Properly print prompt to stderr when err=True is passed

The prompt was still printed to stdout, even when err=True was passed.
Use redirect_stdout to fix this.

Also, update the tests to test for the new behavior.

Author: yorickvP

src/click/termui.py

@@ -7,6 +7,7 @@
 import sys
 import typing as t
 from contextlib import AbstractContextManager
+from contextlib import redirect_stdout
 from gettext import gettext as _
 
 from ._compat import isatty
@@ -147,7 +148,11 @@ def prompt_func(text: str) -> str:
             else:
                 # On non-Windows platforms, pass the full prompt to readline
                 # so it can properly handle line editing and cursor positioning
-                return f(text)
+                if err:
+                    with redirect_stdout(sys.stderr):
+                        return f(text)
+                else:
+                    return f(text)
         except (KeyboardInterrupt, EOFError):
             # getpass doesn't print a newline if the user aborts input with ^C.
             # Allegedly this behavior is inherited from getpass(3).

tests/test_utils.py

@@ -227,7 +227,8 @@ def f(_):
         click.echo("interrupted")
 
     out, err = capsys.readouterr()
-    assert out == "Password:\ninterrupted\n"
+    # On non-Windows, prompt is passed directly to getpass, not echoed separately
+    assert out == "\ninterrupted\n"
 
 
 def test_prompts_eof(runner):
@@ -484,8 +485,8 @@ def emulate_input(text):
     emulate_input("asdlkj\n")
     click.prompt("Prompt to stderr", err=True)
     out, err = capfd.readouterr()
-    assert out == " "
-    assert err == "Prompt to stderr:"
+    assert out == ""
+    assert err == "Prompt to stderr: "
 
     emulate_input("y\n")
     click.confirm("Prompt to stdin")