KT-2838 Type inference failed on passing null as a nullable argument

#KT-2838 fixed

compiler/frontend/src/org/jetbrains/jet/lang/resolve/calls/inference/ConstraintSystemImpl.java

@@ -204,6 +204,13 @@ public class ConstraintSystemImpl implements ConstraintSystem {
         if (constrainingTypeDescriptor instanceof TypeParameterDescriptor) {
             assert typeParameterConstraints.get(constrainingTypeDescriptor) == null : "Constraining type contains type variable " + constrainingTypeDescriptor.getName();
         }
+        if (constraintKind == SUB_TYPE && KotlinBuiltIns.getInstance().isNothingOrNullableNothing(constrainingType)) {
+            // following constraints are always true:
+            // 'Nothing' is a subtype of any type
+            if (!constrainingType.isNullable()) return;
+            // 'Nothing?' is a subtype of nullable type
+            if (subjectType.isNullable()) return;
+        }
         if (!(constrainingTypeDescriptor instanceof ClassDescriptor) || !(subjectTypeDescriptor instanceof ClassDescriptor)) {
             errorConstraintPositions.add(constraintPosition);
             return;

compiler/testData/diagnostics/tests/inference/regressions/kt2838.kt

@@ -0,0 +1,17 @@
+//KT-2838 Type inference failed on passing null as a nullable argument
+package a
+
+fun foo<T>(a: T, b: Map<T, String>?) = b?.get(a)
+fun bar<T>(a: T, b: Map<T, String>) = b.get(a)
+
+fun test(a: Int) {
+    foo(a, null)
+    <!TYPE_INFERENCE_TYPE_CONSTRUCTOR_MISMATCH!>bar<!>(a, null)
+}
+fun test1(a: Int) {
+    <!UNREACHABLE_CODE!>foo(a, throw Exception())<!>
+}
+
+fun test2(a: Int) {
+    <!UNREACHABLE_CODE!>bar(a, throw Exception())<!>
+}

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

@@ -2197,6 +2197,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                     doTest("compiler/testData/diagnostics/tests/inference/regressions/kt2514.kt");
                 }
                 
+                @TestMetadata("kt2838.kt")
+                public void testKt2838() throws Exception {
+                    doTest("compiler/testData/diagnostics/tests/inference/regressions/kt2838.kt");
+                }
+                
                 @TestMetadata("kt2841.kt")
                 public void testKt2841() throws Exception {
                     doTest("compiler/testData/diagnostics/tests/inference/regressions/kt2841.kt");