Test for KT-3711. Passes already in M11 or even earlier

compiler/testData/diagnostics/tests/smartCasts/varnotnull/kt3711.kt

@@ -0,0 +1,9 @@
+data class StringPair(val first: String, val second: String)
+
+fun String.to(second: String) = StringPair(this, second)
+
+fun f(a: String?) {
+    if (a != null) {
+        val <!UNUSED_VARIABLE!>b<!>: StringPair = <!DEBUG_INFO_SMARTCAST!>a<!> to <!DEBUG_INFO_SMARTCAST!>a<!>
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/varnotnull/kt3711.txt

@@ -0,0 +1,16 @@
+package
+
+internal fun f(/*0*/ a: kotlin.String?): kotlin.Unit
+internal fun kotlin.String.to(/*0*/ second: kotlin.String): StringPair
+
+kotlin.data() internal final class StringPair {
+    public constructor StringPair(/*0*/ first: kotlin.String, /*1*/ second: kotlin.String)
+    internal final val first: kotlin.String
+    internal final val second: kotlin.String
+    internal final /*synthesized*/ fun component1(): kotlin.String
+    internal final /*synthesized*/ fun component2(): kotlin.String
+    public final /*synthesized*/ fun copy(/*0*/ first: kotlin.String = ..., /*1*/ second: kotlin.String = ...): StringPair
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/tests/org/jetbrains/kotlin/checkers/JetDiagnosticsTestGenerated.java

@@ -11341,6 +11341,12 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                     doTest(fileName);
                 }
 
+                @TestMetadata("kt3711.kt")
+                public void testKt3711() throws Exception {
+                    String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/smartCasts/varnotnull/kt3711.kt");
+                    doTest(fileName);
+                }
+
                 @TestMetadata("kt3899.kt")
                 public void testKt3899() throws Exception {
                     String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/smartCasts/varnotnull/kt3899.kt");