Keywords: any, null, undefined, enum, assign, assignable, assignability, permissive, strict

From the spec:

S is assignable to a type T, and T is assignable from S, if S has no excess properties with respect to T (3.11.5) and one of the following is true:

• S and T are identical types.
• S or T is the Any type.
• S is the Undefined type.
• S is the Null type and T is not the Undefined type.
• S or T is an enum type and the other is the primitive type Number.

Why is:

• S assignable to T, if S is any and T is not any?
• S assignable to T, if S is a number and T is an enum? Because enum <: number, T should be assignable to S, but why is the reverse true? And why is this not the case for string enums?
• S assignable to T, if S is undefined? (this was a Playground bug where strictNullChecks wasn't actually applied)
• S assignable to T, if S is null and T is not undefined?

Have you guys considered a flag to opt out of some or all of these behaviors? Or are there real uses cases for them besides interop with untyped JS (or TS in non-strict mode)? I read through the spec and searched issues, but had trouble finding a discussion of why TS made these design decisions.