[Bug c++/111132] [11/12/13/14 Regression] Function redeclaration in local scope breaks constant expression evaluation

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111132

--- Comment #4 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:8c9063825ce726fcbbc067d8a6d062cc2d4acf5e

commit r14-9809-g8c9063825ce726fcbbc067d8a6d062cc2d4acf5e
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Apr 2 12:59:38 2024 -0400

    c++: constexpr error with fn redecl in local scope [PR111132]

    We evaluate constexpr functions on the original, pre-genericization bodies.
    That means that the function body we're evaluating will not have gone
    through cp_genericize_r's "Map block scope extern declarations to visible
    declarations with the same name and type in outer scopes if any".  Here:

      constexpr bool bar() { return true; } // #1
      constexpr bool foo() {
        constexpr bool bar(void); // #2
        return bar();
      }

    it means that we:
    1) register_constexpr_fundef (#1)
    2) cp_genericize (#1)
       nothing interesting happens
    3) register_constexpr_fundef (foo)
       does copy_fn, so we have two copies of the BIND_EXPR
    4) cp_genericize (foo)
       this remaps #2 to #1, but only on one copy of the BIND_EXPR
    5) retrieve_constexpr_fundef (foo)
       we find it, no problem
    6) retrieve_constexpr_fundef (#2)
       and here #2 isn't found in constexpr_fundef_table, because
       we're working on the BIND_EXPR copy where #2 wasn't mapped to #1
       so we fail.  We've only registered #1.

    It should work to use DECL_LOCAL_DECL_ALIAS (which used to be
    extern_decl_map).  We evaluate constexpr functions on pre-cp_fold
    bodies to avoid diagnostic problems, but the remapping I'm proposing
    should not interfere with diagnostics.

    This is not a problem for a global scope redeclaration; there we go
    through duplicate_decls which keeps the DECL_UID:
      DECL_UID (olddecl) = olddecl_uid;
    and DECL_UID is what constexpr_fundef_hasher::hash uses.

            PR c++/111132

    gcc/cp/ChangeLog:

            * constexpr.cc (get_function_named_in_call): Use
            cp_get_fndecl_from_callee.
            * cvt.cc (cp_get_fndecl_from_callee): If there's a
            DECL_LOCAL_DECL_ALIAS, use it.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/constexpr-redeclaration3.C: New test.
            * g++.dg/cpp0x/constexpr-redeclaration4.C: New test.