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

[Patrick Palka <ppalka@redhat.com>]

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);
+	      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>;
-- 
2.36.1.195.g8ddf593a25


> 
> > 
> > > > 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
> > > > 
> > > > 
> > > 
> > 
> > 
>