[Bug c++/125575] explicit instantiation of variable template misses type check

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125575

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:f213d6591640f76eb86cafdb4366d87b87747038

commit r17-1355-gf213d6591640f76eb86cafdb4366d87b87747038
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Jun 3 15:01:33 2026 -0400

    c++: CWG 1704, type checking in explicit inst of var tmpl [PR125575]

    While working on something else I noticed that we compile

      template<typename T>
      constexpr int vt = 42;
      template float vt<int>;

    but we shouldn't due to the float/int mismatch.  Andrew found
    98524 which is the same problem.  This turned out to be CWG 1704.

    For explicit specialization of functions, determine_specialization
    already performs the checking; see the various same_type_p, compparms,
    comp_template_parms etc. checks.  Except I do believe it doesn't check
    exception specification as it should:

      template<typename T> void (fn)(T) {}
      template void (fn<int>)(int) noexcept; // ill-formed, we accept

    The fix should be rather easy.

            PR c++/125575
            PR c++/98524

    gcc/cp/ChangeLog:

            * pt.cc (check_explicit_inst_of_var_template): New.
            (check_explicit_specialization): Use it.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/var-templ90.C: New test.
            * g++.dg/cpp1y/var-templ91.C: New test.
            * g++.dg/cpp1y/var-templ92.C: New test.
            * g++.dg/cpp1y/var-templ93.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>