As seen in <https://build.webkit.org/builders/WinCairo%2064-Bit%20Release/builds/52452/steps/compile-webkit/logs/stdio> and <https://build.webkit.org/builders/Apple%20Win%20Release%20%28Build%29/builds/74905/steps/compile-webkit/logs/stdio>, the copy of StdLibExtras.h in WebKitBuild\Release\DerivedSources\ForwardingHeaders\wtf did not contain the changes made by r194496, causing a build failure. In the case of the Apple Windows build, this issue was resolved by deleting the build directory and forcing a clean rebuild.