Introduce FirFunctionReturnTypeMismatchChecker

compiler/testData/diagnostics/tests/classLiteral/genericArrays.fir.kt

@@ -4,7 +4,7 @@ fun <T> f1(): KClass<Array<T>> = <!CLASS_LITERAL_LHS_NOT_A_CLASS!>Array<T>::clas
 fun <T> f2(): KClass<Array<Array<T>>> = <!CLASS_LITERAL_LHS_NOT_A_CLASS!>Array<Array<T>>::class<!>
 inline fun <reified T> f3() = Array<T>::class
 inline fun <reified T> f4() = Array<Array<T>>::class
-fun f5(): KClass<Array<Any>> = <!CLASS_LITERAL_LHS_NOT_A_CLASS!>Array<*>::class<!>
+fun f5(): KClass<Array<Any>> = <!CLASS_LITERAL_LHS_NOT_A_CLASS, RETURN_TYPE_MISMATCH!>Array<*>::class<!>
 fun f6(): KClass<Array<Int?>> = Array<Int?>::class
 fun f7() = <!CLASS_LITERAL_LHS_NOT_A_CLASS!>Array<List<String>>::class<!>
 fun f8() = <!CLASS_LITERAL_LHS_NOT_A_CLASS!>Array<List<String>?>::class<!>

compiler/testData/diagnostics/tests/classLiteral/smartCast.fir.kt

@@ -0,0 +1,23 @@
+// KT-16291 Smart cast doesn't work when getting class of instance
+
+import kotlin.reflect.KClass
+
+class Foo {
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other === null || other::class != this::class) return false
+
+        return true
+    }
+}
+
+fun test(f: Foo?): KClass<out Foo>? = if (f != null) f::class else null
+
+fun test2(): KClass<out Foo>? {
+    var f: Foo? = null
+    if (f != null) {
+        run { f = null }
+        return <!RETURN_TYPE_MISMATCH!><!EXPRESSION_OF_NULLABLE_TYPE_IN_CLASS_LITERAL_LHS!>f<!>::class<!>
+    }
+    return null
+}

compiler/testData/diagnostics/tests/classLiteral/smartCast.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // KT-16291 Smart cast doesn't work when getting class of instance
 
 import kotlin.reflect.KClass