Writable: Fix behavior with index signatures (#1470)

Co-authored-by: Som Shekhar Mukherjee <49264891+som-sm@users.noreply.github.com>

Author: MahinAnowar

source/writable.d.ts

@@ -61,8 +61,8 @@ export type Writable<BaseType, Keys extends keyof BaseType = keyof BaseType> =
 				: Simplify<
 					// Pick just the keys that are not writable from the base type.
 					Except<BaseType, Keys>
-					// Pick the keys that should be writable from the base type and make them writable by removing the `readonly` modifier from the key.
-					& {-readonly [KeyType in keyof Pick<BaseType, Keys>]: Pick<BaseType, Keys>[KeyType]}
+					// Make the specified keys writable.
+					& {-readonly [KeyType in keyof BaseType as KeyType extends Keys ? KeyType : never]: BaseType[KeyType]}
 				>;
 
 export {};

test-d/writable.ts

@@ -48,3 +48,14 @@ expectType<Set<string>>(variation9);
 // Test readonly map
 declare const variation10: Writable<ReadonlyMap<string, number>>;
 expectType<Map<string, number>>(variation10);
+
+// Only strip `readonly` without otherwise changing the structure, so an index signature is preserved (https://github.com/sindresorhus/type-fest/issues/717).
+declare const variation11: Writable<{readonly [key: string]: number}>;
+expectType<{[key: string]: number}>(variation11);
+
+// Preserve an index signature alongside named keys while stripping `readonly`.
+declare const variation12: Writable<{readonly [key: string]: number; readonly foo: number}>;
+expectType<{[key: string]: number; foo: number}>(variation12);
+
+declare const variation13: Writable<{readonly [key: string]: number; readonly foo: number}, 'foo'>;
+expectType<{readonly [key: string]: number; foo: number}>(variation13);