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From: Fabian H <fab.hoersch@gmail.com>
Date: Tue, 19 Sep 2023 10:52:29 +0200
Message-ID: <CABsEQS-hJn+Xk9g1THpxuF-ubk7Wosb6TYJcT5Rcfy2LSQTqLQ@mail.gmail.com>
Subject: usage of __PRETTY_FUNCTION__ in constexpr
To: gcc-help@gcc.gnu.org
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Hi,

I hope this is the correct place to ask such questions.

I'm using a library that uses __PRETTY_FUNCTION__ in a constexpr to
determine the name of a type at compile time.

Which pretty much looks like this:

template<typename Type>
[[nodiscard]] constexpr auto stripped_type_name() noexcept
{
  std::string_view pretty_function{
    __PRETTY_FUNCTION__
  }; // Results in "constexpr auto stripped_type_name() [with Type =
std::vector<entt::entity>]"
  auto first = pretty_function.find_first_not_of(' ',
pretty_function.find_first_of('=') + 1);
  auto value = pretty_function.substr(first,
pretty_function.find_last_of(']') - first);
  return value;
}

template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
[[nodiscard]] static constexpr std::string_view type_name(int) noexcept
{
  constexpr auto value = stripped_type_name<Type>();
  return value;
}

For me that results in flaky behaviour depending on the compilation
unit, optimization level and various unrelated code changes.
It either returns

"std::vector<entt::entity, std::allocator<entt::entity> >"
or
"std::vector<entt::entity>"

when calling
stripped_type_name<std::vector<entt::entity>>()

I was wondering if this is a proper use of __PRETTY_FUNCTION__ and/or
what could explain such behaviour.