[PATCH] c++: ICE initing lifetime-extended constexpr var [PR107079]

[Marek Polacek <polacek@redhat.com>]

(This may not be a complete fix but I got stuck so I'm posting what
I have, which at least fixes the ICE.)

We ICE on the simple:

  struct X { const X* x = this; };
  constexpr const X& x = X{};

where store_init_value initializes 'x' with

  &TARGET_EXPR <D.2768, {.x=(const struct X *) &<PLACEHOLDER_EXPR struct X>}>

but we must lifetime-extend via extend_ref_init_temps and we get

  _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>;, (const struct X &) &_ZGR1x_;

Since 'x' was declared constexpr, we do cxx_constant_init and we hit
the preeval code added in r269003 while evaluating the INIT_EXPR:

  _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>

but we have no ctx.ctor or ctx.object here so lookup_placeholder won't
find anything that could replace X and we ICE on
 7861       /* A placeholder without a referent.  We can get here when
 7862          checking whether NSDMIs are noexcept, or in massage_init_elt;
 7863          just say it's non-constant for now.  */
 7864       gcc_assert (ctx->quiet);
because cxx_constant_init means !ctx->quiet.  It's not correct that
there isn't a referent.  I think the following patch is a pretty
straightforward fix: pass the _ZGR var down to maybe_constant_init so
that it can replace the PLACEHOLDER_EXPR with _ZGR1x_.

What I wasn't able to make work is the commented assert in the test.
It doesn't pass because we complain that _ZGR1x_ isn't a constexpr
variable, but making it so would just result in "used in its own
initializer" (which is true).  I notice that while clang++ compiles
the assert fine, MSVC++/icc reject it as non-constant.  So maybe we
don't have to/shouldn't make it work.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

	PR c++/107079

gcc/cp/ChangeLog:

	* call.cc (set_up_extended_ref_temp): Pass var to maybe_constant_init.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-nsdmi2.C: New test.
---
 gcc/cp/call.cc                                | 2 +-
 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index f7c5d9da94b..a0afab9b26a 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13583,7 +13583,7 @@ set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups,
 
   /* If the initializer is constant, put it in DECL_INITIAL so we get
      static initialization and use in constant expressions.  */
-  init = maybe_constant_init (expr);
+  init = maybe_constant_init (expr, var);
   /* As in store_init_value.  */
   init = cp_fully_fold (init);
   if (TREE_CONSTANT (init))
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
new file mode 100644
index 00000000000..6dbb7eb739a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
@@ -0,0 +1,9 @@
+// PR c++/107079
+// { dg-do compile { target c++11 } }
+
+struct X {
+  const X* x = this;
+};
+constexpr const X& x = X{};
+// TODO: Should the assert pass?
+//static_assert(x.x == &x);

base-commit: 77bb54b1b07add45007c664724b726541d672ef3
-- 
2.39.1