[cfe-dev] Weird diagnostic about non-type template argument

[Abramo Bagnara]

Il 16/08/2010 10:22, Chandler Carruth ha scritto:
> On Mon, Aug 16, 2010 at 12:31 AM, Abramo Bagnara
> <abramo.bagnara at gmail.com <mailto:abramo.bagnara at gmail.com>> wrote:
> 
> 
>     The following typescript show the issue:
> 
>     $ cat t.cc
> 
>     template <int *ptr>
>     struct S1 { };
> 
>     template <bool b>
>     struct S2 { };
> 
>     int v1;
> 
>     S1<(&v1)> x;
>     S2<(2>2)> y;
>     $ clang -c t.cc
>     t.cc:10:4: error: non-type template argument cannot be surrounded by
>     parentheses
>     S1<(&v1)> x;
>       ^~~~~
>     1 error generated.
> 
>     My notes:
> 
>     1) in my opinion the diagnostic is due to a needless too strong
>     interpretation of the standard:
> 
>     - comeau and intel does not mark this as an error
> 
>     - gcc seems to mark this as an error due to some bug (the diagnostic  is
>     very weird)
> 
> 
> The standard seems really really clear here -- the argument is *not* an
> expression, it's an address. You're only provided one mechanism for
> writing an address in this context: '& id-expression', with the '&'
> being optional in certain cases. You can't even write '0' here. I think
> it's actually useful to preclude parentheses as they make this look like
> an expression, which it simply is not. You can't write '(&foo +
> sizeof(int))' to advance past one element of an int array either.
> 

Hmmm... perhaps not so clear...

If you look at standard grammar, you read this:

template-argument:
  constant-expression
  type-id
  id-expression

Of course S1<v1> is in the 'id-expression' case, but both S1<&v1> and
S1<(&v1)> are in 'constant-expression' case.

Don't you think that if the intention was to inhibit parenthesis use the
standard would have been written as:

template-argument:
  constant-expression
  type-id
  '&'opt id-expression

?