Re: [PATCH] c++: improve class NTTP object pretty printing [PR111471]

[Patrick Palka <ppalka@redhat.com>]

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On Tue, 19 Sep 2023, Patrick Palka wrote:

> On Tue, 19 Sep 2023, Jason Merrill wrote:
> 
> > On 9/19/23 12:40, Patrick Palka wrote:
> > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk/13?
> > 
> > OK for trunk.  What's your argument for backporting?
> 
> Thanks.  I don't feel strongly about it, but I was thinking that since
> we typically backport C++20-only correctness fixes to the most recent
> release branch, C++20-only diagnostic improvements might be suitable
> too?
> 
> > 
> > > -- >8 --
> > > 
> > > 1. Move class NTTP object pretty printing to a more general spot in
> > >     the pretty printer.

FWIW this first change isn't just a refactoring, it means we now pretty
print an NTTP object that appears elsewhere besides in a template
argument list, e.g. in a parameter mapping:

Before:

diagnostic19.C:8:15: note: the expression ‘((const A)V).value [with V = _ZTAXtl1AEE]’ evaluated to ‘false’

After:

diagnostic19.C:8:15: note: the expression ‘(V).value [with V = A{false}]’ evaluated to ‘false’

> > > 2. Print the type of an class NTTP object alongside its CONSTRUCTOR
> > >     value, like dump_expr would have done.
> > > 3. Don't pretty print const VIEW_CONVERT_EXPR wrapping class NTTPs.
> > > 
> > > 	PR c++/111471
> > > 
> > > gcc/cp/ChangeLog:
> > > 
> > > 	* cxx-pretty-print.cc (cxx_pretty_printer::expression)
> > > 	<case VAR_DECL>: Print the value of a class NTTP object.
> > > 	<case VIEW_CONVERT_EXPR>: Strip cosnt VIEW_CONVERT_EXPR
> > > 	wrappers for class NTTPs.
> > > 	(pp_cxx_template_argument_list): Don't handle the class
> > > 	NTTP objects here.
> > > 
> > > gcc/testsuite/ChangeLog:
> > > 
> > > 	* g++.dg/concepts/diagnostic19.C: New test.
> > > ---
> > >   gcc/cp/cxx-pretty-print.cc                   | 19 +++++++++++++++++--
> > >   gcc/testsuite/g++.dg/concepts/diagnostic19.C | 20 ++++++++++++++++++++
> > >   2 files changed, 37 insertions(+), 2 deletions(-)
> > >   create mode 100644 gcc/testsuite/g++.dg/concepts/diagnostic19.C
> > > 
> > > diff --git a/gcc/cp/cxx-pretty-print.cc b/gcc/cp/cxx-pretty-print.cc
> > > index 909a9dc917f..7cd43151592 100644
> > > --- a/gcc/cp/cxx-pretty-print.cc
> > > +++ b/gcc/cp/cxx-pretty-print.cc
> > > @@ -1121,6 +1121,15 @@ cxx_pretty_printer::expression (tree t)
> > >         t = OVL_FIRST (t);
> > >         /* FALLTHRU */
> > >       case VAR_DECL:
> > > +      if (DECL_NTTP_OBJECT_P (t))
> > > +	{
> > > +	  /* Print the type followed by the CONSTRUCTOR value of an
> > > +	     NTTP object.  */
> > > +	  simple_type_specifier (cv_unqualified (TREE_TYPE (t)));
> > > +	  expression (DECL_INITIAL (t));
> > > +	  break;
> > > +	}
> > > +      /* FALLTHRU */
> > >       case PARM_DECL:
> > >       case FIELD_DECL:
> > >       case CONST_DECL:
> > > @@ -1261,6 +1270,14 @@ cxx_pretty_printer::expression (tree t)
> > >         pp_cxx_right_paren (this);
> > >         break;
> > >   +    case VIEW_CONVERT_EXPR:
> > > +      if (TREE_CODE (TREE_OPERAND (t, 0)) == TEMPLATE_PARM_INDEX)
> > > +	{
> > > +	  /* Strip const VIEW_CONVERT_EXPR wrappers for class NTTPs.  */
> > > +	  expression (TREE_OPERAND (t, 0));
> > > +	  break;
> > > +	}
> > > +      /* FALLTHRU */
> > >       default:
> > >         c_pretty_printer::expression (t);
> > >         break;
> > > @@ -1966,8 +1983,6 @@ pp_cxx_template_argument_list (cxx_pretty_printer *pp,
> > > tree t)
> > >   	  if (TYPE_P (arg) || (TREE_CODE (arg) == TEMPLATE_DECL
> > >   			       && TYPE_P (DECL_TEMPLATE_RESULT (arg))))
> > >   	    pp->type_id (arg);
> > > -	  else if (VAR_P (arg) && DECL_NTTP_OBJECT_P (arg))
> > > -	    pp->expression (DECL_INITIAL (arg));
> > >   	  else
> > >   	    pp->expression (arg);
> > >   	}
> > > diff --git a/gcc/testsuite/g++.dg/concepts/diagnostic19.C
> > > b/gcc/testsuite/g++.dg/concepts/diagnostic19.C
> > > new file mode 100644
> > > index 00000000000..20cdb63380b
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/concepts/diagnostic19.C
> > > @@ -0,0 +1,20 @@
> > > +// Verify our pretty printing of class NTTP objects.
> > > +// PR c++/111471
> > > +// { dg-do compile { target c++20 } }
> > > +
> > > +struct A { bool value; };
> > > +
> > > +template<A V>
> > > +  requires (V.value) // { dg-message {'\(V\).value \[with V =
> > > A\{false\}\]'} }
> > > +void f();
> > > +
> > > +template<A V> struct B { static constexpr auto value = V.value; };
> > > +
> > > +template<class T>
> > > +  requires T::value // { dg-message {'T::value \[with T = B<A\{false\}>\]'}
> > > }
> > > +void g();
> > > +
> > > +int main() {
> > > +  f<A{false}>(); // { dg-error "no match" }
> > > +  g<B<A{false}>>(); // { dg-error "no match" }
> > > +}
> > 
> > 
>