From f7c5e6cbc99b4258f6f6ef4cfe65cfd9402c4323 Mon Sep 17 00:00:00 2001
From: Liam Miller-Cushon <cushon@google.com>
Date: Tue, 12 Jan 2021 11:29:53 -0800
Subject: [PATCH] Improve handling of type parameter names that start with `_`

PiperOrigin-RevId: 351412076
---
 .../errorprone/names/NamingConventions.java   | 10 ++++++----
 .../bugpatterns/TypeParameterNaming.java      |  3 ++-
 .../bugpatterns/TypeParameterNamingTest.java  | 19 +++++++++++++++++++
 3 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/check_api/src/main/java/com/google/errorprone/names/NamingConventions.java b/check_api/src/main/java/com/google/errorprone/names/NamingConventions.java
index d2d0a1cf9c0..7b991dbdedd 100644
--- a/check_api/src/main/java/com/google/errorprone/names/NamingConventions.java
+++ b/check_api/src/main/java/com/google/errorprone/names/NamingConventions.java
@@ -18,8 +18,8 @@
 
 import static com.google.common.collect.ImmutableList.toImmutableList;
 
+import com.google.common.base.Splitter;
 import com.google.common.collect.ImmutableList;
-import java.util.Arrays;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 
@@ -32,8 +32,9 @@ public class NamingConventions {
   private static final String CASE_TRANSITION = "(?<=[a-z0-9])(?=[A-Z])";
   private static final String TRAILING_DIGITS = "(?<![0-9_])(?=[0-9]+$)";
 
-  private static final Pattern TERM_SPLITTER =
-      Pattern.compile(String.format("%s|%s|%s", UNDERSCORE, CASE_TRANSITION, TRAILING_DIGITS));
+  private static final Splitter TERM_SPLITTER =
+      Splitter.onPattern(String.format("%s|%s|%s", UNDERSCORE, CASE_TRANSITION, TRAILING_DIGITS))
+          .omitEmptyStrings();
 
   /**
    * Split a Java name into terms based on either Camel Case or Underscores. We also split digits at
@@ -47,7 +48,8 @@ public static ImmutableList<String> splitToLowercaseTerms(String identifierName)
       // Degenerate case of names which contain only underscore
       return ImmutableList.of(identifierName);
     }
-    return Arrays.stream(TERM_SPLITTER.split(identifierName))
+    return TERM_SPLITTER
+        .splitToStream(identifierName)
         .map(String::toLowerCase)
         .collect(toImmutableList());
   }
diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/TypeParameterNaming.java b/core/src/main/java/com/google/errorprone/bugpatterns/TypeParameterNaming.java
index dd55d418f7f..724f29d464a 100644
--- a/core/src/main/java/com/google/errorprone/bugpatterns/TypeParameterNaming.java
+++ b/core/src/main/java/com/google/errorprone/bugpatterns/TypeParameterNaming.java
@@ -205,7 +205,8 @@ private static List<TypeVariableSymbol> typeVariablesEnclosing(Symbol sym) {
   }
 
   private static String suggestedSingleLetter(String id, Tree tree) {
-    char firstLetter = id.charAt(0);
+    char firstLetter =
+        Ascii.toUpperCase(NamingConventions.splitToLowercaseTerms(id).get(0).charAt(0));
     Symbol sym = ASTHelpers.getSymbol(tree);
     List<TypeVariableSymbol> enclosingTypeSymbols = typeVariablesEnclosing(sym);
 
diff --git a/core/src/test/java/com/google/errorprone/bugpatterns/TypeParameterNamingTest.java b/core/src/test/java/com/google/errorprone/bugpatterns/TypeParameterNamingTest.java
index a926158d418..c7cf3020d53 100644
--- a/core/src/test/java/com/google/errorprone/bugpatterns/TypeParameterNamingTest.java
+++ b/core/src/test/java/com/google/errorprone/bugpatterns/TypeParameterNamingTest.java
@@ -294,6 +294,25 @@ public void negativeCases_manyNumberedTypes() {
         .doTest();
   }
 
+  @Test
+  public void refactoring_underscore() {
+    refactoring
+        .addInputLines(
+            "in/Test.java", //
+            "class Test {",
+            "  public <_T> void method(_T t) {",
+            "  }",
+            "}")
+        .addOutputLines(
+            "in/Test.java", //
+            "class Test {",
+            "  public <T> void method(T t) {",
+            "  }",
+            "}")
+        .setFixChooser(FixChoosers.FIRST)
+        .doTest(TestMode.TEXT_MATCH);
+  }
+
   @Test
   public void classifyTypeName_singleLetter() {
     assertKindOfName("T").isEqualTo(LETTER_WITH_MAYBE_NUMERAL);