Is the void() in decltype(void()) an expression or is it a function type?

Using a hyperlinked C++ grammar, the parsing of decltype(void()) is:

decltype( expression )
decltype( assignment-expression )
decltype( conditional-expression )

… lots of steps involving order of operations go here …

decltype( postfix-expression )
decltype( simple-type-specifier ( expression-listopt ) )
decltype( void() )

So void() is a kind of expression here, in particular a postfix-expression.

Specifically, quoting section 5.2.3 [expr.type.conf] paragraph 2 of the 2011 ISO C++ standard:

The expression T(), where T is a simple-type-specifier or
typename-specifier for a non-array complete object type or the (possibly cv-qualified) void type, creates a prvalue of the
specified type, which is value-initialized (8.5; no initialization is
done for the void() case).

So void() is an expression of type void, just as int() is an expression of type int (with value 0). Clearly a void expression has no value, but here it’s the operand of decltype, so it’s not evaluated. decltype refers only to its operand’s type, not its value.

decltype(void()) is simply a verbose way of referring to the type void.