[support] ParentIndexIterator: Add subscript and friend plus operators

`ParentIndexIterator` declares `using iterator_category =`
`std::random_access_iterator_tag;`, but did not provide the subscript
operator (`operator[]`) or friend commutative addition
(`friend operator+(difference_type, const ParentIndexIterator&)`).

When `ConstraintAnalysis: Remove older redundant constraints` (#8912)
introduced `std::sort(begin(), end())` on `AndedConstraintSet` (which
inherits from `inplace_vector`), building `binaryen` failed on the
Chromium `emscripten-releases` macOS builders with:

  error: type 'wasm::inplace_vector<...>::Iterator' does not provide
  a subscript operator

This failure did not show up on Binaryen's CI because `libstdc++`
implements `std::sort` using pointer/iterator addition and dereferencing
(`*(first + child)`). In contrast, on macOS with `-stdlib=libc++`,
`libc++` implements `std::sort` (`std::__sift_down`) using
`__first[__child]` and `__first[__start]`, which requires `operator[]`.

This commit adds `decltype(auto) operator[]` and `friend operator+` to
`ParentIndexIterator` to satisfy `RandomAccessIterator` requirements.

Are we missing CI testing for stdlib=libc++?

Author: sbc100

.github/workflows/ci.yml

@@ -127,7 +127,7 @@ jobs:
       run: python check.py --binaryen-bin=out/bin
 
   build-clang:
-    name: clang (LTO)
+    name: clang (LTO + libc++)
     runs-on: ubuntu-latest
     steps:
     - uses: actions/setup-python@v5
@@ -137,7 +137,7 @@ jobs:
       with:
         submodules: true
     - name: install ninja
-      run: sudo apt-get install ninja-build
+      run: sudo apt-get install ninja-build libc++-dev libc++abi-dev
     - name: install v8
       run: |
         npm install jsvu -g
@@ -147,7 +147,7 @@ jobs:
     - name: cmake
       run: |
         mkdir -p out
-        cmake -S . -B out -G Ninja -DCMAKE_INSTALL_PREFIX=out/install -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DBYN_ENABLE_LTO=ON
+        cmake -S . -B out -G Ninja -DCMAKE_INSTALL_PREFIX=out/install -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DBYN_ENABLE_LTO=ON -DCMAKE_CXX_FLAGS="-stdlib=libc++" -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++"
     - name: build
       run: cmake --build out -v
     - name: test binaryen-lit

src/support/parent_index_iterator.h

@@ -24,8 +24,8 @@ namespace wasm {
 
 // A helper for defining iterators that contain references to some parent object
 // and indices into that parent object. Provides operator implementations for
-// equality, inequality, index updates, and index differences, but not for
-// dereferencing.
+// equality, inequality, index updates, index differences, and subscripting,
+// but not for dereferencing (`operator*` or `operator->`).
 //
 // Users of this utility should subclass ParentIndexIterator<...> and define the
 // following members in the subclass:
@@ -96,6 +96,10 @@ template<typename Parent, typename Iterator> struct ParentIndexIterator {
   Iterator operator+(difference_type off) const {
     return Iterator(self()) += off;
   }
+  friend Iterator operator+(difference_type off,
+                            const ParentIndexIterator& it) {
+    return it + off;
+  }
   Iterator& operator-=(difference_type off) {
     index -= off;
     return self();
@@ -107,6 +111,9 @@ template<typename Parent, typename Iterator> struct ParentIndexIterator {
     assert(parent == other.parent);
     return index - other.index;
   }
+  decltype(auto) operator[](difference_type off) const {
+    return *(self() + off);
+  }
 };
 
 } // namespace wasm

test/gtest/inplace_vector.cpp

@@ -110,3 +110,15 @@ TEST_F(InplaceVectorTest, Insert) {
   EXPECT_EQ(vec.size(), 6u);
   EXPECT_EQ(vec[5], 50);
 }
+
+TEST_F(InplaceVectorTest, SortAndIteratorOps) {
+  inplace_vector<int, 5> vec{40, 10, 50, 20, 30};
+
+  // Test iterator subscripting and friend operator+
+  EXPECT_EQ(vec.begin()[1], 10);
+  EXPECT_EQ(*(2 + vec.begin()), 50);
+
+  // Test std::sort on inplace_vector iterators
+  std::sort(vec.begin(), vec.end());
+  EXPECT_EQ(vec, (inplace_vector<int, 5>{10, 20, 30, 40, 50}));
+}