[Bug c++/94960] extern template prevents inlining of standard library objects

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94960

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> ---
C++17 and below said, 

Except for inline functions and variables, declarations with types deduced from
their initializer or return value (10.1.7.4), const variables of literal types,
variables of reference types, and class template specializations, explicit
instantiation declarations have the effect of suppressing the implicit
instantiation of the entity to which they refer. [ Note: The intent is that an
inline function that is the subject of an explicit instantiation declaration
will still be implicitly instantiated when odr-used (6.2) so that the body can
be considered for inlining, but that no out-of-line copy of the inline function
would be generated in the translation unit. — end note ]

This wording was changed in C++20 by P1815, a modules paper, but I believe the
replacement wording still says that the a function is not implicitly
instantiated  after an explicit instantiation declaration unless it is inline
or has a deduced return type.  And if it isn't instantiated, it can't be
inlined.

So if you want an *explicit instantiation declaration* to still be considered
for inlining, you need to declare it inline.  Most templates don't need to be
declared inline, only those that have implicit instantiations suppressed by
'extern template'.

So, I think the string case specifically is a library isssue.