Re: [PATCH 07/10] Fix g++.dg/template/ref3.C

[David Malcolm <dmalcolm@redhat.com>]

On Thu, 2015-12-03 at 15:38 -0500, Jason Merrill wrote:
> On 12/03/2015 09:55 AM, David Malcolm wrote:
> > Testcase g++.dg/template/ref3.C:
> >
> >       1	// PR c++/28341
> >       2
> >       3	template<const int&> struct A {};
> >       4
> >       5	template<typename T> struct B
> >       6	{
> >       7	  A<(T)0> b; // { dg-error "constant|not a valid" }
> >       8	  A<T(0)> a; // { dg-error "constant|not a valid" }
> >       9	};
> >      10
> >      11	B<const int&> b;
> >
> > The output of this test for both c++11 and c++14 is unaffected
> > by the patch kit:
> >   g++.dg/template/ref3.C: In instantiation of 'struct B<const int&>':
> >   g++.dg/template/ref3.C:11:15:   required from here
> >   g++.dg/template/ref3.C:7:11: error: '0' is not a valid template argument for type 'const int&' because it is not an lvalue
> >   g++.dg/template/ref3.C:8:11: error: '0' is not a valid template argument for type 'const int&' because it is not an lvalue
> >
> > However, the c++98 output is changed:
> >
> > Status quo for c++98:
> > g++.dg/template/ref3.C: In instantiation of 'struct B<const int&>':
> > g++.dg/template/ref3.C:11:15:   required from here
> > g++.dg/template/ref3.C:7:11: error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression
> > g++.dg/template/ref3.C:8:11: error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression
> >
> > (line 7 and 8 are at the closing semicolon for fields b and a)
> >
> > With the patchkit for c++98:
> > g++.dg/template/ref3.C: In instantiation of 'struct B<const int&>':
> > g++.dg/template/ref3.C:11:15:   required from here
> > g++.dg/template/ref3.C:7:5: error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression
> > g++.dg/template/ref3.C:7:5: error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression
> >
> > So the 2nd:
> >    "error: a cast to a type other than an integral or enumeration type cannot appear in a constant-expression"
> > moves from line 8 to line 7 (and moves them to earlier, having ranges)
> >
> > What's happening is that cp_parser_enclosed_template_argument_list
> > builds a CAST_EXPR, the first time from cp_parser_cast_expression,
> > the second time from cp_parser_functional_cast; these have locations
> > representing the correct respective caret&ranges, i.e.:
> >
> >     A<(T)0> b;
> >       ^~~~
> >
> > and:
> >
> >     A<T(0)> a;
> >       ^~~~
> >
> > Eventually finish_template_type is called for each, to build a RECORD_TYPE,
> > and we get a cache hit the 2nd time through here in pt.c:
> > 8281	      hash = spec_hasher::hash (&elt);
> > 8282	      entry = type_specializations->find_with_hash (&elt, hash);
> > 8283
> > 8284	      if (entry)
> > 8285		return entry->spec;
> >
> > due to:
> >    template_args_equal (ot=<cast_expr 0x7ffff19bc400>, nt=<cast_expr 0x7ffff19bc480>) at ../../src/gcc/cp/pt.c:7778
> > which calls:
> >    cp_tree_equal (t1=<cast_expr 0x7ffff19bc400>, t2=<cast_expr 0x7ffff19bc480>) at ../../src/gcc/cp/tree.c:2833
> > and returns equality.
> >
> > Hence we get a single RECORD_TYPE for the type A<(T)(0)>, and hence
> > when issuing the errors it uses the TREE_VEC for the first one,
> > using the location of the first line.
> 
> Why does the type sharing affect where the parser gives the error?

I believe what's happening is that the patchkit is setting location_t
values for more expressions than before, including the expression for
the template param.  pt.c:tsubst_expr has this:

  if (EXPR_HAS_LOCATION (t))
    input_location = EXPR_LOCATION (t);

I believe that before (in the status quo), the substituted types didn't
have location_t values, and hence the above conditional didn't fire;
input_location was coming from a *token* where the expansion happened,
hence we got an error message on the relevant line for each expansion.

With the patch, the substituted types have location_t values within
their params, hence the conditional above fires: input_location is
updated to use the EXPR_LOCATION, which comes from that of the param
within the type - but with type-sharing it's using the first place where
the type is created.

Perhaps a better fix is for cp_parser_non_integral_constant_expression
to take a location_t, rather than have it rely on input_location?


> > I'm not sure what the ideal fix for this is; for now I've worked
> > around it by updating the dg directives to reflect the new output.
> >
> > gcc/testsuite/ChangeLog:
> > 	* g++.dg/template/ref3.C: Update locations of dg directives.
> > ---
> >   gcc/testsuite/g++.dg/template/ref3.C | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/gcc/testsuite/g++.dg/template/ref3.C b/gcc/testsuite/g++.dg/template/ref3.C
> > index 976c093..6e568c3 100644
> > --- a/gcc/testsuite/g++.dg/template/ref3.C
> > +++ b/gcc/testsuite/g++.dg/template/ref3.C
> > @@ -4,8 +4,10 @@ template<const int&> struct A {};
> >
> >   template<typename T> struct B
> >   {
> > -  A<(T)0> b; // { dg-error "constant|not a valid" }
> > -  A<T(0)> a; // { dg-error "constant|not a valid" }
> > +  A<(T)0> b; // { dg-error "constant" "" { target c++98_only } }
> > +  // { dg-error "not a valid" "" { target c++11 } 7 }
> > +
> > +  A<T(0)> a; // { dg-error "not a valid" "" { target c++11 } }
> >   };
> >
> >   B<const int&> b;
> >
>