KT-786 Exception on incomplete code with 'when'

compiler/frontend/src/org/jetbrains/jet/lang/types/TypeUtils.java

@@ -361,6 +361,7 @@ public class TypeUtils {
                 fullSubstitution.put(typeParameterDescriptor.getTypeConstructor(), substitutedTypeProjection);
             }
         }
+        if (JetStandardClasses.isNothingOrNullableNothing(context)) return;
         for (JetType supertype : context.getConstructor().getSupertypes()) {
             fillInDeepSubstitutor(supertype, substitutor, substitution, fullSubstitution);
         }

compiler/testData/diagnostics/tests/controlStructures/kt786.jet

@@ -0,0 +1,26 @@
+namespace kt786
+
+//KT-786 Exception on incomplete code with 'when'
+fun foo() : Int {
+    val d = 2
+    var z = 0
+    when(d) {
+        is 5, is 3 => <!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY, UNUSED_CHANGED_VALUE!>z++<!>
+        <!ELSE_MISPLACED_IN_WHEN!>else => { <!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>z = <!UNUSED_VALUE!>-1000<!><!> }<!>
+        return z => <!UNREACHABLE_CODE!>34<!>
+    }
+}
+
+//test unreachable code
+fun fff(): Int {
+    var d = 3
+    when(d) {
+        is 4 => 21
+        return 2 => <!UNREACHABLE_CODE!>return 47<!>
+        <!UNREACHABLE_CODE!>bar()<!> => <!UNREACHABLE_CODE!>45<!>
+        <!UNREACHABLE_CODE!>444<!> => <!UNREACHABLE_CODE!>true<!>
+    }
+    return 34
+}
+
+fun bar(): Int = 8