[ Resolved by paper P2308R1 (Template parameter initialization), adopted in November, 2023. ]

Since non-type template parameters can now have class types, it would seem to make sense to allow a braced-init-list as a template-argument, but the grammar does not permit it.

See also issues 2049 and 2459.

Possible resolution:

The resolution also addresses issue 2049.

1. Change in 7.3.1 [conv.general] paragraph 3 as follows:

   An expression or braced-init-list E can be implicitly converted to a type T if and only if the declaration T t = E; is well-formed, for some invented temporary variable t (9.4 [dcl.init]).

2. Change in 7.7 [expr.const] paragraph 12 as follows:

   A converted constant expression of type T is an expression or braced-init-list, implicitly converted to type T, where the converted expression is a constant expression and the implicit conversion sequence contains only (12.1 [over.pre])

   • user-defined conversions,
   • ...

3. Change in 13.3 [temp.names] paragraph 1 as follows:

   template-argument:
     constant-expression
     type-id
     id-expression
     braced-init-list
   

4. Change in 13.4.2 [temp.arg.type] paragraph 4 as follows:

     template<auto n> struct B { /* ... */ };
     B<5> b1;        // OK, template parameter type is int
     B<'a'> b2;      // OK, template parameter type is char
     B<2.5> b3;      // OK, template parameter type is double
     B<void(0)> b4;  // error: template parameter type cannot be void
     template<int i> struct C { /* ... */ };
     C<{ 42 }> c1; // OK