Does the default constructor of std::pair set basic types (int, etc) to zero?

Yes, that guarantee holds true. Quoting the C++11 standard, §20.3.2/2-3:

        constexpr pair();

2 Requires: is_default_constructible<first_type>::value is true and is_default_constructible<second_type>::value is true.
3 Effects: Value-initializes first and second.

And §8.5/7:

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type with a user-provided constructor, then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
• if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T‘s implicitly-declared default constructor is non-trivial, that constructor is called.
• if T is an array type, then each element is value-initialized;
• otherwise, the object is zero-initialized.

And lastly, §8.5/5:

To zero-initialize an object or reference of type T means:

• if T is a scalar type, the object is set to the value 0 (zero), taken as an integral constant expression, converted to T;
• if T is a (possibly cv-qualified) non-union class type, each non-static data member and each base-class subobject is zero-initialized and padding is initialized to zero bits;
• if T is a (possibly cv-qualified) union type, the object’s first non-static named data member is zero-initialized and padding is initialized to zero bits;
• if T is an array type, each element is zero-initialized;
• if T is a reference type, no initialization is performed.