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
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