C++ Contracts: Fix testsuite failure due to incorrect caching.
Prevents caching of constexpr function results when contract violations occur, fixing a testsuite failure.
The compiler was incorrectly caching the results of constexpr functions even when contract violations occurred. This commit prevents caching of results with contract violations in constexpr_call, which fixes a testsuite failure in basic.contract.eval.p8. This ensures that contract violations are not masked by cached results.
In Details
The issue was in cxx_eval_constant_expression in constexpr.cc. The function was caching results even when contract violations were present, leading to incorrect behavior in constexpr contexts. The commit modifies cxx_eval_constant_expression to avoid caching results with contract violations. The test g++.dg/contracts/cpp26/basic.contract.eval.p8-3.C is added to verify the fix.
For Context
C++26 introduces contracts, which are a way to specify preconditions, postconditions, and invariants for functions. constexpr functions are functions that can be evaluated at compile time. This commit addresses an issue where the results of constexpr functions with contract violations were being incorrectly cached. By preventing the caching of such results, the compiler will now correctly detect and report contract violations even in constexpr contexts.