Why Does `decltype((...))` Return a Reference While `decltype(...)` Doesn\'t?

Understanding the Significance of Parentheses in decltype((...))

The use of double parentheses in decltype((...)) has a subtle but important impact on the type deduced by the decltype operator, as evidenced in the example from the C 17 Function Definition Concurrency (FCD).

When applied to an unparenthesized identifier (id-expression) or a class member access expression, decltype returns the type of the referenced entity. However, when the expression is an lvalue (a variable or expression referring to a memory location), decltype prefixes an additional reference (&) before the entity's type.

In the FCD example:

• decltype(a->x) references a member variable (double x) of a class instance (*a) and thus returns the base type of the member: double.
• decltype((a->x)) wraps the same member access expression within parentheses, treating it as an lvalue. As a result, decltype returns the type of the lvalue, which is a reference to the double type: const double&.

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