[PATCH] c++: implicit dummy object in requires clause [PR103198]

[Jason Merrill]

On 11/17/21 14:52, Patrick Palka wrote:
> On Wed, 17 Nov 2021, Jason Merrill wrote:
> 
>> On 11/11/21 20:25, Patrick Palka wrote:
>>> In the testcase below satisfaction misbehaves for f and g ultimately
>>> because find_template_parameters fails to notice that the constraint
>>> 'val.x' depends on the template parameters of the class template.
>>> In contrast, satisfaction works just fine for h.
>>>
>>> The problem seems to come down to a difference in how any_template_parm_r
>>> handles 'this' vs a dummy object: we walk TREE_TYPE of the former but
>>> not the latter, and this causes us to miss the tparm dependencies in
>>> f/g's constraints since in their case the implicit object parameter
>>> through which we access 'val' is a dummy object.  (For h, since we know
>>> it's a non-static member function when parsing its trailing constraints,
>>> the implicit object parameter is 'this' instead of a dummy object.)
>>>
>>> This patch fixes this inconsistency by making any_template_parm_r also
>>> walk into the TREE_TYPE of a dummy object, as is already done for
>>> 'this'.
>>>
>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, also tested on
>>> cmcstl2 and range-v3, does this look OK for trunk and 11?
>>>
>>> 	PR c++/103198
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* pt.c (any_template_parm_r): Walk the TREE_TYPE of a dummy
>>> 	object.
>>
>> Should we handle CONVERT_EXPR with the various casts in cp_walk_subtrees?
> 
> This seems to work well too.  But I'm not sure about doing this since
> IIUC cp_walk_subtrees is generally supposed to walk subtrees that are
> explicitly written in the source code, but when a CONVERT_EXPR
> corresponds to an implicit conversion then the target type doesn't
> explicitly appear anywhere.

We could check is_dummy_object there as well?

>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/cpp2a/concepts-this1.C: New test.
>>> ---
>>>    gcc/cp/pt.c                                 |  5 ++++
>>>    gcc/testsuite/g++.dg/cpp2a/concepts-this1.C | 30 +++++++++++++++++++++
>>>    2 files changed, 35 insertions(+)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-this1.C
>>>
>>> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
>>> index 82bf7dc26f6..fa55857d783 100644
>>> --- a/gcc/cp/pt.c
>>> +++ b/gcc/cp/pt.c
>>> @@ -10766,6 +10766,11 @@ any_template_parm_r (tree t, void *data)
>>>    	WALK_SUBTREE (TREE_TYPE (t));
>>>          break;
>>>    +    case CONVERT_EXPR:
>>> +      if (is_dummy_object (t))
>>> +	WALK_SUBTREE (TREE_TYPE (t));
>>> +      break;
>>> +
>>>        default:
>>>          break;
>>>        }
>>> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-this1.C
>>> b/gcc/testsuite/g++.dg/cpp2a/concepts-this1.C
>>> new file mode 100644
>>> index 00000000000..d717028201a
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-this1.C
>>> @@ -0,0 +1,30 @@
>>> +// PR c++/103198
>>> +// { dg-do compile { target c++20 } }
>>> +
>>> +template<class T, class = void>
>>> +struct A {
>>> +  T val;
>>> +
>>> +  template<class U>
>>> +    requires requires { val.x; }
>>> +  void f(U);
>>> +
>>> +  static void g(int)
>>> +    requires requires { val.x; };
>>> +
>>> +  void h(int)
>>> +    requires requires { val.x; };
>>> +};
>>> +
>>> +struct B { int x; };
>>> +struct C { };
>>> +
>>> +int main() {
>>> +  A<B>().f(0);
>>> +  A<B>().g(0);
>>> +  A<B>().h(0);
>>> +
>>> +  A<C>().f(0); // { dg-error "no match" }
>>> +  A<C>().g(0); // { dg-error "no match" }
>>> +  A<C>().h(0); // { dg-error "no match" }
>>> +}
>>>
>>
>>
>