Add newer LLVM test, with libc++

Author: alugowski

.github/workflows/tests.yml

@@ -17,6 +17,12 @@ jobs:
             description: "LLVM 5"
             llvm-version: "5"
 
+          - os: ubuntu-22.04
+            # select libc++ as libstdc++ appears to be the default
+            description: "LLVM 16 libc++"
+            llvm-version: "16"
+            cmake-flags: '-DCMAKE_CXX_FLAGS="-stdlib=libc++" -DCMAKE_EXE_LINKER_FLAGS="-lc++abi"'
+
           - os: ubuntu-20.04
             description: "GCC 7"
             gcc-version: "7"

README.md

@@ -14,17 +14,18 @@ std::sort(std::execution::par, vec.begin(), vec.end());
     //    ^^^^^^^^^^^^^^^^^^^ native C++17 parallel Execution Policy      
 ```
 
-Unfortunately compiler support [varies](https://en.cppreference.com/w/cpp/compiler_support/17):
-
-|              |    Linux     |    macOS     |   Windows    |
-|:-------------|:------------:|:------------:|:------------:|
-| GCC 8-       |      ❌      |     ❌       |      ❌      |
-| GCC 9+       | TBB Required | TBB Required | TBB Required |
-| Clang        | TBB Required | TBB Required | TBB Required |
-| Apple Clang  |              |      ❌      |              |
-| MSVC         |              |              |      ✅      |
+Unfortunately compiler support [varies](https://en.cppreference.com/w/cpp/compiler_support/17). Quick summary of compilers' default standard libraries:
+
+|                   |    Linux     |    macOS     |   Windows    |
+|:------------------|:------------:|:------------:|:------------:|
+| GCC 8-            |      ❌      |      ❌      |      ❌      |
+| GCC 9+            | TBB Required | TBB Required | TBB Required |
+| Clang (libstdc++) | TBB Required | TBB Required | TBB Required |
+| Clang (libc++)    |      ❌      |      ❌      |      ❌      |
+| Apple Clang       |              |      ❌      |              |
+| MSVC 15.7+ (2017) |              |              |      ✅      |
 | [Parallel STL](https://www.intel.com/content/www/us/en/developer/articles/guide/get-started-with-parallel-stl.html) | TBB Required | TBB Required | TBB Required |
-| **poolSTL**  |      ✅      |      ✅      |      ✅      |
+| **poolSTL**       |      ✅      |      ✅      |      ✅      |
 
 PoolSTL is a *supplement* to fill in the support gaps. It is small, easy to integrate, and has no external dependencies.
 Note that poolSTL is not a full implementation; only the basics are covered.