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

[Jason Merrill <jason@redhat.com>]

On 6/3/22 10:46, Patrick Palka wrote:
> On Thu, 2 Jun 2022, Jason Merrill wrote:
> 
>> 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?
> 
> I'm afraid I'm not sure how to do that :/  In particular for the case
> where add_capture_p is nonzero and the given lambda lacks a
> lambda_function.  I suppose we can relax the assert in the !add_capture_p
> case but that seems somewhat hacky.
> 
> I noticed that finish_this_expr, another user of lambda_expr_this_capture,
> isn't guarded by resolvable_dummy_lambda.  I believe it gets away with
> this because it checks lambda-ness of TREE_TYPE (current_class_ref) instead
> of current_class_type.  Perhaps we should do the same in maybe_dummy_object?
> This avoids the ICE in lambda-ice11.C without needing to check lambda_function,
> and seems like a cleaner approach overall.
> 
> -- >8 --
> 
> Subject: [PATCH] c++: cv-quals of dummy obj for non-dep memfn call [PR105637]
> 
> 	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                                | 31 ++++++++++++++-----
>   .../g++.dg/cpp0x/lambda/lambda-this22.C       | 20 ++++++++++++
>   .../g++.dg/template/non-dependent23.C         | 25 +++++++++++++++
>   3 files changed, 69 insertions(+), 7 deletions(-)
>   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 2b9cb7e1c7b..183febffb5d 100644
> --- a/gcc/cp/tree.cc
> +++ b/gcc/cp/tree.cc
> @@ -4319,15 +4319,32 @@ maybe_dummy_object (tree type, tree* binfop)
>     if (binfop)
>       *binfop = binfo;
>   
> -  if (current_class_ref
> -      /* current_class_ref might not correspond to current_class_type if
> -	 we're in tsubst_default_argument or a lambda-declarator; in either
> -	 case, we want to use current_class_ref if it matches CONTEXT.  */
> -      && (same_type_ignoring_top_level_qualifiers_p
> -	  (TREE_TYPE (current_class_ref), context)))
> +  /* current_class_ref might not correspond to current_class_type if
> +     we're in tsubst_default_argument or a lambda-declarator; in either
> +     case, we want to use current_class_ref if it matches CONTEXT.  */
> +  tree ctype = current_class_ref ? TREE_TYPE (current_class_ref) : NULL_TREE;
> +  if (ctype
> +      && same_type_ignoring_top_level_qualifiers_p (ctype, 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 (ctype)
> +	{
> +	  int quals = 0;
> +	  if (LAMBDA_TYPE_P (ctype))
> +	    {
> +	      tree lambda = CLASSTYPE_LAMBDA_EXPR (ctype);

And just checking CLASSTYPE_LAMBDA_EXPR (ctype) still isn't enough?

> +	      if (tree cap = lambda_expr_this_capture (lambda, false))
> +		quals = cp_type_quals (TREE_TYPE (TREE_TYPE (cap)));
> +	    }
> +	  else
> +	    quals = cp_type_quals (ctype);
> +	  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>;