Kotlin: how to mock a static Companion method using MockK

Introduction

How do you mock a static companion method in Kotlin using MockK?

I wanted to mock this (static) method in a companion object in Kotlin in class MyClass:

  companion object {
    fun isNegativeAndFromSavingsAccount(amount: BigDecimal, accountType: accountType) = amount < BigDecimal.ZERO && accountType == AccountType.SAVINGS
  }

 Trying it with a regular 'every' like this doesn't work:

      every { Declaration.isNegativeAndFromSavingsAccount(any(), any()) } returns false

Note it does compile fine!
But when running the test, you'll get this error:

    io.mockk.MockKException: Failed matching mocking signature for left matchers: [any(), any()]
    at io.mockk.impl.recording.SignatureMatcherDetector.detect(SignatureMatcherDetector.kt:97)

Setup:

• Kotlin 1.9
• MockK 1.13.7
  

Solution

This is the way it does work:

import io.mockk.mockkObject

    mockkObject(Declaration.Companion) {
      every { MyClass.isNegativeAndFromSavingsAccount(any(), any()) } returns false
    }

Note this did not work, got the same error:

    mockkObject(Declaration::class)
    every { MyClass.isNegativeAndFromSavingsAccount(any(), any()) } returns false

I found several posts, but none of them gave a clear answer and/or were using some older version of MockK. E.g: https://github.com/mockk/mockk/issues/61
and this StackOverflow post.

Some more examples and variants of solutions can be found here, e.g when using @JvmStatic.