[ADT] Deprecate llvm::NoneType

[kazu]

Summary

I've migrated all known uses of NoneType to std::nullopt_t. This
patch deprecates NoneType.

I'm using "typedef" instead of "using" because somehow
"[[deprecated]]" and "using" do not seem to get along.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

[MaskRay]

LGTM. using can be used as well, but the attribute is at a different place.

MSVC supports deprecated for using/typedef but it doesn't warn about function arguments ([Facepalm]). g++/clang++ are good.

using NoneType [[depercated("xxx")]] = std::nullopt_t;
[[depercated("xxx")]] typedef std::nullopt_t NoneType2;

void foo(NoneType a) {}
void foo2(NoneType2 a) {}

void use() {
    foo(std::nullopt);
    foo2(std::nullopt);
}

llvm/include/llvm/ADT/None.h

25–26

typo: std::nullopt_t

I think the period can be omitted.

[kazu]

In D138623#3948072, @MaskRay wrote:

LGTM. using can be used as well, but the attribute is at a different place.

MSVC supports deprecated for using/typedef but it doesn't warn about function arguments ([Facepalm]). g++/clang++ are good.

Ah, thank you for the explanation (and the review, of course)!

llvm/include/llvm/ADT/None.h

 ///  of various (usually templated) types to make such construction more
 ///  terse.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_ADT_NONE_H
 #define LLVM_ADT_NONE_H
 
+#include "llvm/Support/Compiler.h"
 #include <optional>
 
 namespace llvm {
 /// A simple null object to allow implicit construction of Optional<T>
 /// and similar types without having to spell out the specialization's name.
-using NoneType = std::nullopt_t;
+LLVM_DEPRECATED("Use std::nullopt_t instead", "std::nullopt_t")
+typedef std::nullopt_t NoneType;
 inline constexpr std::nullopt_t None = std::nullopt;
 }
 
 #endif