Inconsistent name resolution with generalized field notation in opened namespaces

Prerequisites

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Description

There seems to be some inconsistencies regarding the usage of generalized field notation in namespaces.

Opening a namespace allows the usage of generalized field notation as if the definition was made at the top level:

namespace A

def Nat.f : Nat → Nat := Function.const _ 1

/--
info: 1
-/
#guard_msgs in
open A in
#eval (1 : Nat).f


end A

With recursive definitions this does not work, however. For instance, consider

namespace A
open A
/--
error: Invalid field `f`: The environment does not contain `Nat.f`, so it is not possible to project the field `f` from an expression
  n
of type `Nat`
-/
#guard_msgs in
def Nat.f : Nat → Nat
  | 0 => 0
  | n + 1 => n.f + 1

end A

Context

On Zulip there was some discussion regarding generalized field notation for scoped definitions and during experiment I came across this limitation. Relevant Zulip topic: #new members > generalized field notation vs namespaces

Steps to Reproduce

namespace A
open A

def Nat.aux : Nat → Nat := Function.const _ 1

def Nat.fNonrecursive (x : Nat) : Nat := x.aux

def Nat.fRecursive : Nat → Nat
  | 0 => 0
  | n + 1 => n.fRecursive + 1

end A

Expected behavior: Both fNonrecursive and fRecursive compile without or error or none of them do.

Actual behavior: fNonrecursive compiles without error and the fRecursive gives the following error

/--
error: Invalid field `fRecursive`: The environment does not contain `Nat.fRecursive`, so it is not possible to project the field `fRecursive` from an expression
  n
of type `Nat`
--/

Versions

/--
info: Lean 4.29.0
Target: x86_64-unknown-linux-gnu
-/
#guard_msgs in
#version

Additional Information

[Additional information, configuration or data that might be necessary to reproduce the issue]

Impact

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Start with the minimal reproducer in the issue, comparing the nonrecursive and recursive definitions under namespace A and open A. Trace generalized field notation and name resolution for n.fNonrecursive versus n.fRecursive; done means both forms consistently compile or consistently report an error, with the reproducer's expected behavior covered by a regression test.