[PATCH] c++: implicit dummy object in requires clause [PR103198]

Patrick Palka ppalka@redhat.com
Wed Nov 17 19:52:32 GMT 2021


On Wed, 17 Nov 2021, Jason Merrill wrote:

> On 11/11/21 20:25, Patrick Palka wrote:
> > In the testcase below satisfaction misbehaves for f and g ultimately
> > because find_template_parameters fails to notice that the constraint
> > 'val.x' depends on the template parameters of the class template.
> > In contrast, satisfaction works just fine for h.
> > 
> > The problem seems to come down to a difference in how any_template_parm_r
> > handles 'this' vs a dummy object: we walk TREE_TYPE of the former but
> > not the latter, and this causes us to miss the tparm dependencies in
> > f/g's constraints since in their case the implicit object parameter
> > through which we access 'val' is a dummy object.  (For h, since we know
> > it's a non-static member function when parsing its trailing constraints,
> > the implicit object parameter is 'this' instead of a dummy object.)
> > 
> > This patch fixes this inconsistency by making any_template_parm_r also
> > walk into the TREE_TYPE of a dummy object, as is already done for
> > 'this'.
> > 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, also tested on
> > cmcstl2 and range-v3, does this look OK for trunk and 11?
> > 
> > 	PR c++/103198
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* pt.c (any_template_parm_r): Walk the TREE_TYPE of a dummy
> > 	object.
> 
> Should we handle CONVERT_EXPR with the various casts in cp_walk_subtrees?

This seems to work well too.  But I'm not sure about doing this since
IIUC cp_walk_subtrees is generally supposed to walk subtrees that are
explicitly written in the source code, but when a CONVERT_EXPR
corresponds to an implicit conversion then the target type doesn't
explicitly appear anywhere.

> 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp2a/concepts-this1.C: New test.
> > ---
> >   gcc/cp/pt.c                                 |  5 ++++
> >   gcc/testsuite/g++.dg/cpp2a/concepts-this1.C | 30 +++++++++++++++++++++
> >   2 files changed, 35 insertions(+)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-this1.C
> > 
> > diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> > index 82bf7dc26f6..fa55857d783 100644
> > --- a/gcc/cp/pt.c
> > +++ b/gcc/cp/pt.c
> > @@ -10766,6 +10766,11 @@ any_template_parm_r (tree t, void *data)
> >   	WALK_SUBTREE (TREE_TYPE (t));
> >         break;
> >   +    case CONVERT_EXPR:
> > +      if (is_dummy_object (t))
> > +	WALK_SUBTREE (TREE_TYPE (t));
> > +      break;
> > +
> >       default:
> >         break;
> >       }
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-this1.C
> > b/gcc/testsuite/g++.dg/cpp2a/concepts-this1.C
> > new file mode 100644
> > index 00000000000..d717028201a
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-this1.C
> > @@ -0,0 +1,30 @@
> > +// PR c++/103198
> > +// { dg-do compile { target c++20 } }
> > +
> > +template<class T, class = void>
> > +struct A {
> > +  T val;
> > +
> > +  template<class U>
> > +    requires requires { val.x; }
> > +  void f(U);
> > +
> > +  static void g(int)
> > +    requires requires { val.x; };
> > +
> > +  void h(int)
> > +    requires requires { val.x; };
> > +};
> > +
> > +struct B { int x; };
> > +struct C { };
> > +
> > +int main() {
> > +  A<B>().f(0);
> > +  A<B>().g(0);
> > +  A<B>().h(0);
> > +
> > +  A<C>().f(0); // { dg-error "no match" }
> > +  A<C>().g(0); // { dg-error "no match" }
> > +  A<C>().h(0); // { dg-error "no match" }
> > +}
> > 
> 
> 



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