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.
>
>
> > + 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>;
>
>
