Do not overuse makeNullableAsSpecified in TypeSubstitutor

compiler/testData/diagnostics/tests/platformTypes/methodCall/entrySet.kt

@@ -0,0 +1,24 @@
+// FILE: p/MultiMap.java
+
+package p;
+
+import java.util.*;
+
+public class MultiMap<K, V> {
+    public Set<Collection<V>> entrySet() {
+        return null;
+    }
+}
+
+// FILE: k.kt
+
+import p.*
+
+fun test() {
+    val map = MultiMap<Int, String>()
+    val set = map.entrySet()
+    set.iterator()
+
+    val set1 = map.entrySet()!!
+    set1.iterator()
+}

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

@@ -7779,6 +7779,11 @@ public class JetDiagnosticsTestGenerated extends AbstractJetDiagnosticsTest {
                     JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("compiler/testData/diagnostics/tests/platformTypes/methodCall"), Pattern.compile("^(.+)\\.kt$"), true);
                 }
                 
+                @TestMetadata("entrySet.kt")
+                public void testEntrySet() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/platformTypes/methodCall/entrySet.kt");
+                }
+                
                 @TestMetadata("int.kt")
                 public void testInt() throws Exception {
                     String fileName = JetTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/platformTypes/methodCall/int.kt");

core/descriptors/src/org/jetbrains/jet/lang/types/TypeSubstitutor.java

@@ -191,8 +191,8 @@ public class TypeSubstitutor {
                         substitutedType = typeVariable.substitutionResult(replacement.getType());
                     }
                     else {
-                        boolean resultingIsNullable = type.isNullable() || replacement.getType().isNullable();
-                        substitutedType = TypeUtils.makeNullableAsSpecified(replacement.getType(), resultingIsNullable);
+                        // this is a simple type T or T?: if it's T, we should just take replacement, if T? - we make replacement nullable
+                        substitutedType = type.isNullable() ? TypeUtils.makeNullable(replacement.getType()) : replacement.getType();
                     }
 
                     Variance resultingProjectionKind = combine(originalProjectionKind, replacement.getProjectionKind());