Allow calling a generic function with an existential value (dynamic dispatch) (#1508)

* Allow calling a generic function with an existential value (dynamic dispatch).

* Fixes per review comments.

* Clean up implementation by having `openExistential` return `ExtractExistentialType` instead of a DeclRef to the interface with a `ThisTypeSubstitution`.

* More cleanups

Co-authored-by: Tim Foley <tfoleyNV@users.noreply.github.com>
Co-authored-by: Yong He <yhe@nvidia.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/source/slang/slang-check-conformance.cpp b/source/slang/slang-check-conformance.cpp
index 4dcfb3065..e1904dc71 100644
--- a/source/slang/slang-check-conformance.cpp
+++ b/source/slang/slang-check-conformance.cpp
@@ -268,6 +268,30 @@ namespace Slang
                 }
             }
         }
+        else if (auto extractExistentialType = as<ExtractExistentialType>(subType))
+        {
+            // An ExtractExistentialType from an existential value of type I
+            // is a subtype of I.
+            // We need to check and make sure the interface type of the `ExtractExistentialType`
+            // is equal to `superType`.
+            auto interfaceDeclRef = extractExistentialType->interfaceDeclRef;
+            auto thisTypeSubst = findThisTypeSubstitution(interfaceDeclRef.substitutions.substitutions, interfaceDeclRef.getDecl());
+            SLANG_ASSERT(thisTypeSubst && thisTypeSubst == interfaceDeclRef.substitutions.substitutions);
+            // The interfaceDeclRef in `extractExistentialType` contains a `ThisTypeSubstitution`
+            // to allow member lookup to return correct substituted types. Here we just need
+            // to know if that interface is the same as the superType, so we need to exclude
+            // the `ThisTypeSubstitution` from comparison.
+            interfaceDeclRef.substitutions.substitutions = thisTypeSubst->outer;
+            if (interfaceDeclRef.equals(superTypeDeclRef))
+            {
+                if (outWitness)
+                {
+                    *outWitness = thisTypeSubst->witness;
+                }
+                return true;
+            }
+            return false;
+        }
         else if(auto taggedUnionType = as<TaggedUnionType>(subType))
         {
             // A tagged union type conforms to an interface if all of