Expand deferred givens to lazy abstract givens

That way they can be implemented by operations of the form `given T = f(...)`, which also
map to lazy values. And strict abstract values cannot be overridden by lazy values since
that could undermine realizability and with it type soundness.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1827,7 +1827,7 @@ class Namer { typer: Typer =>
           if !sym.maybeOwner.is(Trait) then
             report.error(em"`deferred` can only be used for givens in traits", mdef.rhs.srcPos)
           else
-            sym.resetFlag(Final | Lazy)
+            sym.resetFlag(Final)
             sym.setFlag(Deferred | HasDefault)
         case _ =>
 

tests/pos/deferred-givens.scala

@@ -0,0 +1,11 @@
+import compiletime.*
+class Ord[Elem]
+
+trait B:
+  type Elem
+  given Ord[Elem] = deferred
+  def foo = summon[Ord[Elem]]
+
+class C extends B:
+  type Elem = String
+  given Ord[Elem] = ???