Sequence concept incorrectly fails for basic_string

[Joel Lathrop <jal6806@…>]

The Sequence concept fails when tested against basic_string (e.g. BOOST_CONCEPT_ASSERT((Sequence<T>)); ) because an extra constructor test is included. This test is for X a(n) where n is the size of the sequence to be created. However, section 23.1.1 table 67 of the C++ standard does not include this test in the definition of the Sequence concept. Because this test is in place, basic_string is rejected by the current implementation of the Sequence concept, since basic_string does not have this constructor. However, section 21.3 paragraph 2 of the C++ standard states, "The class template basic_string conforms to the requirements of a Sequence, as specified in (23.1.1)."

The fix for this should be as simple as removing the test for a constructor of the form X a(n) in the Sequence concept implementation.

[acharles]

Proposed fix: ​https://github.com/boostorg/concept_check/pull/2