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.
>
> > > 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
> > >
> > >
> >
>
>
