Fix erroneous diagnostic when checking a generic differentiable mutable method. (#3192)

* Add a unit test

* Fix.

---------

Co-authored-by: Lifan Wu <lifanw@nvidia.com>
Co-authored-by: Yong He <yhe@nvidia.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/source/slang/slang-check-decl.cpp b/source/slang/slang-check-decl.cpp
index 76ab8655a..1ebd50cbd 100644
--- a/source/slang/slang-check-decl.cpp
+++ b/source/slang/slang-check-decl.cpp
@@ -4869,7 +4869,8 @@ namespace Slang
             maybeRegisterDifferentiableType(m_astBuilder, decl->returnType.type);
             if (as<ConstructorDecl>(decl) || !isEffectivelyStatic(decl))
             {
-                auto parentDeclRef = createDefaultSubstitutionsIfNeeded(m_astBuilder, this, makeDeclRef(decl->parentDecl));
+                auto parentDecl = getParentDecl(decl);
+                auto parentDeclRef = createDefaultSubstitutionsIfNeeded(m_astBuilder, this, makeDeclRef(parentDecl));
                 auto thisType = calcThisType(parentDeclRef);
                 maybeRegisterDifferentiableType(m_astBuilder, thisType);
             }