[0023] Add proposal to adopt C++11 as the base language (#314)

This proposal is for HLSL 202y to adopt C++ 11 as its base C++ language
version and encorporate as many of C++ 11's features as reasonably fit
within HLSL's language design.

proposals/0023-cxx11-base.md

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+# Adopt C++11 Base
+
+* Proposal: [NNNN](NNNN-cxx11-base.md)
+* Author(s): [Chris Bieneman](https://github.com/llvm-beanz)
+* Sponsor: TBD
+* Status: **Under Consideration**
+* Planned Version: 202y
+
+## Introduction
+
+In DXC HLSL is a set of feature extensions on top of a subset of C++98. C++98
+is now over 20 years old and most modern C++ users have adopted newer language
+constructs. This proposal suggests taking the small step of updating HLSL 202y's
+base C++ language to C++11.
+
+## Motivation
+
+C++11 is over a decade old and introduced widely adopted features, many of which
+have been frequently requested additions for HLSL.
+
+## Proposed solution
+
+Adopt a C++11 base language and include the following C++11 features in HLSL 202y:
+* auto
+* decltype
+* constexpr
+* C++11 scoped enumerations
+* variadic templates
+* user-defined literals
+* [C++11 attributes](/proposals/0002-cxx-attributes.md)
+* Lambda expressions
+* Static assert
+* Range-based for loops
+
+## Alternatives considered
+
+### C++20
+
+We could instead adopt an even more recent C++, like C++20. The main drawback of
+that is that it significantly increases the rapid divergence from DXC, and it
+gives us a longer list of features that we need to rectify against HLSL's
+language features. Adopting a C++11 base for 202y does not prevent later
+versions from adopting newer C++ base standards, but it does allow us to phase
+the changes in iteratively as HLSL evolves.
+
+### Target HLSL 202x
+
+While the original Clang 3.7 release did support C++11 fully, the intrusive
+changes to support HLSL broke many of the basic features Clang uses for
+configuring language features and supporting language modes. To restore those
+parts of clang sufficiently to support a C++11 base in DXC would be non-trivial.
+For that reason this is proposed as a Clang-only HLSL 202y feature.
+
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