Address some clang-tidy gripes.

Change-Id: I9cc48dc0619a6903c1b9f1f3a734e366c4e764a6
Reviewed-on: https://code-review.googlesource.com/c/re2/+/40010
Reviewed-by: Paul Wankadia <junyer@google.com>

re2/re2.h

diff --git a/re2/re2.h b/re2/re2.h
index 09f9e67..c39589d 100644
--- a/re2/re2.h
+++ b/re2/re2.h
@@ -305,13 +305,13 @@
   // useful to invoke them directly, but the syntax is awkward, so the 'N'-less
   // versions should be preferred.
   static bool FullMatchN(const StringPiece& text, const RE2& re,
-                         const Arg* const args[], int argc);
+                         const Arg* const args[], int n);
   static bool PartialMatchN(const StringPiece& text, const RE2& re,
-                            const Arg* const args[], int argc);
+                            const Arg* const args[], int n);
   static bool ConsumeN(StringPiece* input, const RE2& re,
-                       const Arg* const args[], int argc);
+                       const Arg* const args[], int n);
   static bool FindAndConsumeN(StringPiece* input, const RE2& re,
-                              const Arg* const args[], int argc);
+                              const Arg* const args[], int n);
 
 #ifndef SWIG
  private:
@@ -323,8 +323,8 @@
   template <typename F, typename SP, typename... A>
   static inline bool Apply(F f, SP sp, const RE2& re, const A&... a) {
     const Arg* const args[] = {&a...};
-    const int argc = sizeof...(a);
-    return f(sp, re, args, argc);
+    const int n = sizeof...(a);
+    return f(sp, re, args, n);
   }
 
  public:

re2/testing/re2_test.cc

diff --git a/re2/testing/re2_test.cc b/re2/testing/re2_test.cc
index 2d692a6..52c9294 100644
--- a/re2/testing/re2_test.cc
+++ b/re2/testing/re2_test.cc
@@ -1461,9 +1461,8 @@
   EXPECT_TRUE(re.Match(null, 0, null.size(), RE2::UNANCHORED,
                        matches, arraysize(matches)));
   for (int i = 0; i < arraysize(matches); i++) {
-    EXPECT_TRUE(matches[i] == StringPiece());
     EXPECT_TRUE(matches[i].data() == NULL);  // always null
-    EXPECT_TRUE(matches[i] == "");
+    EXPECT_TRUE(matches[i].empty());
   }
 
   for (int i = 0; i < arraysize(matches); i++)
@@ -1472,18 +1471,14 @@
   StringPiece empty("");
   EXPECT_TRUE(re.Match(empty, 0, empty.size(), RE2::UNANCHORED,
                        matches, arraysize(matches)));
-  EXPECT_TRUE(matches[0] == StringPiece());
   EXPECT_TRUE(matches[0].data() != NULL);  // empty, not null
-  EXPECT_TRUE(matches[0] == "");
-  EXPECT_TRUE(matches[1] == StringPiece());
+  EXPECT_TRUE(matches[0].empty());
   EXPECT_TRUE(matches[1].data() != NULL);  // empty, not null
-  EXPECT_TRUE(matches[1] == "");
-  EXPECT_TRUE(matches[2] == StringPiece());
+  EXPECT_TRUE(matches[1].empty());
   EXPECT_TRUE(matches[2].data() == NULL);
-  EXPECT_TRUE(matches[2] == "");
-  EXPECT_TRUE(matches[3] == StringPiece());
+  EXPECT_TRUE(matches[2].empty());
   EXPECT_TRUE(matches[3].data() == NULL);
-  EXPECT_TRUE(matches[3] == "");
+  EXPECT_TRUE(matches[3].empty());
 }
 
 // Issue 1816809