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, 2 insertions, 2 deletions

diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index 63367fa97..0cc3a55c5 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -116,11 +116,11 @@ namespace Slang
             openedThisType->witness = openedWitness;
 
             DeclRef<InterfaceDecl> substDeclRef = DeclRef<InterfaceDecl>(interfaceDecl, openedThisType);
-            auto substDeclRefType = DeclRefType::create(m_astBuilder, substDeclRef);
+            openedType->interfaceDeclRef = substDeclRef;
 
             ExtractExistentialValueExpr* openedValue = m_astBuilder->create<ExtractExistentialValueExpr>();
             openedValue->declRef = varDeclRef;
-            openedValue->type = QualType(substDeclRefType);
+            openedValue->type = QualType(openedType);
 
             return openedValue;
         });