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

[Jason Merrill <jason@redhat.com>]

On 12/03/2015 05:08 PM, David Malcolm wrote:
> 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?

Ah, I see, the error is coming from tsubst_copy_and_build, not 
cp_parser_non_integral_constant_expression.  So indeed this is an effect 
of the canonicalization of template instances, and we aren't going to 
fix it in the context of this patchset.  But this is still a bug, so I'd 
rather have an xfail and a PR than change the expected output.

Jason

>>> 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;
>>>
>>
>
>