Enable fbstring test in cmake builds

Summary: [Folly] Enable `fbstring` test in cmake builds. Fix an msvc build failure which would otherwise prevent enabling this test.

Reviewed By: Orvid

Differential Revision: D14595828

fbshipit-source-id: fbc2109830f2b085dffa33d8e61cd7c5e14b1215

CMakeLists.txt

@@ -774,6 +774,7 @@ if (BUILD_TESTS)
       TEST exception_test SOURCES ExceptionTest.cpp
       TEST exception_wrapper_test SOURCES ExceptionWrapperTest.cpp
       TEST expected_test SOURCES ExpectedTest.cpp
+      TEST fbstring_test SOURCES FBStringTest.cpp
       TEST fbvector_test SOURCES FBVectorTest.cpp
       TEST file_test SOURCES FileTest.cpp
       # Open-source linux build can't handle running this.

folly/FBString.h

@@ -2729,7 +2729,7 @@ operator>>(
       _istream_type;
   typename _istream_type::sentry sentry(is);
   size_t extracted = 0;
-  auto err = _istream_type::goodbit;
+  typename _istream_type::iostate err = _istream_type::goodbit;
   if (sentry) {
     auto n = is.width();
     if (n <= 0) {