Allow std::unordered_*.

base::hash_* is, as a transition step, implemented in terms of
std::unordered_*. Later commits will convert existing uses.

Also fix a host of IWYU problems that arose from this CL.

(NOPRESUBMIT because the wstring presubmit check is overzealous
and complains about the reference to wstring in the comment.)

NOPRESUBMIT=true
BUG=576864
TBR=derat@chromium.org,blundell@chromium.org,jbauman@chromium.org,dalecurtis@chromium.org
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1502373009

Cr-Commit-Position: refs/heads/master@{#370553}

1 files changed, 17 insertions, 0 deletions

diff --git a/base/strings/string16.h b/base/strings/string16.h
index e47669c..af44a5c 100644
--- a/base/strings/string16.h
+++ b/base/strings/string16.h
@@ -29,6 +29,8 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
+
+#include <functional>
 #include <string>
 
 #include "base/base_export.h"
@@ -182,6 +184,21 @@ BASE_EXPORT extern void PrintTo(const string16& str, std::ostream* out);
 extern template
 class BASE_EXPORT std::basic_string<base::char16, base::string16_char_traits>;
 
+// Specialize std::hash for base::string16. Although the style guide forbids
+// this in general, it is necessary for consistency with WCHAR_T_IS_UTF16
+// platforms, where base::string16 is a type alias for std::wstring.
+namespace std {
+template<>
+struct hash<base::string16> {
+  std::size_t operator()(const base::string16& s) const {
+    std::size_t result = 0;
+    for (base::char16 c : s)
+      result = (result * 131) + c;
+    return result;
+  }
+};
+}  // namespace std
+
 #endif  // WCHAR_T_IS_UTF32
 
 #endif  // BASE_STRINGS_STRING16_H_