keyof erasure in generic union types #18990
Description
TypeScript Version: 2.2.2, 2.4.0
Code
interface A { a: number; } interface B extends A { b: number } export type IFieldConfig<TData> = { name: keyof TData }; export type TFieldConfigMap<TData> = {[P in keyof TData]: IFieldConfig<TData> & { name: P } }; let AFields: TFieldConfigMap<A> = { a: { name: 'a' } }; let BFieldsFailed: TFieldConfigMap<B> = Object.assign({}, AFields, { b: { name: 'b' } }); let BFieldsFailedToo: TFieldConfigMap<B> = Object.assign({}, AFields, { b: { name: <keyof B>'b' } }); let BFieldsWorkaround: TFieldConfigMap<B> = Object.assign({}, AFields, { b: { name: <keyof Pick<B, 'b'>>'b' } }); const BMap: {[P in keyof B]: P} = { a: 'a', b: 'b' } let BFieldsWorkaround2: TFieldConfigMap<B> = Object.assign({}, AFields, { b: { name: BMap.b } });Expected behavior:
compile flawlessly
Actual behavior:
failure to deduce types of both BFieldsFailed
Type 'TFieldConfigMap & { b: { name: string; }; }' is not assignable to type 'TFieldConfigMap'. Types of property 'b' are incompatible.
Type '{ name: string; }' is not assignable to type 'IFieldConfig & { name: "b"; }'.
Type '{ name: string; }' is not assignable to type 'IFieldConfig'.
Types of property 'name' are incompatible.
Type 'string' is not assignable to type '"a" | "b"'.
and BFieldsFailedToo
Type 'TFieldConfigMap & { b: { name: "a" | "b"; }; }' is not assignable to type 'TFieldConfigMap'.
Types of property 'b' are incompatible.
Type '{ name: "a" | "b"; }' is not assignable to type 'IFieldConfig & { name: "b"; }'.
Type '{ name: "a" | "b"; }' is not assignable to type '{ name: "b"; }'.
Types of property 'name' are incompatible.
Type '"a" | "b"' is not assignable to type '"b"'.
Type '"a"' is not assignable to type '"b"'.
I'm not sure if it is a bug in a second case.