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

[Jason Merrill <jason@redhat.com>]

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).

Does convert_default_arg not run into the same problem, e.g. when calling

   void g(B = {0});

?

> 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{};
> +};