http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60715
Bug ID: 60715
Summary: Narrowing conversions not caught in non-type template
parameters
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: filip.roseen at gmail dot com
Created attachment 32493
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32493&action=edit
testcase.cpp
template< char> void f () {}
template<unsigned int> void g () {}
template<unsigned int> struct A {};
int main () {
f<1024> (); // ill-formed, `char { 1024 }` is a narrowing conversion,
// see [dcl.init.list]p7
g<-123> (); // ill-formed, `unsigned int { -123 }` is a narrowing
// conversion, see [dcl.init.list]p7
A<-123> a; // ill-formed, see previous comment
}
/* In the above we assume `char` is 8 bits */
-------------------------------------------------------------------------
[temp.arg.nontype]p1;
> A template-argument for a non-type, non-template template
> parameeter shall be one of:
>
> - for a non-type template-aprameter of integral or enumeration
> type, a converted constant expression (5.19) of the type of
> the template-parameter
>
> <snip />
[expr.const]p3;
> A converted constant expression of type `T` is an expression,
> implicitly converted to a prvalue of type `T`, where the converted
> expression is a core constant expression and the implicit
> conversion sequence contains only user-defined conversions,
> lvalue-to-rvalue conversions (4.1), integral promotions (4.5), and
> integral conversions (4.7) other than narrowing conversions
> (8.5.4).
-------------------------------------------------------------------------
[ Note: `gcc` accepts testcase.cpp, while `clang` (correctly) issues the
relevant diagnostics. ]
[ Note: `gcc` handles overflow conversions for non-type template
parameters when used in a class template; ie. it shows a diagnostic
for `template<char> struct B {};` instantiated as `B<1024> {};` ]
