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

[Jason Merrill <jason@redhat.com>]

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.

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.

> [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.

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{};
>>> +};
>>
>>
>