[llvm] [ADT] Allow std::next to work on BitVector's set_bits_iterator (PR #80830)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-adt

Author: Jay Foad (jayfoad)

<details>
<summary>Changes</summary>

Without this I would hit errors with libstdc++-12 like:

/usr/include/c++/12/bits/stl_iterator_base_funcs.h:230:5: note: candidate template ignored: substitution failure [with _InputIterator = llvm::const_set_bits_iterator_impl<llvm::BitVector>]: argument may not have 'void' type
    next(_InputIterator __x, typename
    ^


---
Full diff: https://github.com/llvm/llvm-project/pull/80830.diff


2 Files Affected:

- (modified) llvm/include/llvm/ADT/BitVector.h (+1-1) 
- (modified) llvm/unittests/ADT/BitVectorTest.cpp (+3) 


``````````diff
diff --git a/llvm/include/llvm/ADT/BitVector.h b/llvm/include/llvm/ADT/BitVector.h
index e0de1afcc94182..0eaa77b6dd81ce 100644
--- a/llvm/include/llvm/ADT/BitVector.h
+++ b/llvm/include/llvm/ADT/BitVector.h
@@ -42,7 +42,7 @@ template <typename BitVectorT> class const_set_bits_iterator_impl {
 
 public:
   using iterator_category = std::forward_iterator_tag;
-  using difference_type   = void;
+  using difference_type   = std::ptrdiff_t;
   using value_type        = int;
   using pointer           = value_type*;
   using reference         = value_type&;
diff --git a/llvm/unittests/ADT/BitVectorTest.cpp b/llvm/unittests/ADT/BitVectorTest.cpp
index e00e11e4655aa0..79d5dee5ad30fd 100644
--- a/llvm/unittests/ADT/BitVectorTest.cpp
+++ b/llvm/unittests/ADT/BitVectorTest.cpp
@@ -1143,6 +1143,9 @@ TYPED_TEST(BitVectorTest, EmptyVectorGetData) {
 }
 
 TYPED_TEST(BitVectorTest, Iterators) {
+  TypeParam Singleton(1);
+  EXPECT_EQ(std::next(Singleton.set_bits_begin()), Singleton.set_bits_end());
+
   TypeParam Filled(10, true);
   EXPECT_NE(Filled.set_bits_begin(), Filled.set_bits_end());
   unsigned Counter = 0;

``````````

</details>


https://github.com/llvm/llvm-project/pull/80830