Fix issue with synthesizing `Differential` type for self-differential generic types (#4602)

* Fix issue with synthesizing `Differential` type for self-differential generic types

The problem was that we were using the type that was performing the lookup for `.Differential` which can have substitutions based on the local context where the decl is being referenced.

We need to synthesize the type local to the decl itself

* Update auto-differential-type-generic.slang

1 files changed, 6 insertions, 1 deletions

diff --git a/source/slang/slang-check-expr.cpp b/source/slang/slang-check-expr.cpp
index ee36a21fb..f60ead1e9 100644
--- a/source/slang/slang-check-expr.cpp
+++ b/source/slang/slang-check-expr.cpp
@@ -638,7 +638,12 @@ namespace Slang
                     auto typeDef = m_astBuilder->create<TypeAliasDecl>();
                     typeDef->nameAndLoc.name = item.declRef.getName();
                     typeDef->parentDecl = parent;
-                    typeDef->type.type = subType;
+
+                    // Compute the decl's type as if it is referred to from itself. This is important because
+                    // subType may have substitutions from the context it is used in, while this synthesis step
+                    // is local to the decl.
+                    // 
+                    typeDef->type.type = calcThisType(subType->getDeclRef().getDecl()->getDefaultDeclRef());
                     
                     synthesizedDecl = parent;