`open` does not open all overloaded namespaces

Prerequisites

Please put an X between the brackets as you perform the following steps:

• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

Consider the following code:

import Lean

def A.B.foo1 := 3
def B.foo2 := 3

section Test1
namespace A
-- ... [Lean.Syntax.Preresolved.namespace `A.B]
#eval `(ident|B)
end A
end Test1

section Test2
open A
-- ... [Lean.Syntax.Preresolved.namespace `B, Lean.Syntax.Preresolved.namespace `A.B]
#eval `(ident|B)
end Test2

Note that the first identifier B produces a syntax that contains only one preresolved namespace A.B, which is wrong, while the second identifier B produces two preresolved namespaces B and A.B, which is the correct result.

This leads to the fact that open B in the first section not allowing foo2, which is wrong:

import Lean

def A.B.foo1 := 3
def B.foo2 := 3

section Test1
namespace A
open B
#check foo2 -- Unknown identifier
end A
end Test1

section Test2
open A
open B
#check foo2
end Test2

Context

Zulip: lean4 > open not opening a namespace

Versions

Lean 4.26.0-nightly-2025-10-29
Target: x86_64-unknown-linux-gnu

Impact

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Start by running the minimal reproducer against Lean nightly and compare the preresolved namespaces shown for #eval (ident|B)inside and outsidenamespace A. Use the two #check foo2cases as the regression criteria:open BinsideAshould resolvefoo2` from the overloaded namespaces.