This issue has two distinct aspects:

• discussion of an existing documentation bug
• discussion of the problematic fixed default file encoding currently (alpha16) chosen for Core.

Steps to reproduce

'ö' | Set-Content -NoNewline -Encoding ASCII tmp.txt 
'ö' | Add-Content -Encoding ASCII -NoNewline tmp.txt 
Get-Content -Encoding ASCII tmp.txt
(Get-Content -Encoding Byte -TotalCount 2 tmp.txt) | % { '0x{0:x}' -f $_ }
'--'
'ö' | Set-Content -NoNewline tmp.txt   # use default encoding
'ö' | Add-Content -NoNewline tmp.txt   # use default encoding
Get-Content tmp.txt                    # use default encoding
(Get-Content -Encoding Byte -TotalCount 2 tmp.txt) | % { '0x{0:x}' -f $_ }

Expected behavior

??
0x3f
0x3f
--
??
0x3f
0x3f

Actual behavior

??
0x3f
0x3f
--
öö
0xf6
0xf6

That is, ASCII encoding turns a non-ASCII character into literal ? (0x3f)

The fact that Set-Content without an -Encoding argument resulted in ö on reading implies that ASCII encoding wasn't used, and the specific byte value of 0xf6 further implies that that a single-byte, extended-ASCII encoding was used:

• For Windows PowerShell, it is the respective system's legacy codepage ("ANSI"), such as Windows-1252 on US-English systems, or Windows-1251 on Russian systems. In other words: the specific encoding is, to put it in Unix terms, locale-dependent.

• For PowerShell Core, as of alpha 16, it is ISO-8859-1, as @iSazonov helpfully points out (see his comment below for the source-code links).

  • Using a fixed encoding that is limited to 256 code points is problematic, however.
  • See @iSazonov's comment below and the discussion of the RFC about default file encodings.

In contrast, Get-Help Set-Content, Get-Help Add-Content, and Get-Help Get-Content state for parameter -Encoding:

Specifies the file encoding. The default is ASCII.

The help-topic sources (branch live) for the relevant cmdlets can be found here.

Additionally:

• While these cmdlets accept an encoding identifier Default, as used in other cmdlets, the help only mentions String.

• Given that the two appear to result in the same encoding - what is their relationship?

• The description for encoding String in the online help is inadequate:

Uses the encoding type for a string.

Environment data

PowerShell Core v6.0.0-alpha (v6.0.0-alpha.16) on Microsoft Windows 10 Pro (64-bit; v10.0.14393)