Negations supported

idea/src/org/jetbrains/jet/lang/psi/JetPrefixExpression.java

@@ -3,7 +3,7 @@ package org.jetbrains.jet.lang.psi;
 import com.intellij.lang.ASTNode;
 import com.intellij.psi.PsiElement;
 import org.jetbrains.annotations.NotNull;
-import org.jetbrains.jet.JetNodeTypes;
+import org.jetbrains.annotations.Nullable;
 
 /**
  * @author max
@@ -18,13 +18,12 @@ public class JetPrefixExpression extends JetUnaryExpression {
         visitor.visitPrefixExpression(this);
     }
 
-    @NotNull
+    @Nullable @IfNotParsed
     public JetExpression getBaseExpression() {
         PsiElement expression = getOperationSign().getNextSibling();
         while (expression != null && false == expression instanceof JetExpression) {
             expression = expression.getNextSibling();
         }
-        assert expression != null;
         return (JetExpression) expression;
     }
 }

idea/src/org/jetbrains/jet/lang/psi/JetUnaryExpression.java

@@ -2,6 +2,7 @@ package org.jetbrains.jet.lang.psi;
 
 import com.intellij.lang.ASTNode;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.jetbrains.jet.JetNodeTypes;
 
 /**
@@ -12,7 +13,6 @@ public abstract class JetUnaryExpression extends JetExpression {
         super(node);
     }
 
-    @NotNull
     public abstract JetExpression getBaseExpression();
 
     @NotNull

idea/src/org/jetbrains/jet/lang/types/JetTypeInferrer.java

@@ -1403,6 +1403,25 @@ public class JetTypeInferrer {
                         }
                     }
                 }
+
+                @Override
+                public void visitUnaryExpression(JetUnaryExpression expression) {
+                    IElementType operationTokenType = expression.getOperationSign().getReferencedNameElementType();
+                    if (operationTokenType == JetTokens.EXCL) {
+                        JetExpression baseExpression = expression.getBaseExpression();
+                        if (baseExpression != null) {
+                            result[0] = extractDataFlowInfoFromCondition(baseExpression, !conditionValue);
+                        }
+                    }
+                }
+
+                @Override
+                public void visitParenthesizedExpression(JetParenthesizedExpression expression) {
+                    JetExpression body = expression.getExpression();
+                    if (body != null) {
+                        body.accept(this);
+                    }
+                }
             });
             return result[0];
         }

idea/testData/checker/Nullability.jet

@@ -214,3 +214,44 @@ fun f6(s : String?) {
   } while (s == null)
   s.get(0)
 }
+
+fun f7(s : String?, t : String?) {
+  s?.get(0)
+  if (!(s == null)) {
+    s.get(0)
+  }
+  s?.get(0)
+  if (!(s != null)) {
+    s?.get(0)
+  }
+  else {
+    s.get(0)
+  }
+  s?.get(0)
+  if (!!(s != null)) {
+    s.get(0)
+  }
+  else {
+    s?.get(0)
+  }
+  s?.get(0)
+  t?.get(0)
+  if (!(s == null || t == null)) {
+    s.get(0)
+    t.get(0)
+  }
+  else {
+    s?.get(0)
+    t?.get(0)
+  }
+  s?.get(0)
+  t?.get(0)
+  if (!(s == null && s == null)) {
+    s.get(0)
+    t?.get(0)
+  }
+  else {
+    s?.get(0)
+    t?.get(0)
+  }
+}