaration}\iref{dcl.asm};

source/preprocessor.tex

@@ -1850,6 +1850,7 @@
 \defnxname{cpp_conditional_explicit}              & \tcode{201806L} \\ \rowsep
 \defnxname{cpp_constexpr}                         & \tcode{202406L} \\ \rowsep
 \defnxname{cpp_constexpr_dynamic_alloc}           & \tcode{201907L} \\ \rowsep
+\defnxname{cpp_constexpr_exceptions}              & \tcode{202411L} \\ \rowsep
 \defnxname{cpp_constexpr_in_decltype}             & \tcode{201711L} \\ \rowsep
 \defnxname{cpp_consteval}                         & \tcode{202211L} \\ \rowsep
 \defnxname{cpp_constinit}                         & \tcode{201907L} \\ \rowsep

source/support.tex

@@ -607,6 +607,7 @@
 #define @\defnlibxname{cpp_lib_constexpr_cmath}@                   202306L // also in \libheader{cmath}, \libheader{cstdlib}
 #define @\defnlibxname{cpp_lib_constexpr_complex}@                 202306L // also in \libheader{complex}
 #define @\defnlibxname{cpp_lib_constexpr_dynamic_alloc}@           201907L // also in \libheader{memory}
+#define @\defnlibxname{cpp_lib_constexpr_exceptions}@              202411L // also in \libheader{exception}
 #define @\defnlibxname{cpp_lib_constexpr_functional}@              201907L // freestanding, also in \libheader{functional}
 #define @\defnlibxname{cpp_lib_constexpr_iterator}@                201811L // freestanding, also in \libheader{iterator}
 #define @\defnlibxname{cpp_lib_constexpr_memory}@                  202202L // freestanding, also in \libheader{memory}
@@ -2941,7 +2942,7 @@
   class bad_alloc : public exception {
   public:
     // see \ref{exception} for the specification of the special member functions
-    const char* what() const noexcept override;
+    constexpr const char* what() const noexcept override;
   };
 }
 \end{codeblock}
@@ -2954,7 +2955,7 @@
 
 \indexlibrarymember{what}{bad_alloc}%
 \begin{itemdecl}
-const char* what() const noexcept override;
+constexpr const char* what() const noexcept override;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -2972,7 +2973,7 @@
   class bad_array_new_length : public bad_alloc {
   public:
     // see \ref{exception} for the specification of the special member functions
-    const char* what() const noexcept override;
+    constexpr const char* what() const noexcept override;
   };
 }
 \end{codeblock}
@@ -2985,7 +2986,7 @@
 
 \indexlibrarymember{what}{bad_array_new_length}%
 \begin{itemdecl}
-const char* what() const noexcept override;
+constexpr const char* what() const noexcept override;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -3310,7 +3311,7 @@
   class bad_cast : public exception {
   public:
     // see \ref{exception} for the specification of the special member functions
-    const char* what() const noexcept override;
+    constexpr const char* what() const noexcept override;
   };
 }
 \end{codeblock}
@@ -3326,7 +3327,7 @@
 
 \indexlibrarymember{what}{bad_cast}%
 \begin{itemdecl}
-const char* what() const noexcept override;
+constexpr const char* what() const noexcept override;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -3344,7 +3345,7 @@
   class bad_typeid : public exception {
   public:
     // see \ref{exception} for the specification of the special member functions
-    const char* what() const noexcept override;
+    constexpr const char* what() const noexcept override;
   };
 }
 \end{codeblock}
@@ -3360,7 +3361,7 @@
 
 \indexlibrarymember{what}{bad_typeid}%
 \begin{itemdecl}
-const char* what() const noexcept override;
+constexpr const char* what() const noexcept override;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -3775,16 +3776,16 @@
   terminate_handler set_terminate(terminate_handler f) noexcept;
   [[noreturn]] void terminate() noexcept;
 
-  int uncaught_exceptions() noexcept;
+  constexpr int uncaught_exceptions() noexcept;
 
   using exception_ptr = @\unspec@;
 
-  exception_ptr current_exception() noexcept;
-  [[noreturn]] void rethrow_exception(exception_ptr p);
-  template<class E> exception_ptr make_exception_ptr(E e) noexcept;
+  constexpr 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 T> [[noreturn]] void throw_with_nested(T&& t);
-  template<class E> void rethrow_if_nested(const E& e);
+  template<class T> [[noreturn]] constexpr void throw_with_nested(T&& t);
+  template<class E> constexpr void rethrow_if_nested(const E& e);
 }
 \end{codeblock}
 
@@ -3796,11 +3797,11 @@
 namespace std {
   class exception {
   public:
-    exception() noexcept;
-    exception(const exception&) noexcept;
-    exception& operator=(const exception&) noexcept;
-    virtual ~exception();
-    virtual const char* what() const noexcept;
+    constexpr exception() noexcept;
+    constexpr exception(const exception&) noexcept;
+    constexpr exception& operator=(const exception&) noexcept;
+    constexpr virtual ~exception();
+    constexpr virtual const char* what() const noexcept;
   };
 }
 \end{codeblock}
@@ -3833,8 +3834,8 @@
 \indexlibraryctor{exception}%
 \indexlibrarymember{operator=}{exception}%
 \begin{itemdecl}
-exception(const exception& rhs) noexcept;
-exception& operator=(const exception& rhs) noexcept;
+constexpr exception(const exception& rhs) noexcept;
+constexpr exception& operator=(const exception& rhs) noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -3846,7 +3847,7 @@
 
 \indexlibrarydtor{exception}%
 \begin{itemdecl}
-virtual ~exception();
+constexpr virtual ~exception();
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -3858,13 +3859,15 @@
 
 \indexlibrarymember{what}{exception}%
 \begin{itemdecl}
-virtual const char* what() const noexcept;
+constexpr virtual const char* what() const noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
 \pnum
 \returns
-An \impldef{return value of \tcode{exception::what}} \ntbs{}.
+An \impldef{return value of \tcode{exception::what}} \ntbs{},
+which during constant evaluation is encoded with
+the ordinary literal encoding\iref{lex.ccon}.
 
 \pnum
 \remarks
@@ -3885,7 +3888,7 @@
   class bad_exception : public exception {
   public:
     // see \ref{exception} for the specification of the special member functions
-    const char* what() const noexcept override;
+    constexpr const char* what() const noexcept override;
   };
 }
 \end{codeblock}
@@ -3900,7 +3903,7 @@
 
 \indexlibrarymember{what}{bad_exception}%
 \begin{itemdecl}
-const char* what() const noexcept override;
+constexpr const char* what() const noexcept override;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4007,7 +4010,7 @@
 
 \indexlibraryglobal{uncaught_exceptions}%
 \begin{itemdecl}
-int uncaught_exceptions() noexcept;
+constexpr int uncaught_exceptions() noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4068,11 +4071,14 @@
 Changes in the number of \tcode{exception_ptr} objects that refer to a
 particular exception do not introduce a data race.
 \end{note}
+
+\pnum
+All member functions are marked \tcode{constexpr}.
 \end{itemdescr}
 
 \indexlibraryglobal{current_exception}%
 \begin{itemdecl}
-exception_ptr current_exception() noexcept;
+constexpr exception_ptr current_exception() noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4103,7 +4109,7 @@
 
 \indexlibraryglobal{rethrow_exception}%
 \begin{itemdecl}
-[[noreturn]] void rethrow_exception(exception_ptr p);
+[[noreturn]] constexpr void rethrow_exception(exception_ptr p);
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4131,7 +4137,7 @@
 
 \indexlibraryglobal{make_exception_ptr}%
 \begin{itemdecl}
-template<class E> exception_ptr make_exception_ptr(E e) noexcept;
+template<class E> constexpr exception_ptr make_exception_ptr(E e) noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4160,18 +4166,18 @@
 namespace std {
   class nested_exception {
   public:
-    nested_exception() noexcept;
-    nested_exception(const nested_exception&) noexcept = default;
-    nested_exception& operator=(const nested_exception&) noexcept = default;
-    virtual ~nested_exception() = default;
+    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;
 
     // access functions
-    [[noreturn]] void rethrow_nested() const;
-    exception_ptr nested_ptr() const noexcept;
+    [[noreturn]] constexpr void rethrow_nested() const;
+    constexpr exception_ptr nested_ptr() const noexcept;
   };
 
-  template<class T> [[noreturn]] void throw_with_nested(T&& t);
-  template<class E> void rethrow_if_nested(const E& e);
+  template<class T> [[noreturn]] constexpr void throw_with_nested(T&& t);
+  template<class E> constexpr void rethrow_if_nested(const E& e);
 }
 \end{codeblock}
 
@@ -4188,7 +4194,7 @@
 
 \indexlibraryctor{nested_exception}%
 \begin{itemdecl}
-nested_exception() noexcept;
+constexpr nested_exception() noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4199,7 +4205,7 @@
 
 \indexlibrarymember{rethrow_nested}{nested_exception}%
 \begin{itemdecl}
-[[noreturn]] void rethrow_nested() const;
+[[noreturn]] constexpr void rethrow_nested() const;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4211,7 +4217,7 @@
 
 \indexlibrarymember{nested_ptr}{nested_exception}%
 \begin{itemdecl}
-exception_ptr nested_ptr() const noexcept;
+constexpr exception_ptr nested_ptr() const noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4222,7 +4228,7 @@
 
 \indexlibrarymember{throw_with_nested}{nested_exception}%
 \begin{itemdecl}
-template<class T> [[noreturn]] void throw_with_nested(T&& t);
+template<class T> [[noreturn]] constexpr void throw_with_nested(T&& t);
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -4245,7 +4251,7 @@
 
 \indexlibrarymember{rethrow_if_nested}{nested_exception}%
 \begin{itemdecl}
-template<class E> void rethrow_if_nested(const E& e);
+template<class E> constexpr void rethrow_if_nested(const E& e);
 \end{itemdecl}
 
 \begin{itemdescr}