Do not throw an exception when a when-statement with no else makes no match

compiler/backend/src/org/jetbrains/jet/codegen/ExpressionCodegen.java

@@ -3725,7 +3725,17 @@ The "returned" value of try expression with no finally is either the last expres
         }
         if (!hasElse && nextCondition != null) {
             v.mark(nextCondition);
-            throwNewException(CLASS_NO_PATTERN_MATCHED_EXCEPTION);
+            if (!isStatement) {
+                // a result is expected
+                if (Boolean.TRUE.equals(bindingContext.get(BindingContext.EXHAUSTIVE_WHEN, expression))) {
+                    // when() is supposed to be exhaustive
+                    throwNewException(CLASS_NO_PATTERN_MATCHED_EXCEPTION);
+                }
+                else {
+                    // non-exhaustive when() with no else -> Unit must be expected
+                    StackValue.putUnitInstance(v);
+                }
+            }
         }
 
         markLineNumber(expression);

compiler/frontend/src/org/jetbrains/jet/lang/cfg/WhenChecker.java

@@ -65,7 +65,11 @@ public final class WhenChecker {
                 }
             }
         }
-        return isExhaust && notEmpty;
+        boolean exhaustive = isExhaust && notEmpty;
+        if (exhaustive) {
+            trace.record(BindingContext.EXHAUSTIVE_WHEN, expression);
+        }
+        return exhaustive;
     }
 
     private static boolean containsEnumEntryCase(

compiler/frontend/src/org/jetbrains/jet/lang/resolve/BindingContext.java

@@ -110,6 +110,8 @@ public interface BindingContext {
 
     WritableSlice<JetExpression, JetType> AUTOCAST = Slices.createSimpleSlice();
 
+    WritableSlice<JetWhenExpression, Boolean> EXHAUSTIVE_WHEN = Slices.createSimpleSlice();
+
     /**
      * A scope where type of expression has been resolved
      */

compiler/testData/codegen/box/when/noElseAssignedNoMatch.kt

@@ -0,0 +1,9 @@
+fun box(): String {
+    var r = "OK"
+    val x = 3
+    val y: Unit = when (x) {
+        1 -> { r = "Fail 0" }
+        2 -> { r = "Fail 1" }
+    }
+    return r
+}

compiler/testData/codegen/box/when/noElseExhaustive.kt

@@ -0,0 +1,9 @@
+enum class En {
+    A
+    B
+}
+
+fun box(): String = when(En.A) {
+    En.A -> "OK"
+    En.B -> "Fail 1"
+}

compiler/testData/codegen/box/when/noElseExhaustiveStatement.kt

@@ -0,0 +1,12 @@
+enum class En {
+    A
+    B
+}
+
+fun box(): String {
+    when(En.A) {
+        En.A -> "s1"
+        En.B -> "s2"
+    }
+    return "OK"
+}

compiler/testData/codegen/box/when/noElseExhaustiveUnitExpected.kt

@@ -0,0 +1,14 @@
+enum class En {
+    A
+    B
+}
+
+fun box(): String {
+
+  val u: Unit = when(En.A) {
+    En.A -> {}
+    En.B -> {}
+  }
+
+  return "OK"
+}

compiler/testData/codegen/box/when/noElseInRetunedExpressionNoMatch.kt

@@ -0,0 +1,11 @@
+var r = "OK"
+
+fun foo(x: Int) {
+    return when (x) {
+        1 -> { r = "Fail" }
+    }
+}
+fun box(): String {
+    foo(0)
+    return r
+}

compiler/testData/codegen/box/when/noElseNoMatch.kt

@@ -0,0 +1,8 @@
+fun box(): String {
+    val x = 3
+    when (x) {
+        1 -> {}
+        2 -> {}
+    }
+    return "OK"
+}

compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -4413,16 +4413,46 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest("compiler/testData/codegen/box/when/noElseAssigned.kt");
         }
         
+        @TestMetadata("noElseAssignedNoMatch.kt")
+        public void testNoElseAssignedNoMatch() throws Exception {
+            doTest("compiler/testData/codegen/box/when/noElseAssignedNoMatch.kt");
+        }
+        
+        @TestMetadata("noElseExhaustive.kt")
+        public void testNoElseExhaustive() throws Exception {
+            doTest("compiler/testData/codegen/box/when/noElseExhaustive.kt");
+        }
+        
+        @TestMetadata("noElseExhaustiveStatement.kt")
+        public void testNoElseExhaustiveStatement() throws Exception {
+            doTest("compiler/testData/codegen/box/when/noElseExhaustiveStatement.kt");
+        }
+        
+        @TestMetadata("noElseExhaustiveUnitExpected.kt")
+        public void testNoElseExhaustiveUnitExpected() throws Exception {
+            doTest("compiler/testData/codegen/box/when/noElseExhaustiveUnitExpected.kt");
+        }
+        
         @TestMetadata("noElseInRetunedExpression.kt")
         public void testNoElseInRetunedExpression() throws Exception {
             doTest("compiler/testData/codegen/box/when/noElseInRetunedExpression.kt");
         }
         
+        @TestMetadata("noElseInRetunedExpressionNoMatch.kt")
+        public void testNoElseInRetunedExpressionNoMatch() throws Exception {
+            doTest("compiler/testData/codegen/box/when/noElseInRetunedExpressionNoMatch.kt");
+        }
+        
         @TestMetadata("noElseInStatement.kt")
         public void testNoElseInStatement() throws Exception {
             doTest("compiler/testData/codegen/box/when/noElseInStatement.kt");
         }
         
+        @TestMetadata("noElseNoMatch.kt")
+        public void testNoElseNoMatch() throws Exception {
+            doTest("compiler/testData/codegen/box/when/noElseNoMatch.kt");
+        }
+        
         @TestMetadata("nullableWhen.kt")
         public void testNullableWhen() throws Exception {
             doTest("compiler/testData/codegen/box/when/nullableWhen.kt");