more advanced fake covariance

tests/generics/tfakecovariance.nim

@@ -1,11 +1,56 @@
+template accept(x) =
+  static: assert(compiles(x))
+
+template reject(x) =
+  static: assert(not compiles(x))
+
 type
   BaseObj = object of RootObj
   DerivedObj = object of BaseObj
+  NonDerivedObj = object
 
   Container[T] = object
 
-proc doSomething(c: Container[BaseObj or DerivedObj]) = discard
+var base: BaseObj
+var derived: DerivedObj
+var nonDerived: NonDerivedObj
 
-var t: Container[DerivedObj]
-doSomething t
+var baseContainer: Container[BaseObj]
+var derivedContainer: Container[DerivedObj]
+var nonDerivedContainer: Container[NonDerivedObj]
+
+# We can fake covariance by listing some specific derived types that
+# will be allowed with our overload. This is not a real covariance,
+# because there will be multiple instantiations of the proc, but for
+# many purposes, it will suffice:
+
+proc wantsSpecificContainers(c: Container[BaseObj or DerivedObj]) = discard
+
+accept wantsSpecificContainers(baseContainer)
+accept wantsSpecificContainers(derivedContainer)
+
+reject wantsSpecificContainers(nonDerivedContainer)
+reject wantsSpecificContainers(derived)
+
+# Now, let's make a more general solution able to catch all derived types:
+
+type
+  DerivedFrom[T] = concept type A
+    var derived: ref A
+    var base: ref T = derived
+
+proc wantsDerived(x: DerivedFrom[BaseObj]) = discard
+
+accept wantsDerived(base)
+accept wantsDerived(derived)
+
+reject wantsDerived(nonDerived)
+reject wantsDerived(baseContainer)
+
+proc wantsDerivedContainer(c: Container[DerivedFrom[BaseObj]]) = discard
+
+accept wantsDerivedContainer(baseContainer)
+accept wantsDerivedContainer(derivedContainer)
+
+reject wantsDerivedContainer(nonDerivedContainer)