Our vendored version of tcmalloc fails to compile on GCC 14.2.0 against the C++20 standard.

In short, the incompatibility arises due to tcmalloc improperly attempting to initialize a constinit std::atomic<double> with a double literal when the compiler is using the C++20 standard. Compilation succeeds on prior standards because the ABSL_CONST_INIT macro will resolve to empty text when the constinit specifier is not available. When using C++20, constinit becomes available, which causes the macro to resolve to the constinit keyword, and so any variables declared ABSL_CONST_INIT std::atomic<double> and initialized with a double literal result in compile failures.

The reason that initializing a constinit std::atomic<double> with a double literal results in compile failure is that the single-argument constructor for std::atomic<double> is not declared constexpr and internally invokes runtime-dependent behavior.