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Error with this['prop'] type interacting with || operator #15376

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@adufilie

Description

@adufilie

TypeScript Version: 2.2.1, 2.3.0

Code

// A *self-contained* demonstration of the problem follows...
// Compiled with strictNullChecks=true
class Foo {
	test1(foo:string|undefined):string {
		return foo || 'default'; // no error
	}
	test2(params:{foo: string|undefined}):string {
		return params.foo || 'default'; // no error
	}
	test3(params:Pick<{foo: string|undefined}, 'foo'>):string {
		return params.foo || 'default'; // no error
	}

	foo:string|undefined;

	test4(params:Pick<this, 'foo'>):string {
	    return params.foo || 'default'; // error: undefined not assignable to string
	}
	test5(foo:this['foo']):string {
	    return foo || 'default'; // error: undefined not assignable to string
	}
	test6(foo:this['foo']):string {
	    return (foo as string|undefined) || 'default'; // no error
	}
}

Expected behavior:
Expected no errors because the type of the expression foo || 'default' should be string in all cases.

Actual behavior:
When a type is derived indrectly from this it seems to break the type inference of the || operator.

Error:(17, 6) TS2322:Type 'this["foo"]' is not assignable to type 'string'.
  Type 'string | undefined' is not assignable to type 'string'.
    Type 'undefined' is not assignable to type 'string'.
Error:(20, 6) TS2322:Type 'this["foo"]' is not assignable to type 'string'.
  Type 'string | undefined' is not assignable to type 'string'.
    Type 'undefined' is not assignable to type 'string'.

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