Use absl::StripLeadingAsciiWhitespace instead of a lambda using std::isspace

As written the lambda doesn't correctly handle sign-extension.

Closes #2061

PiperOrigin-RevId: 921439321
Change-Id: I952e9f29c957546e91d3c91d9df0bbc45b7a2888

absl/time/format.cc

diff --git a/absl/time/format.cc b/absl/time/format.cc
index bd06f8f..c59129f 100644
--- a/absl/time/format.cc
+++ b/absl/time/format.cc
@@ -17,7 +17,7 @@
 #include <cctype>
 #include <cstdint>
 #include <utility>
-
+#include "absl/strings/ascii.h"
 #include "absl/strings/match.h"
 #include "absl/strings/string_view.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
@@ -99,13 +99,6 @@
 // the fields with respect to the given TimeZone.
 bool ParseTime(absl::string_view format, absl::string_view input,
                absl::TimeZone tz, absl::Time* time, std::string* err) {
-  auto strip_leading_space = [](absl::string_view* sv) {
-    while (!sv->empty()) {
-      if (!std::isspace(sv->front())) return;
-      sv->remove_prefix(1);
-    }
-  };
-
   // Portable toolchains means we don't get nice constexpr here.
   struct Literal {
     const char* name;
@@ -116,12 +109,12 @@
       {kInfiniteFutureStr, strlen(kInfiniteFutureStr), InfiniteFuture()},
       {kInfinitePastStr, strlen(kInfinitePastStr), InfinitePast()},
   };
-  strip_leading_space(&input);
+  input = StripLeadingAsciiWhitespace(input);
   for (const auto& lit : literals) {
     if (absl::StartsWith(input, absl::string_view(lit.name, lit.size))) {
       absl::string_view tail = input;
       tail.remove_prefix(lit.size);
-      strip_leading_space(&tail);
+      tail = StripLeadingAsciiWhitespace(tail);
       if (tail.empty()) {
         *time = lit.value;
         return true;

absl/time/format_test.cc

diff --git a/absl/time/format_test.cc b/absl/time/format_test.cc
index 0e145a8..38c0e3d 100644
--- a/absl/time/format_test.cc
+++ b/absl/time/format_test.cc
@@ -250,6 +250,13 @@
   EXPECT_FALSE(
       absl::ParseTime("%Y", std::string("2026\0payload", 12), &t, &err));
   EXPECT_THAT(err, HasSubstr("Illegal trailing data"));
+
+  // High-bit character test for sign-extension bugs.
+  for (int i = 128; i < 256; ++i) {
+    char c = static_cast<char>(i);
+    std::string input = std::string(1, c) + "2015-01-02";
+    EXPECT_FALSE(absl::ParseTime("%Y-%m-%d", input, &t, &err));
+  }
 }
 
 TEST(ParseTime, ExtendedSeconds) {