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, 2 insertions, 2 deletions

diff --git a/tests/language-feature/interface-as-rhs-error.slang b/tests/language-feature/interface-as-rhs-error.slang
index 9ad71afde..7293b5134 100644
--- a/tests/language-feature/interface-as-rhs-error.slang
+++ b/tests/language-feature/interface-as-rhs-error.slang
@@ -21,13 +21,13 @@ struct AnotherType
 // These should produce errors - interface types as RHS
 bool testIsOperatorWithInterface<T>()
 {
-    //CHECK: ([[# @LINE+1]]): error 30301: 'is' and 'as' operators do not support interface types as the right-hand side
+    //CHECK: ([[# @LINE+1]]): error 30301: cannot use 'is' operator with an interface type as the right-hand side
     return (T is IMyInterface);
 }
 
 void testAsOperatorWithInterface<T>(T value)
 {
-    //CHECK: ([[# @LINE+1]]): error 30301: 'is' and 'as' operators do not support interface types as the right-hand side
+    //CHECK: ([[# @LINE+1]]): error 30302: cannot use 'as' operator with an interface type as the right-hand side
     let result = value as IMyInterface;
 }