[Bug c++/113789] [13/14 Regression] ICE on P2266/C++23 `decltype(throw x)` where x is move-eligible parameter

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113789

--- Comment #8 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:254ff3d0e34835b4de93d5e5763a7366dc7d989d

commit r14-9034-g254ff3d0e34835b4de93d5e5763a7366dc7d989d
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Feb 14 17:53:52 2024 -0500

    c++: implicit move with throw [PR113853]

    Here we have

      template<class T>
      auto is_throwable(T t) -> decltype(throw t, true) { ... }

    where we didn't properly mark 't' as IMPLICIT_RVALUE_P, which caused
    the wrong overload to have been chosen.  Jason figured out it's because
    we don't correctly implement [expr.prim.id.unqual]#4.2, which post-P2266
    says that an id-expression is move-eligible if

    "the id-expression (possibly parenthesized) is the operand of
    a throw-expression, and names an implicitly movable entity that belongs
    to a scope that does not contain the compound-statement of the innermost
    lambda-expression, try-block, or function-try-block (if any) whose
    compound-statement or ctor-initializer contains the throw-expression."

    I worked out that it's trying to say that given

      struct X {
        X();
        X(const X&);
        X(X&&) = delete;
      };

    the following should fail: the scope of the throw is an sk_try, and it's
    also x's scope S, and S "does not contain the compound-statement of the
    *try-block" so x is move-eligible, so we move, so we fail.

      void f ()
      try {
        X x;
        throw x;  // use of deleted function
      } catch (...) {
      }

    Whereas here:

      void g (X x)
      try {
        throw x;
      } catch (...) {
      }

    the throw is again in an sk_try, but x's scope is an sk_function_parms
    which *does* contain the {} of the *try-block, so x is not move-eligible,
    so we don't move, so we use X(const X&), and the code is fine.

    The current code also doesn't seem to handle

      void h (X x) {
        void z (decltype(throw x, true));
      }

    where there's no enclosing lambda or sk_try so we should move.

    I'm not doing anything about lambdas because we shouldn't reach the
    code at the end of the function: the DECL_HAS_VALUE_EXPR_P check
    shouldn't let us go further.

            PR c++/113789
            PR c++/113853

    gcc/cp/ChangeLog:

            * typeck.cc (treat_lvalue_as_rvalue_p): Update code to better
            reflect [expr.prim.id.unqual]#4.2.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/sfinae69.C: Remove dg-bogus.
            * g++.dg/cpp0x/sfinae70.C: New test.
            * g++.dg/cpp0x/sfinae71.C: New test.
            * g++.dg/cpp0x/sfinae72.C: New test.
            * g++.dg/cpp2a/implicit-move4.C: New test.