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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99790] [8/9/10/11 Regression] internal compiler error: in
 expand_expr_real_2 since r7-3811
Date: Mon, 29 Mar 2021 12:37:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99790

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Even more reduced:

struct A;
struct B { void (*fn) (A *); };
template <typename T>
int foo (const T &);
struct A { int a; static constexpr B b{[] (A *n) { n->*&A::a =3D 2; }}; };
int a =3D foo (A::b);

The problem seems to be that the lambda is genericized from
finish_lambda_function -> finish_function before the A class is finalized, =
so
    case PTRMEM_CST:
      /* By the time we get here we're handing off to the back end, so we d=
on't
         need or want to preserve PTRMEM_CST anymore.  */
      *stmt_p =3D cplus_expand_constant (stmt);
doesn't do anything.

If PTRMEM_CSTs are the only thing, one possible fix could be
--- gcc/cp/cp-gimplify.c.jj     2021-03-20 17:01:59.791040946 +0100
+++ gcc/cp/cp-gimplify.c        2021-03-29 14:27:37.532223156 +0200
@@ -660,6 +660,14 @@ cp_gimplify_expr (tree *expr_p, gimple_s
       ret =3D GS_UNHANDLED;
       break;

+    case PTRMEM_CST:
+      *expr_p =3D cplus_expand_constant (*expr_p);
+      if (TREE_CODE (*expr_p) =3D=3D PTRMEM_CST)
+       ret =3D GS_ERROR;
+      else
+       ret =3D GS_OK;
+      break;
+
     case RETURN_EXPR:
       if (TREE_OPERAND (*expr_p, 0)
          && (TREE_CODE (TREE_OPERAND (*expr_p, 0)) =3D=3D INIT_EXPR
i.e. handle PTRMEM_CSTs not just during  genericization, but also during
gimplification where because we are unit at a time the classes should be be=
tter
complete already.=