`Out-String -Stream` unexpectedly does not split multi-line input strings into individual lines too

^{Note: Originally submitted as a suggestion, now reclassified as as bug and rewritten accordingly.}

Due to a presumed oversight, input objects to Out-String -Stream that happen to be multi-line strings are not split into individual lines, even though doing so is the purpose of the -Stream switch - see this Stack Overflow answer for background information.

Steps to reproduce

# Pass a single 3-line string, whose lines should be emitted individually.
"a`nb`nc" | Out-String -Stream | Should -Be a, b, c

Note: As with Get-Content, CR, CRLF, and LF newlines should be recognized interchangeably.

Expected behavior

The test should succeed.

Actual behavior

The test fails:

Expected @('a', 'b', 'c'), but got 'a b c'

That is, the multi-line input string is passed through as-is (Pester converts it to a single-line representation for the sake of a concise error message).

Environment data

PowerShell Core 7.2.0-preview.2

[mklement0]

Thanks, @iSazonov.

Yes, the behavior is consistent with Windows PowerShell, but all that means is that the bug is present there too. While there is buggy / questionable legacy behavior that users indeed either (a) have taken advantage of or (b) have employed workarounds for that a fix would break, neither applies in this case:

• The bug results in unequivocally unhelpful behavior.
• Existing workarounds would not break if we fix this (the workaround is to apply line-splitting manually, which would simple be a (then-unneeded) no-op).

Therefore, I think a fix for 7.2 is perfectly appropriate.

To save others having to follow the link in my previous comment about the intent behind and current behavior of -Stream:

• Out-String by default emits a single multi-line string that is the formatted string representation of all of its input, as you would see it in the console, including a trailing newline (which arguably shouldn’t be added, but that’s a separate discussion - see https://github.com/PowerShell/PowerShell/issues/14444).

• If you add -Stream, the lines of this string are output one by one - except if multi-line strings happen to be among the input objects, which are not split into their lines (they are output as is); this is the bug at hand.

That is, even the lines that make up the formatting-system’s string representation of a single object are output individually; e.g.:

# Use Out-String -Stream and enclose all resulting lines in [...]
PS> Get-Item / | Out-String -Stream | ForEach-Object { "[$($_.TrimEnd())]" }
[]
[]
[    Directory:]
[]
[]
[Mode                 LastWriteTime         Length Name]
[----                 -------------         ------ ----]
[d--hs-         1/21/2021   9:40 AM                C:\]
[]
[]

[mklement0]

Good point, @iSazonov - can you please relabel the issue?