[Bug c++/54948] New: template unnecessarily displayed as "A< template-parameter-1-1 >" not "A<T>"

[Bug c++/54948] New: template unnecessarily displayed as "A< template-parameter-1-1 >" not "A<T>"

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54948

             Bug #: 54948
           Summary: template unnecessarily displayed as "A<
                    template-parameter-1-1 >" not "A<T>"
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: redi@gcc.gnu.org


template<typename> struct A;

template<typename T> struct A { A(Y); };

A<int> a;

This invalid code produces:

tp.cc: In instantiation of 'struct A<int>':
tp.cc:5:8:   required from here
tp.cc:3:36: error: 'A< <template-parameter-1-1> >::Y' has incomplete type
 template<typename T> struct A { A(Y); };
                                    ^
tp.cc:3:29: error: declaration of 'struct A<int>'
 template<typename T> struct A { A(Y); };
                             ^

This is correct, but "A< <template-parameter-1-1> >::Y" is rather ugly.

At the point of the error the parameter has a name, so "A<T>::Y" could be shown
instead.

[Bug c++/54948] template unnecessarily displayed as "A< template-parameter-1-1 >" not "A<T>"

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54948

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #2 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2013-04-20 12:03:29 UTC ---
There are several "bugs" in this case. The first one is that we consider that
the type of:

template<typename> struct A

is "struct A< <template-parameter-1-1> >". It would be nicer to print something
like "struct A<typename>" or "struct A<>". Clang never needs to print made up
names.

The second bug is that I think we record T when parsing, but later we drop it.
I wasn't able to find when we drop it. There is a lot going on between the
moment we parse T and the moment we parse {, so it is rather difficult to
figure out. Stepping in the parser is rather frustrating.

I am afraid that the only person that may know what is going or where to look
is Jason. There is lookup_template_class, but by the time we call it, we
already dropped T (or so it seems to me, maybe it is recorded somewhere else).
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54948

--- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2013-04-20 12:06:10 UTC ---
BTW, the parser may benefit by marking some functions with "skip" to help
debugging. All the cp_lexer_peek_* are useless to step into.
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Subject: [Bug c++/57012] New: pretty-printer does not handle well template parameter packs
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW012

             Bug #: 57012
           Summary: pretty-printer does not handle well template parameter
                    packs
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: manu@gcc.gnu.org


/home/manuel/test3/src/gcc/testsuite/g++.dg/cpp0x/pr31432.C:2:40: error:
parameter pack '<template-parameter-1-1>' must be at the end of the template
parameter list
/home/manuel/test3/src/gcc/testsuite/g++.dg/cpp0x/pr31432.C:7:11: error:
type/value mismatch at argument 1 in template parameter list for
'template<<declaration error>, class> struct A'
/home/manuel/test3/src/gcc/testsuite/g++.dg/cpp0x/pr31432.C:7:11: error:
expected a constant of type '<type error>', got 'int'
/home/manuel/test3/src/gcc/testsuite/g++.dg/cpp0x/pr31432.C:7:14: error:
invalid type in declaration before ';' token
/home/manuel/test3/src/gcc/testsuite/g++.dg/cpp0x/pr31432.C:8:11: error:
type/value mismatch at argument 1 in template parameter list for
'template<<declaration error>, class> struct A'
/home/manuel/test3/src/gcc/testsuite/g++.dg/cpp0x/pr31432.C:8:11: error:
expected a constant of type '<type error>', got 'char'
/home/manuel/test3/src/gcc/testsuite/g++.dg/cpp0x/pr31432.C:8:14: error:
invalid type in declaration before ';' token

Clang++ by comparison:

/home/manuel/test3/src/gcc/testsuite/g++.dg/cpp0x/pr31432.C:2:10: error:
template parameter pack must be the last template parameter
template<typename..., typename> struct A // { dg-error "parameter pack" }
         ^

[Bug c++/54948] template unnecessarily displayed as "A< template-parameter-1-1 >" not "A<T>"

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54948

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=104094

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This continues to be annoying. Libstdc++ has loads of forward declarations of
templates, e.g. in <type_traits>:

  // Forward declarations
  template<typename>
    struct is_reference;
  template<typename>
    struct is_function;
  template<typename>
    struct is_void;
  template<typename>
    struct remove_cv;
  template<typename>
    struct is_const;

Which means we then get diagnostics containing:

std::is_same<_U1, typename std::remove_cv< <template-parameter-1-1> >::type&>

I agree with Manu that this could just show "typename" here. We don't refer to
the template-parameter-1-1 type again in the diagnostic, so why do we care
about uniquely identifying it as 1-1?

But why is it even using the name from the remove_cv declaration? That should
not be appearing in the *use* of remove_cv here. I've filed that as PR 104094.