Fix eq() matcher NPE with nullable value class and null value

[KonaEspresso94]

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Summary

This PR fixes the NPE issue in eq() matcher when used with nullable value class and null value.

Fixes #577

Problem

After #545 added value class support to eq(), using eq(null) with a nullable value class type throws NullPointerException:

@JvmInline
value class MyValueClass(val value: Long)

interface Foo {
    fun bar(x: MyValueClass?)
}

// This throws NPE in 6.2.0+
verify(foo).bar(eq(null))

Root Cause

In eqValueClass(), the code accesses value::class before checking if value is null:

inline fun <reified T> eqValueClass(value: T): T {
    require(value::class.isValue) { ... }  // NPE when value is null
    ...
}

Solution

Add a null check at the beginning of eqValueClass():

if (value == null) {
    @Suppress("UNCHECKED_CAST")
    return ArgumentMatchers.eq<T>(null) as T
}

This follows the existing pattern used in toKotlinType() in ValueClassSupport.kt.

Changes

• Matchers.kt: Add null check in eqValueClass()
• MatchersTest.kt: Add test cases for nullable value class with null value

[TimvdLippe]

@m-koops FYI the gift that keeps on giving

[m-koops]

@KonaEspresso94 Thank you very much for raising the NPE issue and providing your fix in this PR. While reviewing the change, I had to make some slight adjustments.

Therefore I decided to prepare a new commit/PR (#579). @TimvdLippe can you review that new PR?

[KonaEspresso94]

@TimvdLippe @m-koops Thanks for your quick feedback. I’ll close this PR.