I believe this is causing another issue now:

#1475

When I try to add a || in JvmMockFactoryHelper line 181 to accept nullable value classes then it passes the test cases mentioned in #1475

/ For nullable value classes, we should NOT call boxedClass as they are not inlined in JVM bytecode
        // This prevents infinite loops in value class type resolution (issue #1103)
        val returnType: KClass<*> = if (returnTypeClass.isValue || !returnTypeNullable) returnTypeClass.boxedClass else returnTypeClass

Otherwise the current code in master will cause verification to fail this test case:

@Test
    fun `when mock with optional value then test passes`() {
        val mocked: Mocked = mockk()
        every { mocked.something(any()) } returns Id("bar")

        val tested = Tested(mocked)

        tested.test()

        verify { mocked.something(Id("foo")) }
    }

Where:

@JvmInline
        value class Id(val id: String)

        interface Mocked {
            fun something(data: Id): Id?
        }

        class Tested(
            private val mocked: Mocked
        ) {
            fun test() {
                val result = mocked.something(Id("foo"))
            }
        }

Perhaps we just need to use the innermostBoxedClass() extension function that was introduced, within JvmMockFactoryHelper then it wouldn't matter if the value class was nullable or not, @sdetilly what are your thoughts on this?

I was able to reproduce the issue. The problem occurs during verification with exact value matching when the return type is nullable.

I've implemented a fix and will a open a new PR:

• Made innermostBoxedClass public in ValueClassSupport.kt
• Updated JvmMockFactoryHelper.kt to use innermostBoxedClass() for all cases (nullable and non-nullable)
• Added your test case to ValueClassTest.kt and it now passes (So I don't think I broke anything)

The innermostBoxedClass approach is better because it has built-in recursion protection while still properly handling both nullable and nested value classes. This resolves issues #1103, #1308, and #1475 together.

All value class tests pass with this fix. Thank you for the issue report and suggested solution 🎉