Support empty if-statements

Type-check "if (...) ;" to Unit, report "implicit cast to Unit", propagate data
flow info out of its condition

 #KT-2478 Fixed

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

@@ -357,7 +357,7 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> implem
         return genQualified(receiver, expression.getBaseExpression());
     }
 
-    private static boolean isEmptyExpression(JetElement expr) {
+    private static boolean isEmptyExpression(@Nullable JetElement expr) {
         if (expr == null) {
             return true;
         }
@@ -383,10 +383,6 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> implem
         JetExpression thenExpression = expression.getThen();
         JetExpression elseExpression = expression.getElse();
 
-        if (thenExpression == null && elseExpression == null) {
-            throw new CompilationException("Both brunches of if/else are null", null, expression);
-        }
-
         if (isEmptyExpression(thenExpression)) {
             if (isEmptyExpression(elseExpression)) {
                 condition.put(asmType, v);

compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/ControlStructureTypingVisitor.java

@@ -48,7 +48,9 @@ import org.jetbrains.jet.lang.types.checker.JetTypeChecker;
 import org.jetbrains.jet.lang.types.lang.KotlinBuiltIns;
 import org.jetbrains.jet.util.slicedmap.WritableSlice;
 
-import java.util.*;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
 
 import static org.jetbrains.jet.lang.diagnostics.Errors.*;
 import static org.jetbrains.jet.lang.resolve.BindingContext.*;
@@ -108,7 +110,8 @@ public class ControlStructureTypingVisitor extends ExpressionTypingVisitor {
                 return getTypeInfoWhenOnlyOneBranchIsPresent(
                         thenBranch, thenScope, thenInfo, elseInfo, contextWithExpectedType, ifExpression, isStatement);
             }
-            return JetTypeInfo.create(null, context.dataFlowInfo);
+            return DataFlowUtils.checkImplicitCast(KotlinBuiltIns.getInstance().getUnitType(), ifExpression, contextWithExpectedType,
+                                                   isStatement, thenInfo.or(elseInfo));
         }
         if (thenBranch == null) {
             return getTypeInfoWhenOnlyOneBranchIsPresent(

compiler/testData/codegen/box/controlStructures/emptyIf.kt

@@ -0,0 +1,12 @@
+fun box(): String {
+    if (true);
+    if (false);
+    val iftrue = if (true);
+    val iffalse = if (false);
+
+    var state = 0
+    val k = if (state++==1);
+    if (state != 1) return "Fail: $state"
+
+    return "OK"
+}

compiler/testData/diagnostics/tests/controlStructures/emptyIf.kt

@@ -0,0 +1,16 @@
+fun foo(x: Unit) = x
+
+fun test() {
+    if (false);
+    if (true);
+
+    val x = <!IMPLICIT_CAST_TO_UNIT_OR_ANY!>if (false)<!>;
+    foo(x)
+
+    val y: Unit = if (false);
+    foo(y)
+
+    foo({if (1==1);}())
+
+    return if (true);
+}

compiler/testData/diagnostics/tests/dataFlow/EmptyIf.kt

@@ -0,0 +1,22 @@
+fun f1(s: String?) {
+    if (s!! == "");
+    s : String
+}
+
+fun f2(s: Number?) {
+    if (s is Int);
+    <!TYPE_MISMATCH!>s<!> : Int
+    if (s as Int == 42);
+    s : Int
+}
+
+fun f3(s: Number?) {
+    if (s is Int && s as Int == 42);
+    <!TYPE_MISMATCH!>s<!> : Int
+}
+
+fun f4(s: Int?) {
+    var u = <!IMPLICIT_CAST_TO_UNIT_OR_ANY!>if (s!! == 42)<!>;
+    if (u == Unit.VALUE) u = if (s == 239);
+    return u
+}

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

@@ -1565,6 +1565,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), "org.jetbrains.jet.generators.tests.GenerateTests", new File("compiler/testData/diagnostics/tests/controlStructures"), Pattern.compile("^(.+)\\.kt$"), true);
             }
             
+            @TestMetadata("emptyIf.kt")
+            public void testEmptyIf() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/controlStructures/emptyIf.kt");
+            }
+            
             @TestMetadata("forLoopWithNullableRange.kt")
             public void testForLoopWithNullableRange() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/controlStructures/forLoopWithNullableRange.kt");
@@ -1757,6 +1762,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 doTest("compiler/testData/diagnostics/tests/dataFlow/CalleeExpression.kt");
             }
             
+            @TestMetadata("EmptyIf.kt")
+            public void testEmptyIf() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/dataFlow/EmptyIf.kt");
+            }
+            
             @TestMetadata("IsExpression.kt")
             public void testIsExpression() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/dataFlow/IsExpression.kt");
@@ -3917,7 +3927,7 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 public void testContains() throws Exception {
                     doTest("compiler/testData/diagnostics/tests/inline/binaryExpressions/contains.kt");
                 }
-
+                
                 @TestMetadata("mathOperations.kt")
                 public void testMathOperations() throws Exception {
                     doTest("compiler/testData/diagnostics/tests/inline/binaryExpressions/mathOperations.kt");

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

@@ -1427,6 +1427,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest("compiler/testData/codegen/box/controlStructures/doWhileFib.kt");
         }
         
+        @TestMetadata("emptyIf.kt")
+        public void testEmptyIf() throws Exception {
+            doTest("compiler/testData/codegen/box/controlStructures/emptyIf.kt");
+        }
+        
         @TestMetadata("finallyOnEmptyReturn.kt")
         public void testFinallyOnEmptyReturn() throws Exception {
             doTest("compiler/testData/codegen/box/controlStructures/finallyOnEmptyReturn.kt");