9 Declarations [dcl.dcl]

A function-specifier can be used only in a function declaration.

The virtual specifier shall be used only in the initial declaration of a non-static class member function; see [class.virtual].

An explicit-specifier shall be used only in the declaration of a constructor or conversion function within its class definition; see [class.conv.ctor] and [class.conv.fct].

In an explicit-specifier, the constant-expression, if supplied, shall be a contextually converted constant expression of type bool ([expr.const]).

The explicit-specifier explicit without a constant-expression is equivalent to the explicit-specifier explicit(true).

If the constant expression evaluates to true, the function is explicit.

Otherwise, the function is not explicit.

A ( token that follows explicit is parsed as part of the explicit-specifier.