Cannot mock callable interface

[vincentvanderweele]

Describe the bug
Some third-party libraries (typically js-native with ts types added later) provide callable interfaces. It seems that methods on those callable interfaces cannot be mocked with this library (or I haven't figured out how).

To Reproduce

interface Library {
  subSection: Subsection;
  coreMethod: () => string;
}

interface Subsection {
  (): string;
  subMethod: () => string;
}

const lib: SubstituteOf<Library> = Substitute.for<Library>();

// I can mock coreMethod()
lib.coreMethod().returns('hello');

// I can mock subSection()
lib.subSection().returns('hello');

// I can't mock subSection:

// Property 'returns' does not exist on type 'NoArgumentFunctionSubstitute<string> & Subsection'.
lib.subSection.returns({ subMethod: () => 'hello' });

// Property 'returns' does not exist on type 'string'.
lib.subSection.subMethod().returns('hello');

Expected behavior
I would like to be able subMethod in some way. I'm okay with a workaround but not sure what that should be.

Environment

• Typescript version: 4.1.2
• substitute.js version: 1.179.0
• Node version: v14.12.0