Odd behavior with partial and pick in TypeScript

partial_and_pick.ts

interface BaseObj_t
{
	baseMember: string,
	baseNumber: number
}

interface DerivedObj_t extends BaseObj_t
{
	derivedMember: string
}

declare function PickBase<T, K extends keyof T>( obj: T, pick: Pick<T, K> ): void
declare function PartialBase<T>( obj: T, partial: Partial<T> ) : void;

function SimplePartial( obj: BaseObj_t )
{
	let s: string = obj.baseMember;
	PickBase( obj, { baseMember: 'foo' } );
	PartialBase( obj, { baseMember: 'foo' } );
}

function DerivedPartial( obj: DerivedObj_t )
{
	let s: string = obj.baseMember;
	PickBase( obj, { baseMember: 'foo' } );
	PartialBase( obj, { baseMember: 'foo' } );

	let asBaseObj: BaseObj_t = obj;
	PartialBase( asBaseObj, { baseMember: 'foo' } );

	let baseObj: BaseObj_t = { baseMember: 'foo', baseNumber: 12 };
	PartialBase( obj, baseObj );
}

function DerivedPartialGeneric<T extends BaseObj_t>( obj: T )
{
	let s: string = obj.baseMember;
	PickBase( obj, { baseMember: 'foo' } );
	PartialBase( obj, { baseMember: 'foo' } );	// TS2345: Argument of type '{ baseMember: "foo"; }' is not assignable to parameter of type 'Partial<T>'.

	let baseObj: BaseObj_t = { baseMember: 'foo', baseNumber: 12 };
	PartialBase( obj, baseObj );	// TS2345: Argument of type 'BaseObj_t' is not assignable to parameter of type 'Partial<T>'

	// this doesn't need a cast, and works fine
	let asBaseObj: BaseObj_t = obj;
	PartialBase( asBaseObj, { baseMember: 'foo' } );
	
	// same idea, no error
	SimplePartial( obj );
}

function IntersectionPartial<T>( obj: T & BaseObj_t )
{
	let s: string = obj.baseMember;
	PickBase( obj, { baseMember: 'foo' } );
	PartialBase( obj, { baseMember: 'foo' } );	// TS2345: Argument of type '{ baseMember: "foo"; }' is not assignable to parameter of type 'Partial<T & BaseObj_t>'.

	let baseObj: BaseObj_t = { baseMember: 'foo', baseNumber: 12 };
	PartialBase( obj, baseObj );	// TS2345: Argument of type 'BaseObj_t' is not assignable to parameter of type 'Partial<T & BaseObj_t>'.

	// this doesn't need a cast, and works fine
	let asBaseObj: BaseObj_t = obj;
	PartialBase( asBaseObj, { baseMember: 'foo' } );
	
	// same idea, no error
	SimplePartial( obj );
}