[dcl.fct.spec]

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

At most one explicit-specifier and at most one virtual keyword shall appear in a decl-specifier-seq.

function-specifier:
virtual
explicit-specifier

explicit-specifier:
explicit ( constant-expression )
explicit

The virtual specifier shall be used only in the initial declaration of a non-static 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.

[Example 1: struct S { explicit(sizeof(char[2]) S(char); / error: narrowing conversion of value 2 to type bool explicit(sizeof(char) S(bool); / OK, conversion of value 1 to type bool is non-narrowing }; — end example]

9 Declarations [dcl]

9.2 Specifiers [dcl.spec]

9.2.3 Function specifiers [dcl.fct.spec]