"the method must provide an equal or more permissive precondition than in its parent trait" when marking a predicate opaque

[cpitclaudel]

The code:

trait T {
  predicate Valid(x: int)
  method MyMethod(x: int) requires Valid(x)
}

class C extends T {
  predicate {:opaque} Valid(x: int) { true }
  method MyMethod(x: int) requires Valid(x) {
  }
}

Incorrectly shows the error:

the method must provide an equal or more permissive precondition than in its parent trait

Boogie code:

    assume _module.T.Valid(this, x#0);
    assert _module.C.Valid(StartFuelAssert__module.C.Valid, this, x#0);

With `{:opaque} the following axioms are missing:

// layer synonym axiom
axiom (forall $ly: LayerType, this: ref, x#0: int ::
  { _module.C.Valid($LS($ly), this, x#0) }
  _module.C.Valid($LS($ly), this, x#0) == _module.C.Valid($ly, this, x#0));

// fuel synonym axiom
axiom (forall $ly: LayerType, this: ref, x#0: int ::
  { _module.C.Valid(AsFuelBottom($ly), this, x#0) }
  _module.C.Valid($ly, this, x#0) == _module.C.Valid($LZ, this, x#0));

… but they are not enough (since we don't know the value of StartFuelAssert__module.C.Valid

Reported by @MikaelMayer .