[Bug c++/110000] New: GCC should implement exclude_from_explicit_instantiation

["nikolasklauser at berlin dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110000

            Bug ID: 110000
           Summary: GCC should implement
                    exclude_from_explicit_instantiation
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nikolasklauser at berlin dot de
  Target Milestone: ---

`exclude_from_explicit_instantiation` is an attribute implemented by clang. It
tells the compiler that a function should not be part of an explicit
instantiation. This allows libraries to have greater control over which
functions are part of their ABI and which aren't. It is used extensively in
libc++ to keep the ABI surface as small as possible. Currently, libc++ uses
always_inline if exclude_from_explicit_instantiation isn't available, resulting
in almost every function in the library being declared as always_inline.
Replacing always_inline with exclude_from_explicit_instantiation would
approximately halve the time it takes to run the libc++ test suite with GCC.
(removing always_inline brings the time down to about the same as clang takes)