KT-6774 Cannot find equals() when comparing with null

#KT-6774 Fixed

compiler/testData/diagnostics/tests/controlStructures/commonSupertypeOfT.kt

@@ -0,0 +1,30 @@
+// KT-6774 Cannot find equals() when comparing with null
+
+fun <T: Any> fn(t1: T, t2: T?) {
+    val x = if (true) t1 else t2
+    x == null
+    x?.equals(null)
+    x?.hashCode()
+    x.toString()
+    x!!.hashCode()
+
+    val y = t2 ?: t1
+    y == t1
+    y.equals(null)
+    y.hashCode()
+    y.toString()
+    y.hashCode()
+}
+
+trait Tr {
+    fun foo()
+}
+
+fun <T: Tr> fn(t1: T, t2: T?) {
+    val x = if (true) t1 else t2
+    x?.foo()
+    x!!.foo()
+
+    val y = t2 ?: t1
+    y.foo()
+}

compiler/testData/diagnostics/tests/controlStructures/commonSupertypeOfT.txt

@@ -0,0 +1,11 @@
+package
+
+internal fun </*0*/ T : Tr> fn(/*0*/ t1: T, /*1*/ t2: T?): kotlin.Unit
+internal fun </*0*/ T : kotlin.Any> fn(/*0*/ t1: T, /*1*/ t2: T?): kotlin.Unit
+
+internal trait Tr {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    internal abstract fun foo(): kotlin.Unit
+    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

@@ -1950,6 +1950,12 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                 JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("compiler/testData/diagnostics/tests/controlStructures"), Pattern.compile("^(.+)\\.kt$"), true);
             }
 
+            @TestMetadata("commonSupertypeOfT.kt")
+            public void testCommonSupertypeOfT() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/controlStructures/commonSupertypeOfT.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("emptyIf.kt")
             public void testEmptyIf() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/controlStructures/emptyIf.kt");

core/descriptors/src/org/jetbrains/kotlin/types/CommonSupertypes.java

@@ -22,7 +22,7 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns;
 import org.jetbrains.kotlin.descriptors.ClassDescriptor;
-import org.jetbrains.kotlin.descriptors.DeclarationDescriptor;
+import org.jetbrains.kotlin.descriptors.ClassifierDescriptor;
 import org.jetbrains.kotlin.descriptors.TypeParameterDescriptor;
 import org.jetbrains.kotlin.descriptors.annotations.Annotations;
 import org.jetbrains.kotlin.resolve.scopes.JetScope;
@@ -223,12 +223,14 @@ public class CommonSupertypes {
             nullable |= type.isMarkedNullable();
         }
 
-        // TODO : attributes?
         JetScope newScope = JetScope.Empty.INSTANCE$;
-        DeclarationDescriptor declarationDescriptor = constructor.getDeclarationDescriptor();
+        ClassifierDescriptor declarationDescriptor = constructor.getDeclarationDescriptor();
         if (declarationDescriptor instanceof ClassDescriptor) {
             newScope = ((ClassDescriptor) declarationDescriptor).getMemberScope(newProjections);
         }
+        else if (declarationDescriptor instanceof TypeParameterDescriptor) {
+            newScope = ((TypeParameterDescriptor) declarationDescriptor).getUpperBoundsAsType().getMemberScope();
+        }
         return new JetTypeImpl(Annotations.EMPTY, constructor, nullable, newProjections, newScope);
     }