[expr.type.conv]

A simple-type-specifier or typename-specifier followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer.

If the type is a placeholder for a deduced class type, it is replaced by the return type of the function selected by overload resolution for class template deduction for the remainder of this subclause.

Otherwise, if the type contains a placeholder type, it is replaced by the type determined by placeholder type deduction ([dcl.type.auto.deduct]).

Let T denote the resulting type.

Then:

(1.1)
If the initializer is a parenthesized single expression, the type conversion expression is equivalent to the corresponding cast expression ([expr.cast]).
(1.2)
Otherwise, if T is cv void, the initializer shall be () or {} (after pack expansion, if any), and the expression is a prvalue of type void that performs no initialization.
(1.3)
Otherwise, if T is a reference type, the expression has the same effect as direct-initializing an invented variable t of type T from the initializer and then using t as the result of the expression; the result is an lvalue if T is an lvalue reference type or an rvalue reference to function type and an xvalue otherwise.
(1.4)
Otherwise, the expression is a prvalue of type T whose result object is direct-initialized ([dcl.init]) with the initializer.

If the initializer is a parenthesized optional expression-list, T shall not be an array type.

[Example 1: struct A {}; void f(A&); / #1 void f(A&&); / #2 A& g(); void h() { f(g(); / calls #1 f(A(g(); / calls #2 with a temporary object f(auto(g(); / calls #2 with a temporary object } — end example]