eagerly-instantiated entities whose templates are defined after the first point of instantiation don't get instantiated at all

[zygoloid]

Bugzilla Link	24128
Version	trunk
OS	Linux
CC	@dwblaikie,@DougGregor,@ldalessa

Extended Description

If a template specialization is referenced in a way that requires a definition, but the definition of the template appears after the use, the instantiation is never performed.

Examples through the ages:

// C++98:
template struct X { static const int n; };
int k = X::n;
template const int X::n = 5;

// C++11:
template constexpr int f();
int k = f();
template constexpr int f() { return 5; }

// C++14:
template extern const int n;
int k = n;
template const int n = 5;

These are valid, and should cause the instantiation of the relevant template, according to C++14 [temp]/6, which requires the template to "be defined in every translation unit in which it is implicitly instantiated", but doesn't require the definition to precede the use.

[zygoloid]

*** Bug #16380 has been marked as a duplicate of this bug. ***

[zygoloid]

*** Bug llvm/llvm-bugzilla-archive#50207 has been marked as a duplicate of this bug. ***

[zygoloid]

mentioned in issue llvm/llvm-bugzilla-archive#50207

[jwakely]

I think this is causing problems for constexpr std::string in libstdc++:
https://gcc.gnu.org/pipermail/libstdc++/2021-December/053605.html

[jwakely]

*** Bug llvm/llvm-bugzilla-archive#50207 has been marked as a duplicate of this bug. ***

That is #49551 in new money.