16 Library introduction [library]

16.5 Library-wide requirements [requirements]

16.5.4 Constraints on programs [constraints]

16.5.4.11 Semantic requirements [res.on.requirements]

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.