[PATCH] D151198: [llvm][ADT] Fix invlid `reference` type of depth-first, breadth-first and post order iterators

[Markus Böck via Phabricator via llvm-commits]

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C++s iterator concept requires `operator*` to return the same type as is specified by the iterators `reference` type. This functionality is especially important for older generic code that did not yet make use of `auto`. 
An example from within LLVM is `iterator_adaptor_base` which uses the `reference` type of the iterator it is wrapping as its return type for `operator*` (this class is used as base for a lot of other functionality like filter iterators and so on).
Using any of the graph traversal iterators listed above with it would previously fail to compile due to `reference` being non-const while `operator*` returned a const reference.

This patch fixes that by correctly specifying `reference` and using it as the return type of `operator*` explicitly to prevent further issues in the future.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D151198

Files:
  llvm/include/llvm/ADT/BreadthFirstIterator.h
  llvm/include/llvm/ADT/DepthFirstIterator.h
  llvm/include/llvm/ADT/PostOrderIterator.h
  llvm/unittests/ADT/BreadthFirstIteratorTest.cpp
  llvm/unittests/ADT/DepthFirstIteratorTest.cpp
  llvm/unittests/ADT/PostOrderIteratorTest.cpp

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