[clang] [flang] [flang] Add UNSIGNED (PR #113504)

[Peter Klausler via cfe-commits]

================
@@ -0,0 +1,119 @@
+<!--===- docs/Unsigned.md 
+  
+   Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+   See https://llvm.org/LICENSE.txt for license information.
+   SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+  
+-->
+
+# Fortran Extensions supported by Flang
+
+```{contents}
+---
+local:
+---
+```
+
+For better compatibility with GNU Fortran and Sun Fortran,
+this compiler supports an option (`-funsigned`) that enables
+the `UNSIGNED` data type, constants, intrinsic functions,
+its use with intrinsic operations and `SELECT CASE`, and C
+language interoperability.
+
+## `UNSIGNED` type
+
+`UNSIGNED` is a numeric type with the same kinds as `INTEGER`.
+It may appear as a type-spec in any context, including
+a type declaration statement, a type-decl in an array
+constructor or `ALLOCATE` statement, `IMPLICIT`, or a
+function statement's prefix.
+
+`UNSIGNED` constants are nonempty strings of decimal digits
+followed by the letter `U` and optionally a kind suffix with
+an underscore.
+
+## `UNSIGNED` operations
+
+`UNSIGNED` operands are accepted for unary negation (`-`),
+the basic four binary arithmetic intrinsic operations `+`, `-`, `*`, and `/`,
+and for numeric relational operators.
+The power operator `**` does not accept `UNSIGNED` operands.
+
----------------
klausler wrote:

Yes, and I disagreed with that restriction, since `-x` just means `0u-x`, which is well-defined.

https://github.com/llvm/llvm-project/pull/113504