Re: [PATCH] c++: Don't reject pointer to virtual method during constant evaluation [PR117615]

[Jason Merrill]

On 12/4/24 5:52 AM, Simon Martin wrote:

Hi Jason,

On 3 Dec 2024, at 22:37, Jason Merrill wrote:

On 12/3/24 12:25 PM, Simon Martin wrote:

We currently reject the following valid code:

=== cut here ===
struct Base {
      virtual void doit (int v) const {}
};
struct Derived : Base {
      void doit (int v) const {}
};
using fn_t = void (Base::*)(int) const;
struct Helper {
      fn_t mFn;
      constexpr Helper (auto && fn) : mFn(static_cast<fn_t>(fn)) {}
};
void foo () {
      constexpr Helper h (&Derived::doit);
}
=== cut here ===

The problem is that since r6-4014-gdcdbc004d531b4, &Derived::doit is
represented with an expression with type pointer to method and using
an
INTEGER_CST (here 1), and that cxx_eval_constant_expression rejects
any
such expression with a non-null INTEGER_CST.

This patch uses the same strategy as r12-4491-gf45610a45236e9 (fix
for
PR c++/102786), and simply lets such expressions go through.

Successfully tested on x86_64-pc-linux-gnu.

        PR c++/117615

gcc/cp/ChangeLog:

        * constexpr.cc (cxx_eval_constant_expression): Don't reject
        INTEGER_CSTs with type POINTER_TYPE to METHOD_TYPE.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/constexpr-virtual22.C: New test.

---
   gcc/cp/constexpr.cc                           | 27
++++++++++++-------
   .../g++.dg/cpp2a/constexpr-virtual22.C        | 22 +++++++++++++++
   2 files changed, 40 insertions(+), 9 deletions(-)
   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-virtual22.C

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 5a87fa485c6..d9636bad683 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8277,15 +8277,24 @@ cxx_eval_constant_expression (const
constexpr_ctx *ctx, tree t,
              }
            else
              {
-               /* This detects for example:
-                    reinterpret_cast<void*>(sizeof 0)
-               */
-               if (!ctx->quiet)
-                 error_at (loc, "%<reinterpret_cast<%T>(%E)%> is not "
-                           "a constant expression",
-                           type, op);
-               *non_constant_p = true;
-               return t;
+               if (TYPE_PTR_P (type)
+                   && TREE_CODE (TREE_TYPE (type)) == METHOD_TYPE)
+                 /* INTEGER_CST with pointer-to-method type is only used
+                    for a virtual method in a pointer to member function.
+                    Don't reject those.  */
+                 ;

This could be "else if" added before the existing else so it doesn't
need to be reindented.  OK with that change.

Thanks, merged as r15-5920-g72a2380a306a1c with the suggested change.

Since this is a reject-valid, also OK for active branches?

Yes.

Jason