Fix substitution of generic types in contracts

#KT-41078 Fixed

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/contractWithDeepGenerics.fir.kt

@@ -0,0 +1,31 @@
+// !USE_EXPERIMENTAL: kotlin.contracts.ExperimentalContracts
+// ISSUE: KT-43260
+
+import kotlin.contracts.ExperimentalContracts
+import kotlin.contracts.contract
+
+sealed class Either<L : Any, R : Any> {
+    data class Left<L : Any, R : Any>(val leftValue: L) : Either<L, R>()
+    data class Right<L : Any, R : Any>(val rightValue: R) : Either<L, R>()
+}
+
+inline fun <reified L : Any, reified R : Any> Either<L, R>.isLeft(): Boolean {
+    contract {
+        returns(true) implies (this@isLeft is Either.Left<L, R>)
+    }
+    return this is Either.Left<L, R>
+}
+
+inline fun <reified L : Any, reified R : Any> Either<L, R>.isRight(): Boolean {
+    contract {
+        returns(true) implies (this@isRight is Either.Right<L, R>)
+    }
+    return this is Either.Right<L, R>
+}
+
+fun test() {
+    val result: Either<Exception, Unit> = Either.Left(RuntimeException("simulating missing code"))
+    if (result.isLeft()) {
+        val cause = result.leftValue.cause
+    }
+}