Message ID | 20230208210140.391014-1-polacek@redhat.com |
---|---|
State | New |
Headers | show |
Series | c++: ICE initing lifetime-extended constexpr var [PR107079] | expand |
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
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);