Re: [PATCH] c++: get_nsdmi in template context [PR108116]

[Patrick Palka <ppalka@redhat.com>]

On Thu, 22 Dec 2022, Jason Merrill wrote:

> On 12/22/22 11:31, Patrick Palka wrote:
> > On Wed, 21 Dec 2022, Jason Merrill wrote:
> > 
> > > On 12/21/22 09:52, Patrick Palka wrote:
> > > > Here during ahead of time checking of C{}, we indirectly call get_nsdmi
> > > > for C::m from finish_compound_literal, which in turn calls
> > > > break_out_target_exprs for C::m's (non-templated) initializer, during
> > > > which we end up building a call to A::~A and checking expr_noexcept_p
> > > > for it (from build_vec_delete_1).  But this is all done with
> > > > processing_template_decl set, so the built A::~A call is templated
> > > > (whose form r12-6897-gdec8d0e5fa00ceb2 recently changed) which
> > > > expr_noexcept_p doesn't expect and we crash.
> > > > 
> > > > In r10-6183-g20afdcd3698275 we fixed a similar issue by guarding a
> > > > expr_noexcept_p call with !processing_template_decl, which works here
> > > > too.  But it seems to me since the initializer we obtain in get_nsdmi is
> > > > always non-templated, it should be calling break_out_target_exprs with
> > > > processing_template_decl cleared since otherwise the function might end
> > > > up mixing templated and non-templated trees.
> > > > 
> > > > I'm not sure about this though, perhaps this is not the best fix here.
> > > > Alternatively, when processing_template_decl we could make get_nsdmi
> > > > avoid calling break_out_target_exprs at all or something.  Additionally,
> > > > perhaps break_out_target_exprs should be a no-op more generally when
> > > > processing_template_decl since we shouldn't see any TARGET_EXPRs inside
> > > > a template?
> > > 
> > > Hmm.
> > > 
> > > Any time we would call break_out_target_exprs we're dealing with
> > > non-dependent
> > > expressions; if we're in a template, we're building up an initializer or a
> > > call that we'll soon throw away, just for the purpose of checking or type
> > > computation.
> > > 
> > > Furthermore, as you say, the argument is always a non-template tree,
> > > whether
> > > in get_nsdmi or convert_default_arg.  So having processing_template_decl
> > > cleared would be correct.
> > > 
> > > I don't think we can get away with not calling break_out_target_exprs at
> > > all
> > > in a template; if nothing else, we would lose immediate invocation
> > > expansion.
> > > However, we could probably skip the bot_manip tree walk, which should
> > > avoid
> > > the problem.
> > > 
> > > Either way we end up returning non-template trees, as we do now, and
> > > callers
> > > have to deal with transient CONSTRUCTORs containing such (as we do in
> > > massage_init_elt).
> > 
> > Ah I see, makes sense.
> > 
> > > 
> > > Does convert_default_arg not run into the same problem, e.g. when calling
> > > 
> > >    void g(B = {0});
> > 
> > In practice it seems not, because we don't call convert_default_arg
> > when processing_template_decl is set (verified with an assert to
> > that effect).  In build_over_call for example we exit early when
> > processing_template_decl is set, and return a templated CALL_EXPR
> > that doesn't include default arguments at all.  A consequence of
> > this is that we don't reject ahead of time a call that would use
> > an ill-formed dependent default argument, e.g.
> > 
> >    template<class T>
> >    void g(B = T{0});
> > 
> >    template<class>
> >    void f() {
> >      g<void>();
> >    }
> > 
> > since the default argument instantiation would be the responsibility
> > of convert_default_arg.
> > 
> > Thinking hypothetically here, if we do in the future want to include default
> > arguments in the templated form of a CALL_EXPR,
> 
> We definitely do not want to; the templated form should be as close as
> possible to the source.

Ah, sounds good.

> 
> We might want to perform non-dependent conversions to get any errors (such as
> this one) before throwing away the result.  Which would be parallel to what we
> currently do in calling get_nsdmi, and would want the same behavior.

*nod*

> 
> > [snip]
> 
> > shall we go with the original approach to clear
> > processing_template_decl directly from get_nsdmi?
> 
> OK, but then we should also checking_assert !processing_template_decl in
> b_o_t_e.

Unfortunately we'd trigger that assert from maybe_constant_value, which
potentially calls b_o_t_e with processing_template_decl set.

> 
> Jason
> 
> > > ?
> > > 
> > > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > > > 
> > > > 	PR c++/108116
> > > > 
> > > > gcc/cp/ChangeLog:
> > > > 
> > > > 	* init.cc (get_nsdmi): Clear processing_template_decl before
> > > > 	processing the non-templated initializer.
> > > > 
> > > > gcc/testsuite/ChangeLog:
> > > > 
> > > > 	* g++.dg/cpp0x/nsdmi-template24.C: New test.
> > > > ---
> > > >    gcc/cp/init.cc                                |  8 ++++++-
> > > >    gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C | 22
> > > > +++++++++++++++++++
> > > >    2 files changed, 29 insertions(+), 1 deletion(-)
> > > >    create mode 100644 gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C
> > > > 
> > > > diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> > > > index 73e6547c076..c4345ebdaea 100644
> > > > --- a/gcc/cp/init.cc
> > > > +++ b/gcc/cp/init.cc
> > > > @@ -561,7 +561,8 @@ perform_target_ctor (tree init)
> > > >      return init;
> > > >    }
> > > >    -/* Return the non-static data initializer for FIELD_DECL MEMBER.  */
> > > > +/* Return the non-static data initializer for FIELD_DECL MEMBER.
> > > > +   The initializer returned is always non-templated.  */
> > > >      static GTY((cache)) decl_tree_cache_map *nsdmi_inst;
> > > >    @@ -670,6 +671,11 @@ get_nsdmi (tree member, bool in_ctor,
> > > > tsubst_flags_t
> > > > complain)
> > > >          current_class_ptr = build_address (current_class_ref);
> > > >        }
> > > >    +  /* Since INIT is always non-templated clear
> > > > processing_template_decl
> > > > +     before processing it so that we don't interleave templated and
> > > > +     non-templated trees.  */
> > > > +  processing_template_decl_sentinel ptds;
> > > > +
> > > >      /* Strip redundant TARGET_EXPR so we don't need to remap it, and
> > > >         so the aggregate init code below will see a CONSTRUCTOR.  */
> > > >      bool simple_target = (init && SIMPLE_TARGET_EXPR_P (init));
> > > > diff --git a/gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C
> > > > b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C
> > > > new file mode 100644
> > > > index 00000000000..202c67d7321
> > > > --- /dev/null
> > > > +++ b/gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C
> > > > @@ -0,0 +1,22 @@
> > > > +// PR c++/108116
> > > > +// { dg-do compile { target c++11 } }
> > > > +
> > > > +#include <initializer_list>
> > > > +
> > > > +struct A {
> > > > +  A(int);
> > > > +  ~A();
> > > > +};
> > > > +
> > > > +struct B {
> > > > +  B(std::initializer_list<A>);
> > > > +};
> > > > +
> > > > +struct C {
> > > > +  B m{0};
> > > > +};
> > > > +
> > > > +template<class>
> > > > +void f() {
> > > > +  C c = C{};
> > > > +};
> > > 
> > > 
> > 
> 
>