[gcc r13-5763] c++: ICE initing lifetime-extended constexpr var [PR107079]

[gcc r13-5763] c++: ICE initing lifetime-extended constexpr var [PR107079]

[Marek Polacek]

https://gcc.gnu.org/g:67b82bc1b29b82e4577df9491793624f3a8eb12f

commit r13-5763-g67b82bc1b29b82e4577df9491793624f3a8eb12f
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Feb 8 14:02:48 2023 -0500

    c++: ICE initing lifetime-extended constexpr var [PR107079]
    
    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_.
    
    The commented assert in the test doesn't pass: we complain that _ZGR1x_
    isn't a constexpr variable because we don't implement DR2126 (PR101588).
    
            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.

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

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index f7c5d9da94b..a349d8e79db 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, /*manifestly_const_eval=*/true);
   /* 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..d711b8051c0
--- /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: The assert should pass once we implement DR2126 (c++/101588).
+//static_assert(x.x == &x);