diff --git a/compiler/semobjconstr.nim b/compiler/semobjconstr.nim
index eb9b2edad3..3ef1ca7ac2 100644
--- a/compiler/semobjconstr.nim
+++ b/compiler/semobjconstr.nim
@@ -82,6 +82,7 @@ proc semConstrField(c: PContext, flags: TExprFlags,
     var initValue = semExprFlagDispatched(c, assignment[1], flags, field.typ)
     if initValue != nil:
       initValue = fitNodeConsiderViewType(c, field.typ, initValue, assignment.info)
+    initValue.flags.incl nfSkipFieldChecking
     assignment[0] = newSymNode(field)
     assignment[1] = initValue
     assignment.flags.incl nfSem
diff --git a/tests/generics/m3770.nim b/tests/generics/m3770.nim
index 2c6a2bd113..7f5714a2b1 100644
--- a/tests/generics/m3770.nim
+++ b/tests/generics/m3770.nim
@@ -1,6 +1,11 @@
 type
   Noice* = object
     hidden: int
-  
+
 template jjj*: Noice =
-  Noice(hidden: 15)
\ No newline at end of file
+  Noice(hidden: 15)
+
+type Opt* = object
+  o: int
+
+template none*(O: type Opt): Opt = Opt(o: 0)
diff --git a/tests/generics/t3770.nim b/tests/generics/t3770.nim
index fa9c97df83..ffccbeeb55 100644
--- a/tests/generics/t3770.nim
+++ b/tests/generics/t3770.nim
@@ -7,3 +7,7 @@ proc someGeneric(_: type) =
   doAssert $jjj() == "(hidden: 15)" # fails: "Error: the field 'hidden' is not accessible."
 
 someGeneric(int)
+
+# bug #20900
+proc c(y: int | int, w: Opt = Opt.none) = discard
+c(0)