[clang] Revert "[DebugInfo] Ignore undefined constexpr constructors in constructor homing." (PR #221566)

[Avi Kivity via cfe-commits]

avikivity wrote:

The original case where I got hit with this was some abseil map, the type without debugging information was std::pair.

libstdc++ has these constructors for pair:

```c++
      constexpr pair(const pair&) = default;    ///< Copy constructor
      constexpr pair(pair&&) = default;         ///< Move constructor

      template<typename... _Args1, typename... _Args2>
        _GLIBCXX20_CONSTEXPR
        pair(piecewise_construct_t, tuple<_Args1...>, tuple<_Args2...>);

      template<typename... _Args1, size_t... _Indexes1,
               typename... _Args2, size_t... _Indexes2>
        _GLIBCXX20_CONSTEXPR
        pair(tuple<_Args1...>&, tuple<_Args2...>&,
             _Index_tuple<_Indexes1...>, _Index_tuple<_Indexes2...>);

      constexpr
      explicit(__not_<__and_<__is_implicitly_default_constructible<_T1>,
                             __is_implicitly_default_constructible<_T2>>>())
      pair()
      noexcept(is_nothrow_default_constructible_v<_T1>
                && is_nothrow_default_constructible_v<_T2>)
      requires is_default_constructible_v<_T1>
               && is_default_constructible_v<_T2>
      : first(), second()
      { }

```

I see that abseil uses the piecewise_construct constructor, which at the point of the template definition, is declared but not defined. It is defined later.

Could it be that at the point the code checks whether the constexpr constructor is defined or not, the compiler hasn't instantiated the constructor definition?



https://github.com/llvm/llvm-project/pull/221566