[Bug c++/6023] Unhelpful error message with forgotten "template"

["reichelt at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=6023



------- Additional Comments From reichelt at gcc dot gnu dot org  2003-08-05 23:08 -------
Actually the original testcase misses the template keyword *three* times:

As Nathan already explained in line 34 and 35 (right after "->"):

    tmp = h->calc<VEC>(x);
    tmp = h->do_calc<VEC>(x);

And it's missing in line 19 (right before "MyTraits"):

  T do_calc(const typename Otto<T>::MyTraits<T, VEC>::T2& x)


Let's have a look at each of the missing templates:

1) As pointed out by Nathan, line 34 is accepted by gcc in error.
   This can be demonstrated with the following code snippet,
   which compiles, but shouldn't (i.e. we have an accepts-invalid bug
   - it affects all versions since gcc 2.95.x):

   -------------------------------------------
   template <typename> struct A
   {
       template <typename> void foo();
   };

   template <typename T> struct B
   {
       template <int> void foo()
       {
           A<T>* p;
           p->foo<int>();
       }
   };
   -------------------------------------------

   If one removes "template <int>" the compiler issues an error which
   proves Nathan's analysis.

2) Line 35: Nothing fancy here - we get a sensible error message
   (which might not be optimal, but it's not misleading either).

3) Line 19: This is another accepts-invalid bug for gcc 2.95.x - 3.3.x,
   but that is fixed on mainline, i.e. we get an error message.

   But as pointed out by Nathanael, the error message is a mess.
   The good news is that the duplicate "expected" got fixed.
   The bad news, however, is that we now even get an ICE after the error
   message:

     bug.cc:19: error: expected primary-expression
     bug.cc:19: error: expected primary-expression
     bug.cc:19: error: `::T2' has not been declared
     bug.cc:19: error: `x' was not declared in this scope
     bug.cc:20: error: ISO C++ forbids initialization of member `do_calc'
     bug.cc:20: error: making `do_calc' static
     bug.cc:20: error: expected `;'
     bug.cc:20: error: template declaration of `T Hugo<T>::do_calc'
     bug.cc: In member function `T Hugo<T>::calc(VEC)':
     bug.cc:14: internal compiler error: in cp_parser_template_id, at
cp/parser.c:
        7516
     Please submit a full bug report, [etc.]

   This is rewarded with three keywords: "ice-on-invalid-code",
   "error-recovery" (since the ICE happens after a error message),
   and "diagnostic" (since the error message is not very helpful -
   we are missing just one "template", remember).

   Here's a shorter testcase that also demonstrates the problem:

   -------------------------------------------
   template <typename> struct A;

   template <typename T> struct B
   {
       template <int> void foo(const typename A<T>::X<T,T>::Y&);
       template <typename U> void bar(U x) { foo<0>(x); }
   };
   -------------------------------------------

   The corresponding error message is:

     bug.cc:5: error: expected primary-expression
     bug.cc:5: error: expected primary-expression
     bug.cc:5: error: `::Y' has not been declared
     bug.cc:5: error: expected primary-expression
     bug.cc:5: error: variable or field `foo' declared void
     bug.cc:5: error: ISO C++ forbids initialization of member `foo'
     bug.cc:5: error: making `foo' static
     bug.cc:5: error: ISO C++ forbids in-class initialization of non-const
static 
        member `foo'
     bug.cc:5: error: template declaration of `int B<T>::foo'
     bug.cc: In member function `void B<T>::bar(U)':
     bug.cc:6: internal compiler error: in cp_parser_template_id, at
cp/parser.c:
        7516
     Please submit a full bug report, [etc.]

  By varying the declaration of foo (i.e. removing "&", "const", or "::Y"
  one can change the error message a little.


Since all this is a little too much for one PR, I'm going to spin off two
smaller ones for the accepts-invalid in 1) and the mainline problems in 3).

The accepts-invalid stuff in 3) for gcc 2.95.x - 3.3.x will not be fixed on
the 3.3 branch (since it's not a regression and already fixed in mainline).