Stop adding a trailing line feed to clip-chomped block scalars when the stream has no final break #52

Context and request

Observed behavior: When a YAML stream ends without a final line break, a clip-chomped block scalar (the default, no - or + indicator) gains a trailing line feed that the source never contained.

"k: |`n  a`n  b" | ConvertFrom-Yaml   # -> "a`nb`n"
"k: >`n  a`n  b" | ConvertFrom-Yaml   # -> "a b`n"

Expected behavior: No trailing line feed, because there is no final break to clip.

"k: |`n  a`n  b" | ConvertFrom-Yaml   # -> "a`nb"
"k: >`n  a`n  b" | ConvertFrom-Yaml   # -> "a b"

YAML 1.2.2 defines b-chomped-last(clip) ::= b-as-line-feed | <end-of-input>. At end of input that production yields nothing, so clip chomping has no break to keep. Strip (-) and keep (+) chomping are already correct for the same inputs, and every case where the stream does end with a line break is correct.

Reproduction:

Import-Module Yaml
("k: |`n  a`n  b" | ConvertFrom-Yaml).k -replace "`n", '\n'   # actual: a\nb\n   expected: a\nb
("k: >`n  a`n  b" | ConvertFrom-Yaml).k -replace "`n", '\n'   # actual: a b\n   expected: a b
("k: |2`n  a`n  b" | ConvertFrom-Yaml).k -replace "`n", '\n'  # actual: a\nb\n   expected: a\nb
("k: |-`n  a`n  b" | ConvertFrom-Yaml).k -replace "`n", '\n'  # correct: a\nb
("k: |+`n  a`n  b" | ConvertFrom-Yaml).k -replace "`n", '\n'  # correct: a\nb

Environment: PowerShell 7.6 Core, Yaml at release/v1.0.0. Platform independent — the divergence is in Read-YamlBlockScalar.ps1, not in file or encoding handling.

Regression: No. This has been present for the whole life of the block-scalar reader and is not introduced by any recent change.

Workaround: Ensure the YAML stream ends with a line break before parsing. Import-Yaml reads whole files, which normally end with a break, so this mostly affects strings built in memory.

Acceptance criteria:

• Clip-chomped block scalars produce no trailing line feed when the stream ends without a final break, for both literal and folded styles and with or without an explicit indentation indicator.
• Strip and keep chomping behavior is unchanged.
• All streams that do end with a line break are unchanged.
• The 402-input yaml-test-suite conformance accounting in tests/Conformance.Tests.ps1 stays at 400 syntax passes, 0 failures, and the 2 documented duplicate-key policy differences.

Technical decisions

The root cause is bounded to the chomping tail of src/functions/private/Read-YamlBlockScalar.ps1. The reader records a per-line hasLineBreak flag, and the clip branch appends "n"whenever any content line exists, without consulting whether the final content line actually had a break. The correction is to make the clip branch honorhasLineBreak` for the last content line rather than assuming one.

This is a value-level correctness fix only; no public parameter, output type, or error contract changes.

Implementation plan

1. Add a failing regression test in tests/ConvertFrom-Yaml.Tests.ps1 covering literal, folded, and explicit-indent clip cases with no final break, plus strip and keep controls.
2. Make the clip branch respect the last content line's hasLineBreak.
3. Run tests/ConvertFrom-Yaml.Tests.ps1, tests/Format-Yaml.Tests.ps1, tests/Specification.Tests.ps1, and tests/Conformance.Tests.ps1.

Notes

Found while validating #47 with a differential harness against a reference parser. Two other block/flow scalar defects found in the same sweep were corrected inside that pull request; this one is deliberately deferred because it is pre-existing and outside that release's scope.