[JVM IR] Add tests for KT-15971.

compiler/testData/codegen/box/defaultArguments/function/kt15971.kt

@@ -0,0 +1,19 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND: JVM
+interface Q {
+    fun foo(a: Double): Double
+}
+
+interface Z {
+    fun foo(a: Double = 1.0): Double
+}
+
+class R : Q, Z {
+    override fun foo(a: Double) = a
+}
+
+fun box(): String {
+    if (R().foo() != 1.0) return "fail 1"
+    if (R().foo(2.0) != 2.0) return "fail 2"
+    return "OK"
+}

compiler/testData/codegen/box/defaultArguments/function/kt15971_2.kt

@@ -0,0 +1,19 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND: JVM
+interface Q {
+    fun foo(a: Double) = 0.0
+}
+
+interface Z {
+    fun foo(a: Double = 1.0): Double
+}
+
+class R : Q, Z {
+    override fun foo(a: Double) = a
+}
+
+fun box(): String {
+    if (R().foo() != 1.0) return "fail 1"
+    if (R().foo(2.0) != 2.0) return "fail 2"
+    return "OK"
+}

compiler/testData/codegen/box/defaultArguments/function/kt15971_3.kt

@@ -0,0 +1,19 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND: JVM
+interface Q {
+    fun foo(a: Double): Double
+}
+
+interface Z {
+    fun foo(a: Double = 1.0) = 0.0
+}
+
+class R : Q, Z {
+    override fun foo(a: Double) = a
+}
+
+fun box(): String {
+    if (R().foo() != 1.0) return "fail 1"
+    if (R().foo(2.0) != 2.0) return "fail 2"
+    return "OK"
+}