Fix Windows hang in testing

.github/workflows/main_ci.yml

@@ -63,16 +63,10 @@ jobs:
       run: |
         cmake --build "${{ env.BUILD_DIR }}" --target test
 
-## XXX(RLB): Unit tests are currently disabled on Windows because of two
-## conflicting bugs.  On the one hand, doctest has a bug that causes
-## doctest_discover_tests to fail when tests are built with sanitizers.  On the
-## other hand, if tests are not built with sanitizers, then the unit tests hang
-## in the middle of the test run.
-##
-##    - name: Unit Test (Windows)
-##      if: matrix.os == 'windows-latest'
-##      run: |
-##        cmake --build "${{ env.BUILD_DIR }}" --target RUN_TESTS
+    - name: Unit Test (Windows)
+      if: matrix.os == 'windows-latest'
+      run: |
+        cmake --build "${{ env.BUILD_DIR }}" --target RUN_TESTS
 
   interop-test:
     if: github.event.pull_request.draft == false

include/mls/common.h

@@ -83,6 +83,14 @@ template<class... Ts>
 struct overloaded : Ts...
 {
   using Ts::operator()...;
+
+  // XXX(RLB) MSVC has a bug where it incorrectly computes the size of this
+  // type.  Microsoft claims they have fixed it in the latest MSVC, and GitHub
+  // claims they are running a version with the fix.  But in practice, we still
+  // hit it.  Including this dummy variable is a work-around.
+  //
+  // https://developercommunity.visualstudio.com/t/runtime-stack-corruption-using-stdvisit/346200
+  int dummy = 0;
 };
 
 // clang-format off