Is it possible to declare constexpr class in a header and define it in a separate .cpp file?

If a constexpr function is not defined inside the header, the compiler can not see the definition of the constexpr functions while compiling all the other source files.

Obviously, if it can't see the definition of the functions, it can't perform the steps necessary to calculate them at compile-time. Thus all constexpr functions must be defined everywhere they are used.

Thanks @IgorTandetnik:
[dcl.constexpr] §7.1.5/2

constexpr functions and constexpr constructors are implicitly inline.

[basic.def.odr] §3.2/4

An inline function shall be defined in every translation unit in which it is odr-used.

What you're asking can be accomplished, with a significant limitation: the constexpr function can then only be called from inside the translation unit (i.e. source file) where it is defined. This is not appropriate for the example you gave, since the constructor is meant to be part of the class' public interface. However it can be very useful in other cases, e.g. to define private methods as constexpr and then use their return values in expressions that need to be known at compile time, e.g. template instantiations, switch statement labels, etc.