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

[Jason Merrill <jason@redhat.com>]

On 12/22/22 16:41, Patrick Palka wrote:
> 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.

maybe_constant_value could also clear processing_template_decl; entries 
in cv_cache are non-templated.

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