java completion: don't consider underscore lowercase (IDEA-210869)

java/java-impl/src/com/intellij/codeInsight/completion/LimitedAccessibleClassPreprocessor.java

@@ -17,7 +17,6 @@ package com.intellij.codeInsight.completion;
 
 import com.intellij.openapi.diagnostic.Logger;
 import com.intellij.openapi.util.registry.Registry;
-import com.intellij.openapi.util.text.StringUtil;
 import com.intellij.psi.PsiClass;
 import com.intellij.psi.PsiElement;
 import com.intellij.psi.impl.source.tree.java.PsiReferenceExpressionImpl;
@@ -70,7 +69,9 @@ class LimitedAccessibleClassPreprocessor implements Processor<PsiClass> {
       if (PsiReferenceExpressionImpl.seemsScrambled(psiClass)) {
         return true;
       }
-      if (!StringUtil.isCapitalized(psiClass.getName()) && !Registry.is("ide.completion.show.lower.case.classes")) {
+      String name = psiClass.getName();
+      if (name != null && name.length() > 0 && Character.isLowerCase(name.charAt(0)) &&
+          !Registry.is("ide.completion.show.lower.case.classes")) {
         return true;
       }
     }

java/java-tests/testData/codeInsight/completion/className/nameCompletion/java/ClassStartsWithUnderscore-result.java

@@ -0,0 +1,5 @@
+import foo._SomeClass;
+
+class C {
+    _SomeClass<caret>
+}

java/java-tests/testData/codeInsight/completion/className/nameCompletion/java/ClassStartsWithUnderscore-source.java

@@ -0,0 +1,3 @@
+class C {
+    _SomeCl<caret>
+}

java/java-tests/testSrc/com/intellij/java/codeInsight/completion/ClassNameCompletionTest.java

@@ -291,6 +291,11 @@ public class ClassNameCompletionTest extends LightFixtureCompletionTestCase {
 
   public void testNestedAnonymousTab() { doJavaTest('\t');}
 
+  public void testClassStartsWithUnderscore() {
+    myFixture.addClass("package foo; public class _SomeClass {}");
+    doJavaTest('\n');
+  }
+
   private void doJavaTest(char toType) {
     final String path = "/nameCompletion/java";
     myFixture.configureByFile(path + "/" + getTestName(false) + "-source.java");