P3820R0
Split constexpr uncaught_exceptions into distinct runtime and consteval functions

1. Background

[P3068] applied the constexpr specifier to the functions in the header <exception>. uncaught_exceptions is one of these functions, however at runtime its return value depends on mutable global state. It is impossible to implement to return a constexpr value that is consistent with the runtime state. When evaluated in a constant expression, the current implementations of [P3068] return the number of uncaught exceptions in the current constant evaluation instead of the current thread.

The inconsistency of the semantics of uncaught_exceptions depending on whether it is called from a constant expression or at runtime causes a breaking change when the function is called in trial constant evaluation.

const integer variables are usable in constant expressions when they are constant-initialized, so when initializing them the initializer is constant evaluated first, and if that fails then the variable is initialized at runtime. Consider:

#include <exception>

int i = 1;

int main () {
  try {
    struct S {
      ~S() {
        // x is initialized to 0 in trial constant evaluation
        const int x = std::uncaught_exceptions();
        i = x;
      }
    };
    S s;
    throw 0;
  } catch (...) {
    return i;
  }
}

The program returns 0, which is a breaking change compared to C++23, where it returns 1.

2. Proposal

1. Remove the constexpr specifier from uncaught_exceptions().

2. Add a consteval int consteval_uncaught_exceptions() noexcept; function with similar semantics to uncaught_exceptions(), but counting exceptions within the current constant evaluation.

3. Motivation

uncaught_exceptions() is useful to write scope guards and other patterns to conditionally execute code when normally exiting a scope or due to stack unwinding. Now that we have exception facilities available in constant evaluation, this functionality is similarly useful in that context. Putting the functionality in a separate consteval function makes this functionality available in constant evaluation without a breaking change.

4. Implementation experience

There is previous implementation experience for [P3068] including constexpr int uncaught_exceptions(). Splitting the functionality into separate runtime and consteval functions is a trivial change.

5. Wording

Wording is relative to [N5013].

5.1. [exception.syn]

constexpr int uncaught_exceptions() noexcept;
consteval int consteval_uncaught_exceptions() noexcept;

5.2. [uncaught.exceptions]

constexpr int uncaught_exceptions() noexcept;

Returns: The number of uncaught exceptions ([except.throw]) in the current thread.

Remarks: When uncaught_exceptions() > 0, throwing an exception can result in a call of the function std::terminate.