From d655302465457c5d3285ae5339201a0769cc38dc Mon Sep 17 00:00:00 2001
From: Yong He <yonghe@outlook.com>
Date: Thu, 5 Sep 2024 11:24:19 -0700
Subject: 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.
---
 source/slang/slang-check-conversion.cpp | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

(limited to 'source/slang/slang-check-conversion.cpp')

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.
         //
-- 
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