Safe mode rejects unchecked annotations on synthetic symbols

[noti0na1]

Compiler version

3.8.4-RC1-bin-20260331-724e9c3-NIGHTLY
3.nightly

Minimized code

Stest_1.scala

@caps.assumeSafe object A:
  def f(
      a: List[String] = List.empty,
      b: Option[String] = None,
      c: Long = 30000
    ) = ()

Stest_2.scala

import language.experimental.safe

def test(i: Int) =
  A.f(a = List("bar"), c = 1000)

Compile: scala compile -S 3.nightly -experimental Stest_1.scala Stest_2.scala

The code is written in this way to:

1. construct uncheckedVariance annotations on parameters;
2. force the desugar to lift the arguments.

Output

-- Error: Work/dotty/Stest_1.scala:116:26 ---------------------
116 |      b: Option[String] = None,
    |                          ^
    |Cannot refer to class uncheckedVariance in package scala.annotation.unchecked from safe code since it is tagged @rejectSafe
1 error found

It's also weird the error is on the first file, not on the synthesised symbol.

Expectation

Should compile