Move from llvm::makeArrayRef to ArrayRef deduction guides - part 1

[serge-sans-paille]

Summary

Since we're now requiring C++17, Let's get rid of makeXXX functions like makeArrayRef, and use deduction guides instead.

Apart from codebase modernization, there isn't much benefit from that
move, but I can still mention that it would slightly (probably
negligibly) decrease the number of symbols / debug info, as deduction
guides don't generate new code.

The only non-automatic changes have been:

1. Write the deduction guides
2. ArrayRef(some_uint8_pointer, 0) needs to be changed into ArrayRef(some_uint8_pointer, (size_t)0) to avoid an ambiguous call with ArrayRef((uint8_t*), (uint8_t*))
3. CVSymbol sym(makeArrayRef(symStorage)); needed to be rewritten as CVSymbol sym{ArrayRef(symStorage)}; otherwise the compiler is confused and thinks we have a (bad) function prototype. There was a few similar situation across the codebase.
4. ADL doesn't seem to work the same for deductio-guides and functions, so at some point the llvm namespace must be explicitly stated.
5. The "reference mode" of makeArrayRef(ArrayRef<T> &) that acts as no-op is not supported (a constructor cannot achieve that).

[mehdi_amini]

llvm/include/llvm/ADT/ArrayRef.h

502

Can we keep the makeArrayRef functions for now and mark them deprecated?

[jpienaar]

llvm/include/llvm/ADT/ArrayRef.h

502

+1 that would also allow for this change to broken up so that the deduction guides land first and then the updates (potentially even 3 hops, 1) add guides, 2) update, 3) mark deprecated - 1 & 2 could be combined but it'll greatly simplify review/enable sharding it)

[serge-sans-paille]

+ keep deprecated version, flagged appropriately

[MaskRay]

LGTM if makeArrayRef is kept in this patch. Do we foresee compiler bugs (for supported compilers) in this area? If there may be a risk, consider migrate a part of the code base first.

[serge-sans-paille]

Split the original patch in pieces: first just introduce the deduction guide.

llvm/include/llvm/ADT/ArrayRef.h

OwningArrayRef &operator=(OwningArrayRef &&Other) {
       this->MutableArrayRef<T>::operator=(Other);
       Other.MutableArrayRef<T>::operator=(MutableArrayRef<T>());
       return *this;
     }
 
     ~OwningArrayRef() { delete[] this->data(); }
   };
 
-  /// @name ArrayRef Convenience constructors
+  /// @name ArrayRef Deduction guides
   /// @{
+  /// Deduction guide to construct an ArrayRef from a single element.
+  template <typename T> ArrayRef(const T &OneElt) -> ArrayRef<T>;
+
+  /// Deduction guide to construct an ArrayRef from a pointer and length
+  template <typename T> ArrayRef(const T *data, size_t length) -> ArrayRef<T>;
+
+  /// Deduction guide to construct an ArrayRef from a range
+  template <typename T> ArrayRef(const T *data, const T *end) -> ArrayRef<T>;
+
+  /// Deduction guide to construct an ArrayRef from a SmallVector
+  template <typename T> ArrayRef(const SmallVectorImpl<T> &Vec) -> ArrayRef<T>;
+
+  /// Deduction guide to construct an ArrayRef from a SmallVector
+  template <typename T, unsigned N>
+  ArrayRef(const SmallVector<T, N> &Vec) -> ArrayRef<T>;
+
+  /// Deduction guide to construct an ArrayRef from a std::vector
+  template <typename T> ArrayRef(const std::vector<T> &Vec) -> ArrayRef<T>;
+
+  /// Deduction guide to construct an ArrayRef from a std::array
+  template <typename T, std::size_t N>
+  ArrayRef(const std::array<T, N> &Vec) -> ArrayRef<T>;
+
+  /// Deduction guide to construct an ArrayRef from an ArrayRef (no-op) (const)
+  template <typename T> ArrayRef(const ArrayRef<T> &Vec) -> ArrayRef<T>;
+
+  /// Deduction guide to construct an ArrayRef from an ArrayRef (no-op)
+  template <typename T> ArrayRef(ArrayRef<T> &Vec) -> ArrayRef<T>;
+
+  /// Deduction guide to construct an ArrayRef from a C array.
+  template <typename T, size_t N> ArrayRef(const T (&Arr)[N]) -> ArrayRef<T>;
 
+  /// @}
+
+  /// @name ArrayRef Convenience constructors
+  /// @{
   /// Construct an ArrayRef from a single element.
   template<typename T>
   ArrayRef<T> makeArrayRef(const T &OneElt) {
     return OneElt;
   }
 
   /// Construct an ArrayRef from a pointer and length.
   template<typename T>