compiler option "strictNullChecks" working unexpected for Union Type

[YuJianrong]

TypeScript Version: 2.0.10 / nightly (2.2.0-dev.201xxxxx)

Code

let x: {[v: string]: number} | {y?: number} = {a : null};
// Both the following assigns raise compile error
// let x: {[v: string]: number} = {a : null};
// let x: {y?: number} = {a : null};

Expected behavior:
Compile error raised when "strictNullChecks" is set to true.

Actual behavior:
No compile error. Please note compile error will be raised when union type is not used.

[aluanhaddad]

That seems correct. One of the constituents of the union is {} which is a valid assignment target.

[YuJianrong]

@aluanhaddad , but the following code is not working:

let x: {y?: number} = {a : null};

{y?:number} should be same as {}, but I get the following compile error:

message: 'Type '{ a: null; }' is not assignable to type '{ y?: number | undefined; }'.
  Object literal may only specify known properties, and 'a' does not exist in type '{ y?: number | undefined; }'.'
source: 'ts'

[mhegazy]

seems to be the same request in #12745