try intersection of upper bounds as a suggestion

not each bound itself
while resolving constraint system

compiler/testData/diagnostics/tests/inference/upperBounds/intersectUpperBounds.kt

@@ -0,0 +1,30 @@
+package s
+
+trait In<in T>
+
+trait A
+trait B
+trait C: A, B
+
+fun <T> foo(in1: In<T>, in2: In<T>): T = throw Exception("$in1 $in2")
+
+fun test(inA: In<A>, inB: In<B>, inC: In<C>) {
+
+    <!TYPE_INFERENCE_CONFLICTING_SUBSTITUTIONS!>foo<!>(inA, inB)
+
+    val r = foo(inA, inC)
+    r: C
+
+    val c: C = foo(inA, inB)
+
+    use(c)
+}
+
+fun <T: C> bar(in1: In<T>): T = throw Exception("$in1")
+
+fun test(inA: In<A>) {
+    val r = bar(inA)
+    r: C
+}
+
+fun use(vararg a: Any?) = a

compiler/testData/diagnostics/tests/inference/upperBounds/nonNullUpperBound.kt

@@ -0,0 +1,13 @@
+fun <R : Any> unescape(value: Any): R? = throw Exception("$value")
+
+fun <T: Any> foo(v: Any): T? = unescape(v)
+
+//--------------
+
+trait A
+
+fun <R : A> unescapeA(value: Any): R? = throw Exception("$value")
+
+
+fun <T: A> fooA(v: Any): T? = unescapeA(v)
+

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

@@ -3542,11 +3542,21 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                     doTest("compiler/testData/diagnostics/tests/inference/upperBounds/doNotInferFromBoundsOnly.kt");
                 }
                 
+                @TestMetadata("intersectUpperBounds.kt")
+                public void testIntersectUpperBounds() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/inference/upperBounds/intersectUpperBounds.kt");
+                }
+                
                 @TestMetadata("kt2856.kt")
                 public void testKt2856() throws Exception {
                     doTest("compiler/testData/diagnostics/tests/inference/upperBounds/kt2856.kt");
                 }
                 
+                @TestMetadata("nonNullUpperBound.kt")
+                public void testNonNullUpperBound() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/inference/upperBounds/nonNullUpperBound.kt");
+                }
+                
                 @TestMetadata("useBoundsIfUnknownParameters.kt")
                 public void testUseBoundsIfUnknownParameters() throws Exception {
                     doTest("compiler/testData/diagnostics/tests/inference/upperBounds/useBoundsIfUnknownParameters.kt");

core/descriptors/src/org/jetbrains/jet/lang/resolve/calls/inference/TypeConstraintsImpl.java

@@ -186,9 +186,10 @@ public class TypeConstraintsImpl implements TypeConstraints {
         ContainerUtil.addIfNotNull(superTypeOfLowerBounds, values);
 
         Set<JetType> upperBounds = filterBounds(constraints, BoundKind.UPPER_BOUND, values);
-        for (JetType upperBound : upperBounds) {
-            if (trySuggestion(upperBound, constraints)) {
-                return Collections.singleton(upperBound);
+        JetType intersectionOfUpperBounds = TypeUtils.intersect(JetTypeChecker.INSTANCE, upperBounds);
+        if (!upperBounds.isEmpty() && intersectionOfUpperBounds != null) {
+            if (trySuggestion(intersectionOfUpperBounds, constraints)) {
+                return Collections.singleton(intersectionOfUpperBounds);
             }
         }
         //todo