[temp.arg.nontype] Clarify the grammar meaning of template non-type argument

Except for template non-type arguments, other kinds of template arguments are less or more to be given the corresponding grammar meaning. A template type argument is type-id, as per

A template-argument for a template-parameter which is a type shall be a type-id.

A template template argument is expresses as id-expression, as per

A template-argument for a template template-parameter shall be the name of a class template or an alias template, expressed as id-expression.

We just lack to specify the grammar meaning for non-type arguments. Maybe, we should give a similar rule that:

A template-argument for a non-type template-parameter shall be a converted constant expression ([expr.const]) of the type of the template-parameter, expressed as constant-expression.

which conforms to

template-argument

• constant-expression
• type-id
• id-expression

The modification can also clarify the example in [expr.const] p14 by the way.

X<std::is_constant_evaluated()> x;                      // type X<true>

The actual reason is that std::is_constant_evaluated() is evaluated in the context of constant-expression

Start by reading the [temp.arg.nontype] wording and compare it with the grammar and discussion in [expr.const]. Clarify the grammar meaning of non-type template arguments, including whether the proposed constant-expression wording fits the existing template-argument alternatives and the is_constant_evaluated() example.