[PATCH] iterator_range rebinding

[Aaron Ballman]

I have a situation that comes up fairly frequently in clang where we
have an iterator, and then a templated "specific" version of the
iterator. For instance decl_iterator and specific_decl_iterator,
attr_iterator and specific_attr_iterator, etc.

This patch is proposing a template constructor for iterator_range that
allows you to rebind an existing iterator_range of a different type.

This would allow me to do things like:

// In DeclBase.h
template <typename SpecificDecl>
using specific_decl_range =
    llvm::iterator_range<specific_decl_iterator<SpecificDecl>>;

decl_range decls() const;


// In Decl.h
typedef specific_decl_range<EnumConstantDecl> enumerator_range;

enumerator_range enumerators() const {
  const EnumDecl *E = getDefinition();
  if (!E)
    E = this;
  return enumerator_range(E->decls());
}

Is this a reasonable approach, or is there a better way?  Thanks!

~Aaron
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