Re: [PATCH] c++ modules: ICE with templated friend and std namespace [PR100134]

[Jason Merrill <jason@redhat.com>]

On 10/11/22 13:40, Nathan Sidwell wrote:
> On 10/11/22 11:35, Patrick Palka wrote:
>> IIUC the function depset::hash::add_binding_entity has an assert
>> verifying that if a namespace contains an exported entity, then
>> the namespace must have been opened in the module purview:
>>
>>    if (data->hash->add_namespace_entities (decl, data->partitions))
>>      {
>>        /* It contains an exported thing, so it is exported.  */
>>        gcc_checking_assert (DECL_MODULE_PURVIEW_P (decl));
>>        DECL_MODULE_EXPORT_P (decl) = true;
>>      }
>>
>> We're tripping over this assert in the below testcase because by
>> instantiating and exporting std::A<int>, we end up in turn defining
>> and exporting the hidden friend std::f without ever having opening
>> the enclosing namespace std within the module purview and thus
>> DECL_MODULE_PURVIEW_P (std_node) is false.
>>
>> Note that it's important that the enclosing namespace is std here: if we
>> use a different namespace then the ICE disappears.  This probably has
>> something to do with the fact that we predefine std via push_namespace
>> from cxx_init_decl_processing (which makes it look like we've opened the
>> namespace in the TU), whereas with another namespace we would instead
>> lazily obtain the NAMESPACE_DECL from add_imported_namespace.
>>
>> Since templated frined functions are special in that they give us a way
>> to declare a new namespace-scope function without having to explicitly
>> open the namespace, this patch proposes to fix this issue by propagating
>> DECL_MODULE_PURVIEW_P from a friend function to the enclosing namespace
>> when instantiating the friend.
> 
> ouch.  This is ok, but I think we have a bug -- what is the module 
> ownership of the friend introduced by the instantiation?
> 
> Haha, there's a note on 13.7.5/3 -- the attachment is to the same module 
> as the befriending class.
> 
> That means we end up creating and writing out entities that exist in the 
> symbol table (albeit hidden) whose module ownership is neither the 
> global module or the tu's module.  That's not something the module 
> machinery anticipates. We'll get the mangling wrong for starters. Hmm.
> 
> These are probably rare.  Thinking about the right solution though ...

This seems closely connected to

  https://cplusplus.github.io/CWG/issues/2588.html

Jason

> nathan
> 
> 
>>
>> Tested on x86_64-pc-linux-gnu, does this look like the right fix?  Other
>> solutions that seem to work are to set DECL_MODULE_PURVIEW_P on std_node
>> after the fact from declare_module, or simply to suppress the assert for
>> std_node.
>>
>>     PR c++/100134
>>
>> gcc/cp/ChangeLog:
>>
>>     * pt.cc (tsubst_friend_function): Propagate DECL_MODULE_PURVIEW_P
>>     from the new declaration to the enclosing namespace scope.
>>
>> gcc/testsuite/ChangeLog:
>>
>>     * g++.dg/modules/tpl-friend-8_a.H: New test.
>>     * g++.dg/modules/tpl-friend-8_b.C: New test.
>> ---
>>   gcc/cp/pt.cc                                  | 7 +++++++
>>   gcc/testsuite/g++.dg/modules/tpl-friend-8_a.H | 9 +++++++++
>>   gcc/testsuite/g++.dg/modules/tpl-friend-8_b.C | 8 ++++++++
>>   3 files changed, 24 insertions(+)
>>   create mode 100644 gcc/testsuite/g++.dg/modules/tpl-friend-8_a.H
>>   create mode 100644 gcc/testsuite/g++.dg/modules/tpl-friend-8_b.C
>>
>> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
>> index 5b9fc588a21..9e3085f3fa6 100644
>> --- a/gcc/cp/pt.cc
>> +++ b/gcc/cp/pt.cc
>> @@ -11448,6 +11448,13 @@ tsubst_friend_function (tree decl, tree args)
>>            by duplicate_decls.  */
>>         new_friend = old_decl;
>>       }
>> +
>> +      /* We've just added a new namespace-scope entity to the purview 
>> without
>> +     necessarily having opened the enclosing namespace, so make sure the
>> +     enclosing namespace is in the purview now too.  */
>> +      if (TREE_CODE (DECL_CONTEXT (new_friend)) == NAMESPACE_DECL)
>> +    DECL_MODULE_PURVIEW_P (DECL_CONTEXT (new_friend))
>> +      |= DECL_MODULE_PURVIEW_P (STRIP_TEMPLATE (new_friend));
>>       }
>>     else
>>       {
>> diff --git a/gcc/testsuite/g++.dg/modules/tpl-friend-8_a.H 
>> b/gcc/testsuite/g++.dg/modules/tpl-friend-8_a.H
>> new file mode 100644
>> index 00000000000..bd2290460b5
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/modules/tpl-friend-8_a.H
>> @@ -0,0 +1,9 @@
>> +// PR c++/100134
>> +// { dg-additional-options -fmodule-header }
>> +// { dg-module-cmi {} }
>> +
>> +namespace std {
>> +  template<class T> struct A {
>> +    friend void f(A) { }
>> +  };
>> +}
>> diff --git a/gcc/testsuite/g++.dg/modules/tpl-friend-8_b.C 
>> b/gcc/testsuite/g++.dg/modules/tpl-friend-8_b.C
>> new file mode 100644
>> index 00000000000..76d7447c2eb
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/modules/tpl-friend-8_b.C
>> @@ -0,0 +1,8 @@
>> +// PR c++/100134
>> +// { dg-additional-options -fmodules-ts }
>> +// { dg-module-cmi pr100134 }
>> +export module pr100134;
>> +
>> +import "tpl-friend-8_a.H";
>> +
>> +export std::A<int> a;
>