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

[Jason Merrill <jason@redhat.com>]

On 2/8/23 13:01, Marek Polacek wrote:
> (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, 

That sounds like we aren't (correctly) implementing

  https://eel.is/c++draft/expr.const#4.7

> but making it so would just result in "used in its own
> initializer" (which is true).

True, but not in the sense it means; its initializer doesn't depend on 
its (uninitialized) value.

> 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);

We should also pass true for manifestly_const_eval as in store_init_value.

>     /* 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