type BaseContext<TConfig> = {
  foo: number;
  config: TConfig;
};

type FullContext<TConfig, TContext> = TContext & BaseContext<TConfig>;

type ExtractAdditionalContext<TContext extends FullContext<any, any>> =
  TContext extends FullContext<any, infer TAdditionalContext>
    ? TAdditionalContext
    : never;

type Config = Record<string, string>;
type AdditionalContext = { customValue: string };

type Context = FullContext<Config, AdditionalContext>;
type ExtractedAdditionalContext = ExtractAdditionalContext<Context>;

declare const additionalContext: AdditionalContext;
const test1: ExtractedAdditionalContext = additionalContext;