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

[Jason Merrill <jason@redhat.com>]

On 6/2/22 15:57, Patrick Palka wrote:
> On Thu, 2 Jun 2022, Jason Merrill wrote:
> 
>> On 5/27/22 09:57, Patrick Palka wrote:
>>> On Thu, 26 May 2022, Patrick Palka wrote:
>>>
>>>> On Thu, 26 May 2022, Jason Merrill wrote:
>>>>
>>>>> 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)?
>>>>
>>>> That sadly doesn't seem to work -- the object type is BaseClass which is
>>>> not necessarily a base of the dependent TopClass<T>, so
>>>> resolvable_dummy_lambda returns NULL_TREE.  I guess it would work at
>>>> instantiation time though.
>>>
>>> Ah, what seems to work well is directly using lambda_expr_this_capture
>>> instead of maybe_resolve_dummy.  And we might as well handle this in
>>> maybe_dummy_object for benefit of all callers.  How does the following
>>> look?  Smoke tested with RUNTESTFLAGS="dg.exp=*.C", full bootstrap and
>>> regtesting in progress.
>>>
>>> -- >8 --
>>>
>>> Subject: [PATCH] c++: cv-quals of dummy obj for non-dep memfn call
>>> [PR105637]
>>>
>>> In non-dependent23.C below we expect the BaseClass::baseDevice calls to
>>> 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 'this'
>>> dependence 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 maybe_dummy_object respect the cv-quals
>>> of (the non-lambda) 'this' when returning a dummy object.  Thus, ahead
>>> of time OR using a dummy object will give us the right answer that is
>>> consistent with the instantiation time answer.
>>>
>>> An earlier version of this patch didn't handle 'this'-capturing lambdas
>>> correctly, which caused us to mishandle lambda-this22.C below.
>>>
>>> 	PR c++/105637
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	* tree.cc (maybe_dummy_object): When returning a dummy
>>> 	object, respect the cv-quals of 'this' if available.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	* g++.dg/cpp0x/lambda/lambda-this22.C: New test.
>>> 	* g++.dg/template/non-dependent23.C: New test.
>>> ---
>>>    gcc/cp/tree.cc                                | 19 +++++++++++++-
>>>    .../g++.dg/cpp0x/lambda/lambda-this22.C       | 20 +++++++++++++++
>>>    .../g++.dg/template/non-dependent23.C         | 25 +++++++++++++++++++
>>>    3 files changed, 63 insertions(+), 1 deletion(-)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-this22.C
>>>    create mode 100644 gcc/testsuite/g++.dg/template/non-dependent23.C
>>>
>>> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
>>> index 09162795801..679bf05b721 100644
>>> --- a/gcc/cp/tree.cc
>>> +++ b/gcc/cp/tree.cc
>>> @@ -4330,7 +4330,24 @@ maybe_dummy_object (tree type, tree* binfop)
>>>    	  (TREE_TYPE (current_class_ref), context)))
>>>        decl = current_class_ref;
>>>      else
>>> -    decl = build_dummy_object (context);
>>> +    {
>>> +      /* Return a dummy object whose cv-quals are consistent with (the
>>> +	 non-lambda) 'this' if available.  */
>>> +      if (current_class_ref)
>>> +	{
>>> +	  int quals = 0;
>>> +	  if (current == current_class_type)
>>> +	    quals = cp_type_quals (TREE_TYPE (current_class_ref));
>>> +	  else if (lambda_function (current_class_type))
>>> +	    {
>>> +	      tree lambda = CLASSTYPE_LAMBDA_EXPR (current_class_type);
>>
>> How about
>>
>>   else if (tree lambda = CLASSTYPE_LAMBDA_EXPR (current_class_type))
>>
>> ?  OK with that change.
> 
> Unfortunately the lambda_function test is necessary to avoid crashing
> on lambda-ice11.C; the test mirrors what resolvable_dummy_lambda does
> ever since r207999 / r208028 to avoid the crash.

Hmm, how about adjusting lambda_expr_this_capture to avoid the crash?

>>> +	      if (tree cap = lambda_expr_this_capture (lambda, false))
>>> +		quals = cp_type_quals (TREE_TYPE (TREE_TYPE (cap)));
>>> +	    }
>>> +	  context = cp_build_qualified_type (context, quals);
>>> +	}
>>> +      decl = build_dummy_object (context);
>>> +    }
>>>        return decl;
>>>    }
>>> diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-this22.C
>>> b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-this22.C
>>> new file mode 100644
>>> index 00000000000..c9e512b1621
>>> --- /dev/null
>>> +++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-this22.C
>>> @@ -0,0 +1,20 @@
>>> +// PR c++/105637
>>> +// { dg-do compile { target c++11 } }
>>> +
>>> +struct BaseClass {
>>> +  void baseDevice();                // #1
>>> +  void baseDevice() const = delete; // #2
>>> +};
>>> +
>>> +template<class T>
>>> +struct TopClass : T {
>>> +  void failsToCompile() {
>>> +    [this] { BaseClass::baseDevice(); }(); // should select #2, not #1
>>> +  }
>>> +
>>> +  void failsToCompile() const {
>>> +    [this] { BaseClass::baseDevice(); }(); // { dg-error "deleted" }
>>> +  }
>>> +};
>>> +
>>> +template struct TopClass<BaseClass>;
>>> 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>;
>>
>>
>