Add C++17 version for Mac. (#635)

.github/workflows/main.yml

@@ -44,6 +44,10 @@ jobs:
             dependencies: "rm -f /usr/local/bin/2to3 ; brew update && brew install ninja"
           - os: "macos-12"
             dependencies: "rm -f /usr/local/bin/2to3 ; brew update && brew install ninja"
+          - os: "macos-12"
+            variant: "C++17"
+            dependencies: "rm -f /usr/local/bin/2to3 ; brew update && brew install ninja"
+            cmake-flags: "-DSNMALLOC_USE_CXX17=ON"
           # Add the self-host build, using the bounds-checked memcpy in
           # maximally paranoid mode (checking loads and stores)
           - os: "ubuntu-latest"

src/snmalloc/backend_helpers/buddy.h

@@ -15,9 +15,9 @@ namespace snmalloc
   template<typename Rep, size_t MIN_SIZE_BITS, size_t MAX_SIZE_BITS>
   class Buddy
   {
-    std::array<RBTree<Rep>, MAX_SIZE_BITS - MIN_SIZE_BITS> trees;
+    std::array<RBTree<Rep>, MAX_SIZE_BITS - MIN_SIZE_BITS> trees{};
     // All RBtrees at or above this index should be empty.
-    size_t empty_at_or_above = 0;
+    size_t empty_at_or_above{0};
 
     size_t to_index(size_t size)
     {