Fix NameExprType returning deleted canonical type when it's in a generic parent.

fixes #339

`NamedExpressionType::CreateCanonicalType()` may return a deleted pointer. The original implementation is as follows:
```
    Type* NamedExpressionType::CreateCanonicalType()
    {
        return GetType(declRef)->GetCanonicalType();
    }
```
If `GetType()` returns a newly constructed Type (this happens when the `typedef` is defined inside a generic parent, which triggers a non-trivial substitution), the temporary type will be deleted when the function returns. The fix is to store the temporary type as a field of NamedExpressionType (`innerType`).

A relevant fix (though not the true cause of issue #339) is to have `Type::GetCanonicalType()` also hold a `RefPtr` to the constructed canonical type, when the canonical type is not `this`. This prevents a returned canonical type being assigned to a RefPtr, which makes it possible for that RefPtr to be the sole owner of the canonical type and deleteing the canonical type when that RefPtr is destroyed.

1 files changed, 2 insertions, 1 deletions

diff --git a/source/slang/type-defs.h b/source/slang/type-defs.h
index d0b2ebac1..e7310768a 100644
--- a/source/slang/type-defs.h
+++ b/source/slang/type-defs.h
@@ -420,9 +420,10 @@ END_SYNTAX_CLASS()
 
 // A type alias of some kind (e.g., via `typedef`)
 SYNTAX_CLASS(NamedExpressionType, Type)
-    DECL_FIELD(DeclRef<TypeDefDecl>, declRef)
+DECL_FIELD(DeclRef<TypeDefDecl>, declRef)
 
 RAW(
+    RefPtr<Type> innerType;
     NamedExpressionType()
     {}
     NamedExpressionType(