'unordered_map' file not found error when compiling with Xcode 7.3.1

A file in my project fails to compile with

'unordered_map' file not foundIn file included from ...

This is despite I compile with -std=c++ and a directory that has unordered_map in it is in system include paths.

clang -std=c++14 -x c++ -v -E /dev/null

results in the output below. /Applications/Xcode_7.http://ift.tt/2ehLkIn does contain unordered_map.

Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.6.0
Thread model: posix
InstalledDir: /Applications/Xcode_7.http://ift.tt/2dO8gTo
"/Applications/Xcode_7.http://ift.tt/2ehK1JG" -cc1 -triple x86_64-apple-macosx10.11.0 -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -E -disable-free -disable-llvm-verifier -main-file-name null -mrelocation-model pic -pic-level 2 -mthread-model posix -mdisable-fp-elim -masm-verbose -munwind-tables -target-cpu core2 -target-linker-version 264.3.102 -v -dwarf-column-info -resource-dir /Applications/Xcode_7.http://ift.tt/2dO5zkZ -stdlib=libc++ -std=c++14 -fdeprecated-macro -fdebug-compilation-dir /Users/ovz -ferror-limit 19 -fmessage-length 80 -stack-protector 1 -fblocks -fobjc-runtime=macosx-10.11.0 -fencode-extended-block-signature -fcxx-exceptions -fexceptions -fmax-type-align=16 -fdiagnostics-show-option -fcolor-diagnostics -o - -x c++ /dev/null
clang -cc1 version 7.3.0 (clang-703.0.31) default target x86_64-apple-darwin15.6.0
ignoring nonexistent directory "/usr/include/c++/v1"
#include "..." search starts here: 
#include <...> search starts here: /Applications/Xcode_7.http://ift.tt/2ehLkIn
/usr/local/include
/Applications/Xcode_7.http://ift.tt/2ehMPGA
/Applications/Xcode_7.http://ift.tt/2dO8MRt
/usr/include
/System/Library/Frameworks (framework directory)
/Library/Frameworks (framework directory)
End of search list.