I couldn't find any documentation on this breaking change between TS 2.3.4 and TS 2.4.0.

I'm not sure whether this is a bug or simply an undocumented change. I suspect the latter--in which case my suggestion is to add documentation regarding that change.

TypeScript Version: >= 2.4.0

Ran tsc with flags --experimentalDecorators --emitDecoratorMetadata.

Using reflect-metadata v0.1.10

Code

import 'reflect-metadata'

const dec = (obj: {}, prop: string) => undefined

class Foo {
  @dec public foo: string | null // seems to happen with any complex type
  @dec public bar: string
}

console.log(Reflect.getMetadata('design:type', Foo.prototype, 'foo'))
console.log(Reflect.getMetadata('design:type', Foo.prototype, 'bar'))

Expected behavior:

output in 2.3.4:

[Function: String]
[Function: String]

Actual behavior:

output in 2.4.0:

[Function: Object]
[Function: String]