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

[Jason Merrill <jason@redhat.com>]

On 12/23/22 10:48, Patrick Palka wrote:
> On Thu, 22 Dec 2022, Patrick Palka wrote:
> 
>> On Thu, 22 Dec 2022, Jason Merrill wrote:
>>
>>> 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.
>>
>> Aha!  I'll try that.
> 
> How does this look?  Bootstrapped and regtested on x86_64-pc-linux-gnu.

OK.

> -- >8 --
> 
> Subject: [PATCH] c++: get_nsdmi in template context [PR108116]
> 
> 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 build a call to A::~A and check 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 was recently
> changed by r12-6897-gdec8d0e5fa00ceb2) which expr_noexcept_p doesn't
> expect, and we crash.
> 
> This patch fixes this by clearing processing_template_decl before
> the call to break_out_target_exprs from get_nsdmi.  And since it more
> generally seems we shouldn't be seeing (or producing) non-templated
> trees from break_out_target_exprs, this patch also adds an assert to
> that effect.
> 
> 	PR c++/108116
> 
> gcc/cp/ChangeLog:
> 
> 	* constexpr.cc (maybe_constant_value): Clear
> 	processing_template_decl before calling break_out_target_exprs.
> 	* init.cc (get_nsdmi): Likewise.
> 	* tree.cc (break_out_target_exprs): Assert processing_template_decl
> 	is cleared.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/nsdmi-template24.C: New test.
> ---
>   gcc/cp/constexpr.cc                           |  4 ++++
>   gcc/cp/init.cc                                |  4 ++++
>   gcc/cp/tree.cc                                |  4 ++++
>   gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C | 22 +++++++++++++++++++
>   4 files changed, 34 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/nsdmi-template24.C
> 
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index d99c49bdbe2..414af7a6d4c 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -8507,6 +8507,10 @@ maybe_constant_value (tree t, tree decl /* = NULL_TREE */,
>         r = *cached;
>         if (r != t)
>   	{
> +	  /* Clear processing_template_decl for sake of break_out_target_exprs;
> +	     entries in the cv_cache are non-templated.  */
> +	  processing_template_decl_sentinel ptds;
> +
>   	  r = break_out_target_exprs (r, /*clear_loc*/true);
>   	  protected_set_expr_location (r, EXPR_LOCATION (t));
>   	}
> diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> index 73e6547c076..b49a7ca9169 100644
> --- a/gcc/cp/init.cc
> +++ b/gcc/cp/init.cc
> @@ -670,6 +670,10 @@ get_nsdmi (tree member, bool in_ctor, tsubst_flags_t complain)
>         current_class_ptr = build_address (current_class_ref);
>       }
>   
> +  /* Clear processing_template_decl for sake of break_out_target_exprs;
> +     INIT is always non-templated.  */
> +  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/cp/tree.cc b/gcc/cp/tree.cc
> index 33bde16f128..faf01616f87 100644
> --- a/gcc/cp/tree.cc
> +++ b/gcc/cp/tree.cc
> @@ -3342,6 +3342,10 @@ break_out_target_exprs (tree t, bool clear_location /* = false */)
>     static int target_remap_count;
>     static splay_tree target_remap;
>   
> +  /* We shouldn't be called on templated trees, nor do we want to
> +     produce them.  */
> +  gcc_checking_assert (!processing_template_decl);
> +
>     if (!target_remap_count++)
>       target_remap = splay_tree_new (splay_tree_compare_pointers,
>   				   /*splay_tree_delete_key_fn=*/NULL,
> 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{};
> +};