Support `where` clause and type equality constraint. (#4986)

* Support `where` clause.

* Fix.

* Fix parser.

* Enhance test to cover traditional __generic syntax.

* Update user-guide.

* Support `where` clause on associatedtype.

* Fix.

* Put in more comments.

1 files changed, 16 insertions, 0 deletions

diff --git a/source/slang/slang-check-conversion.cpp b/source/slang/slang-check-conversion.cpp
index 85040dc55..c0d7feaff 100644
--- a/source/slang/slang-check-conversion.cpp
+++ b/source/slang/slang-check-conversion.cpp
@@ -1019,6 +1019,22 @@ namespace Slang
                 *outCost = kConversionCost_CastToInterface;
             return true;
         }
+        else if (auto fromIsToWitness = tryGetSubtypeWitness(toType, fromType))
+        {
+            // Is toType and fromType the same via some type equality witness?
+            // If so there is no need to do any conversion.
+            //
+            if (isTypeEqualityWitness(fromIsToWitness))
+            {
+                if (outToExpr)
+                {
+                    *outToExpr = createCastToSuperTypeExpr(toType, fromExpr, fromIsToWitness);
+                }
+                if (outCost)
+                    *outCost = 0;
+                return true;
+            }
+        }
 
         // Disallow converting to a ParameterGroupType.
         //