Which part of the C++ standard allow to declare variable in parenthesis?

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[dcl.meaning] in the Standard says:

In a declaration T D where D has the form ( D1 ) the type of the contained declarator-id is the same as that of the contained declarator-id in the declaration T D1.

Parentheses do not alter the type of the embedded declarator-id, but they can alter the binding of complex declarators.

More simply, you can put parentheses around anything considered a “declarator” in the C++ grammar. (Loosely speaking, a declarator is a part of a declaration without the initial specifiers and types which contains one name.)

In your example, the identifier s is a declarator, so you’re allowed to put parentheses around it and the meaning doesn’t change.

The reason for this, as the second quoted sentence hints, is that it can be necessary when things get more complicated. One example:

int * a [10];     // a is an array of ten pointers to int.
int ( * b ) [10]; // b is a pointer to an array of ten ints.

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