Make 'Omit' a distributive conditional type to support union prop types. Fixes #39300. (#39323)

types/styled-components/index.d.ts

@@ -346,7 +346,7 @@ export type ThemedCssFunction<T extends object> = BaseThemedCssFunction<
 >;
 
 // Helper type operators
-type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;
+type Omit<T, K extends keyof T> = T extends any ? Pick<T, Exclude<keyof T, K>> : never;
 type WithOptionalTheme<P extends { theme?: T }, T> = Omit<P, "theme"> & {
     theme?: T;
 };

types/styled-components/test/index.tsx

@@ -1052,8 +1052,7 @@ function unionTest() {
         font-size: ${props => props.kind === 'book' ? 16 : 14}
     `;
 
-    // undesired, fix was reverted because of https://github.com/Microsoft/TypeScript/issues/30663
-    <StyledReadable kind="book" author="Hejlsberg" />; // $ExpectError
+    <StyledReadable kind="book" author="Hejlsberg" />;
     <StyledReadable kind="magazine" author="Hejlsberg" />; // $ExpectError
 }