Hi,

In both C++98 and C++11, the following code appears to be ill-formed:

   template<bool b>  struct S {};
   S<0>  s;

However, EDG, g++ and clang all accept it. The reason this is ill-formed is
that the non-type template argument is a converted constant expression of type
'bool' (see [temp.arg.nontype]p5), and the second conversion in the implicit
conversion sequence is a boolean conversion, which is not allowed in the
conversion for a converted constant expression (see [expr.const]p3).
Conversions in the other direction (from 'bool' to integer types) are
permitted here, since they're integral promotions.

Suggested resolution: In [expr.const]p3, add "boolean conversions from
integral or unscoped enumeration type other than narrowing conversions" to the
list of conversions allowed in a converted constant expression.

Thanks,
Richard


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Am 20.10.2011 00:24, schrieb Richard Smith:
>  Hi,
>
>  In both C++98 and C++11, the following code appears to be ill-formed:
>
>      template<bool b>    struct S {};
>      S<0>    s;
>
>  However, EDG, g++ and clang all accept it. The reason this is ill-formed is
>  that the non-type template argument is a converted constant expression of type
>  'bool' (see [temp.arg.nontype]p5), and the second conversion in the implicit
>  conversion sequence is a boolean conversion, which is not allowed in the
>  conversion for a converted constant expression (see [expr.const]p3).
>  Conversions in the other direction (from 'bool' to integer types) are
>  permitted here, since they're integral promotions.
>
>  Suggested resolution: In [expr.const]p3, add "boolean conversions from
>  integral or unscoped enumeration type other than narrowing conversions" to the
>  list of conversions allowed in a converted constant expression.

Forwarded to CWG.

It seems to me that the actual issue is, whether it is clear or not that
[conv.integral] p4 already implies what you are suggesting here.

Greetings from Bremen,

Daniel Kr   gler



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