fixes #16270 (#18388)

compiler/ast.nim

@@ -499,7 +499,7 @@ type
     nfFirstWrite# this node is a first write
 
   TNodeFlags* = set[TNodeFlag]
-  TTypeFlag* = enum   # keep below 32 for efficiency reasons (now: ~40)
+  TTypeFlag* = enum   # keep below 32 for efficiency reasons (now: 43)
     tfVarargs,        # procedure has C styled varargs
                       # tyArray type represeting a varargs list
     tfNoSideEffect,   # procedure type does not allow side effects
@@ -566,6 +566,7 @@ type
       # (for importc types); type is fully specified, allowing to compute
       # sizeof, alignof, offsetof at CT
     tfExplicitCallConv
+    tfIsConstructor
 
   TTypeFlags* = set[TTypeFlag]
 

compiler/semexprs.nim

@@ -2412,6 +2412,7 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
 proc semSetConstr(c: PContext, n: PNode): PNode =
   result = newNodeI(nkCurly, n.info)
   result.typ = newTypeS(tySet, c)
+  result.typ.flags.incl tfIsConstructor
   if n.len == 0:
     rawAddSon(result.typ, newTypeS(tyEmpty, c))
   else:

compiler/sigmatch.nim

@@ -1341,8 +1341,13 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
         result = isSubtype
       else:
         result = typeRel(c, f[0], a[0], flags)
-        if result <= isConvertible:
-          result = isNone     # BUGFIX!
+        if result < isGeneric:
+          if result <= isConvertible:
+            result = isNone
+          elif tfIsConstructor notin a.flags:
+            # set constructors are a bit special...
+            result = isNone
+
   of tyPtr, tyRef:
     skipOwned(a)
     if a.kind == f.kind: