First attempt

script/prepare-llvm-mingw.sh

@@ -38,8 +38,8 @@ echo -n " -DSTAGE0_CMAKE_C_COMPILER=clang -DSTAGE0_CMAKE_CXX_COMPILER=clang++"
 echo -n " -DLEAN_EXTRA_CXX_FLAGS='--sysroot $PWD/llvm -idirafter /clang64/include/'"
 echo -n " -DLEANC_INTERNAL_FLAGS='--sysroot ROOT -nostdinc -isystem ROOT/include/clang' -DLEANC_CC=ROOT/bin/clang.exe"
 echo -n " -DLEANC_INTERNAL_LINKER_FLAGS='-L ROOT/lib -static-libgcc -Wl,-Bstatic -lgmp $(pkg-config --static --libs libuv) -lunwind -Wl,-Bdynamic -fuse-ld=lld'"
-# when not using the above flags, link GMP dynamically/as usual
-echo -n " -DLEAN_EXTRA_LINKER_FLAGS='-lgmp $(pkg-config --libs libuv) -lucrtbase'"
+# when not using the above flags, link GMP dynamically/as usual. Always link ICU dynamically.
+echo -n " -DLEAN_EXTRA_LINKER_FLAGS='-licu -lgmp $(pkg-config --libs libuv) -lucrtbase'"
 # do not set `LEAN_CC` for tests
 echo -n " -DAUTO_THREAD_FINALIZATION=OFF -DSTAGE0_AUTO_THREAD_FINALIZATION=OFF"
 echo -n " -DLEAN_TEST_VARS=''"

src/CMakeLists.txt

@@ -297,6 +297,21 @@ if(NOT LEAN_STANDALONE)
   string(APPEND LEAN_EXTRA_LINKER_FLAGS " ${LIBUV_LIBRARIES}")
 endif()
 
+# Windows SDK (for ICU)
+if("${CMAKE_SYSTEM_NAME}" MATCHES "Windows")
+  find_package(WindowsSDK REQUIRED)
+  get_windowssdk_include_dirs(${WINDOWSSDK_LATEST_DIR} WINDOWSSDK_INCLUDE_DIR)
+
+  # To successfully build against Windows SDK headers, the Windows SDK headers must have lower
+  # priority than other system headers, so use `-idirafter`. Unfortunately, CMake does not
+  # support this using `include_directories`.
+  string(APPEND CMAKE_CXX_FLAGS " -idirafter \"${WINDOWSSDK_INCLUDE_DIR}\"")
+  set(WINDOWSSDK_LIBRARIES "-licu")
+  if(NOT LEAN_STANDALONE)
+    string(APPEND LEAN_EXTRA_LINKER_FLAGS " ${WINDOWSSDK_LIBRARIES}")
+  endif()
+endif()
+
 # ccache
 if(CCACHE AND NOT CMAKE_CXX_COMPILER_LAUNCHER AND NOT CMAKE_C_COMPILER_LAUNCHER)
   find_program(CCACHE_PATH ccache)