Minimal optional constraints (#7422)

* Parse optional witness syntax

* Allow failing optional constraint

* Make `is` work with optional constraint

* Allow using optional constraint in checked if statements

* Fix tests

* Make it work with structs

* Fix MSVC build error

* Disallow using `as` with optional constraints

* Update test to match split is/as errors

* Add tests

* Fix uninitialized variables in tests

* Add tests of incorrect uses & fix related bugs

* Mention optional constraints in docs

* format code

* Fix type unification with NoneWitness

* Fix formatting

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>
Co-authored-by: Nathan V. Morrical <natemorrical@gmail.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/docs/user-guide/06-interfaces-generics.md b/docs/user-guide/06-interfaces-generics.md
index bb19fd776..7c957f685 100644
--- a/docs/user-guide/06-interfaces-generics.md
+++ b/docs/user-guide/06-interfaces-generics.md
@@ -144,6 +144,28 @@ struct MyType<T, U>
 }
 ```
 
+Optional conformances can be expressed compactly using the `where optional` syntax:
+```csharp
+// Together, these two overloads...
+int myGenericMethod<T>(T arg)
+{
+}
+
+int myGenericMethod<T>(T arg) where T: IFoo
+{
+    arg.myMethod(1.0);
+}
+
+// ... are equivalent to:
+int myGenericMethod<T>(T arg) where optional T: IFoo
+{
+    if (T is IFoo)
+    {
+        arg.myMethod(1.0); // OK in a block that checks for T: IFoo conformance.
+    }
+}
+```
+
 Supported Constructs in Interface Definitions
 -----------------------------------------------------