Check trait constructor for accessibility even if not called at Typer

This allows to restrict inheritance of traits even if the constructor is not
called, or not called at Typer.

Fixes #17089

Author: odersky

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

@@ -2511,6 +2511,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           checkSimpleKinded(parent)
             // allow missing type parameters if there are implicit arguments to pass
             // since we can infer type arguments from them
+        val constr = psym.primaryConstructor
+        if constr.exists then
+          ensureAccessible(constr.termRef, superAccess = true, tree.srcPos)
       else
         checkParentCall(result, cls)
       if cls is Case then

tests/neg/i17089.scala

@@ -0,0 +1,4 @@
+object o:
+  trait T private[o]()
+
+def test = new o.T { } // error