first implementation of overloading of '='; missing: rewriting let/var sections

compiler/ast.nim

@@ -472,7 +472,7 @@ type
                       # T and I here can bind to both typedesc and static types
                       # before this is determined, we'll consider them to be a
                       # wildcard type.
-    tfGuarded         # guarded pointer
+    tfHasAsgn         # type has overloaded assignment operator
     tfBorrowDot       # distinct type borrows '.'
 
   TTypeFlags* = set[TTypeFlag]
@@ -805,6 +805,7 @@ type
                               # mean that there is no destructor.
                               # see instantiateDestructor in semdestruct.nim
     deepCopy*: PSym           # overriden 'deepCopy' operation
+    assignment*: PSym         # overriden '=' operator
     size*: BiggestInt         # the size of the type in bytes
                               # -1 means that the size is unkwown
     align*: int16             # the type's alignment requirements
@@ -1219,6 +1220,7 @@ proc assignType*(dest, src: PType) =
   dest.align = src.align
   dest.destructor = src.destructor
   dest.deepCopy = src.deepCopy
+  dest.assignment = src.assignment
   dest.lockLevel = src.lockLevel
   # this fixes 'type TLock = TSysLock':
   if src.sym != nil:
@@ -1335,6 +1337,13 @@ proc propagateToOwner*(owner, elem: PType) =
   if elem.isMetaType:
     owner.flags.incl tfHasMeta
 
+  if tfHasAsgn in elem.flags:
+    let o2 = elem.skipTypes({tyGenericInst})
+    if o2.kind in {tyTuple, tyObject, tyArray, tyArrayConstr,
+                   tySequence, tySet, tyDistinct}:
+      o2.flags.incl tfHasAsgn
+      owner.flags.incl tfHasAsgn
+
   if owner.kind notin {tyProc, tyGenericInst, tyGenericBody,
                        tyGenericInvocation}:
     let elemB = elem.skipTypes({tyGenericInst})