Re: [PATCH] c++: cv-quals of dummy obj for non-dep memfn call [PR105637]

[Jason Merrill <jason@redhat.com>]

On 5/26/22 14:57, Patrick Palka wrote:
> On Thu, 26 May 2022, Patrick Palka wrote:
> 
>> Here we expect the calls to BaseClass::baseDevice resolve to the second,
>> third and fourth overloads respectively in light of the cv-qualifiers
>> of 'this' in each case.  But ever since r12-6075-g2decd2cabe5a4f, the
>> calls incorrectly resolve to the first overload at instantiation time.
>>
>> This happens because the calls to BaseClass::baseDevice are all deemed
>> non-dependent (ever since r7-755-g23cb72663051cd made us ignore the
>> dependentness of 'this' when considering the dependence of a non-static
>> memfn call), hence we end up checking the call ahead of time, using as
>> the object argument a dummy object of type BaseClass.  Since this object
>> argument is cv-unqualified, the calls incoherently resolve to the first
>> overload of baseDevice.  Before r12-6075, this incorrect result would
>> just get silently discarded and we'd end up redoing OR at instantiation
>> time using 'this' as the object argument.  But after r12-6075, we now
>> reuse this incorrect result at instantiation time.
>>
>> This patch fixes this by making finish_call_expr request from
>> maybe_dummy_object a cv-qualified object consistent with the cv-quals of
>> 'this'.  That way, ahead of time OR using a dummy object will give us
>> the right answer and we could safely reuse it at instantiation time.
>>
>> NB: r7-755 is also the cause of the related issue PR105742.  Not sure
>> if there's a fix that could resolve both PRs at once..
>>
>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
>> for trunk/12?
>>
>> 	PR c++/105637
>>
>> gcc/cp/ChangeLog:
>>
>> 	* semantics.cc (finish_call_expr): Pass a cv-qualified object
>> 	type to maybe_dummy_object that is consistent with the
>> 	cv-qualifiers of 'this' if available.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 	* g++.dg/template/non-dependent23.C: New test.
>> ---
>>   gcc/cp/semantics.cc                           | 15 ++++++++---
>>   .../g++.dg/template/non-dependent23.C         | 25 +++++++++++++++++++
>>   2 files changed, 37 insertions(+), 3 deletions(-)
>>   create mode 100644 gcc/testsuite/g++.dg/template/non-dependent23.C
>>
>> diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
>> index cd7a2818feb..1d9348c6cf1 100644
>> --- a/gcc/cp/semantics.cc
>> +++ b/gcc/cp/semantics.cc
>> @@ -2802,16 +2802,25 @@ finish_call_expr (tree fn, vec<tree, va_gc> **args, bool disallow_virtual,
>>   	[class.access.base] says that we need to convert 'this' to B* as
>>   	part of the access, so we pass 'B' to maybe_dummy_object.  */
>>   
>> +      tree object_type = BINFO_TYPE (BASELINK_ACCESS_BINFO (fn));
>>         if (DECL_MAYBE_IN_CHARGE_CONSTRUCTOR_P (get_first_fn (fn)))
>>   	{
>>   	  /* A constructor call always uses a dummy object.  (This constructor
>>   	     call which has the form A::A () is actually invalid and we are
>>   	     going to reject it later in build_new_method_call.)  */
>> -	  object = build_dummy_object (BINFO_TYPE (BASELINK_ACCESS_BINFO (fn)));
>> +	  object = build_dummy_object (object_type);
>>   	}
>>         else
>> -	object = maybe_dummy_object (BINFO_TYPE (BASELINK_ACCESS_BINFO (fn)),
>> -				     NULL);
>> +	{
>> +	  if (current_class_ref)
>> +	    {
>> +	      /* Make sure that if maybe_dummy_object gives us a dummy object,
>> +		 it'll have the same cv-quals as '*this'.  */
>> +	      int quals = cp_type_quals (TREE_TYPE (current_class_ref));
>> +	      object_type = cp_build_qualified_type (object_type, quals);
>> +	    }
>> +	  object = maybe_dummy_object (object_type, NULL);
>> +	}
>>   
>>         result = build_new_method_call (object, fn, args, NULL_TREE,
>>   				      (disallow_virtual
> 
> Drat, this fix doesn't interact well with 'this'-capturing lambdas:
> 
>      struct BaseClass {
>        void baseDevice();                // #1
>        void baseDevice() const = delete; // #2
>      };
> 
>      template<class T>
>      struct TopClass : T {
>        void failsToCompile() {
>          [this] { BaseClass::baseDevice(); }();
>        }
>      };
> 
>      template struct TopClass<BaseClass>;
> 
> Here after the fix, we'd incorrectly select the const #2 overload at
> template definition time because current_class_ref is the const 'this'
> for the lambda rather than the non-const 'this' for TopClass..  I suppose
> we need something like current_nonlambda_class_type for getting at the
> innermost non-lambda 'this'?

Do you want maybe_resolve_dummy (ob, false)?

>> diff --git a/gcc/testsuite/g++.dg/template/non-dependent23.C b/gcc/testsuite/g++.dg/template/non-dependent23.C
>> new file mode 100644
>> index 00000000000..ef95c591b75
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/template/non-dependent23.C
>> @@ -0,0 +1,25 @@
>> +// PR c++/105637
>> +
>> +struct BaseClass {
>> +  void baseDevice();                // #1
>> +  void baseDevice() const;          // #2
>> +  void baseDevice() volatile;       // #3
>> +  void baseDevice() const volatile; // #4
>> +};
>> +
>> +template<class T>
>> +struct TopClass : T {
>> +  void failsToCompile() const {
>> +    BaseClass::baseDevice(); // should select #2, not #1
>> +  }
>> +
>> +  void failsToCompile() volatile {
>> +    BaseClass::baseDevice();  // should select #3, not #1
>> +  }
>> +
>> +  void failsToCompile() const volatile {
>> +    BaseClass::baseDevice();  // should select #4, not #1
>> +  }
>> +};
>> +
>> +template struct TopClass<BaseClass>;
>> -- 
>> 2.36.1.182.g6afdb07b7b
>>
>>
>