A sequence Args of template arguments is said to
model a concept C
if Args
satisfies C ([temp.constr.decl]) and
meets all semantic requirements (if any)
given in the specification of C.
If the validity or meaning of a program
depends on whether a sequence of template arguments models a concept, and
the concept is satisfied but not modeled,
the program is ill-formed, no diagnostic required.
If the semantic requirements of a declaration's
constraints ([structure.requirements]) are not modeled at the point of use,
the program is ill-formed, no diagnostic required.