Rewriting a document changes unquoted numbers: `version: 1.10` becomes `1.1`, `zip: 01234` becomes `1234`, `lat: 51.507351` becomes `51.50735`

[matt-edmondson]

What's wrong

Since #174 (the fix for #137, commit 4504544), YamlSerializer's deserializer uses WithAttemptingUnquotedStringTypeDeserialization() (Frontmatter/YamlSerializer.cs:30). Plain scalars now deserialize to the smallest .NET numeric type that fits (Byte, Int16, or Single, a 32-bit float) instead of string.

SerializeYamlObject (Frontmatter/YamlSerializer.cs:135-136) then writes those numbers back out. As a result, every method that re-serializes the header changes the text of numeric-looking values: AddFrontmatter, ReplaceFrontmatter and CombineFrontmatter.

Before #174, every scalar was read as a string, so these values survived byte-for-byte. This is a regression.

Reproduction (at d8d1348)

string doc = "---\nversion: 1.10\nrelease: 1.0\nzip: 01234\nhex: 0x1F\nbig: 123456789012345678901234\nlat: 51.507351\nprice: 1234567.89\n---\nBody\n";
Frontmatter.AddFrontmatter(doc, new() { ["extra"] = "x" });

Output header:

version: 1.1
release: 1
zip: 1234
hex: 31
big: 1.2345679E+23
lat: 51.50735
price: 1234567.9
extra: x

ExtractFrontmatter(doc) reports these types:

Key	Type
version	Single
zip	Int16
hex	Byte
big	Single

CombineFrontmatter(doc, AsIs, AsIs, None) produces the same output, with no merging or renaming involved.

Why it matters

Adding one unrelated key silently rewrites values the caller never touched:

• Version strings change meaning: 1.10 becomes 1.1, and 1.0 becomes 1.
• IDs and zip codes lose their leading zeros.
• Coordinates and prices lose precision because they go through float.
• Large integers become floats in scientific notation.

Hand-written frontmatter usually leaves these values unquoted (for example Hugo weight: 1.10 or version: 2.0), so the "quote it" workaround from #137 does not help existing documents.

Suggested fix / acceptance criteria

• A re-serialized plain scalar produces the same text it was read from. Options:
  • Keep the original scalar text for numbers.
  • Parse only bool, null and int, and fall back to string whenever reformatting would change the text.
  • At minimum, parse floats as double/decimal and integers as long, and keep the raw string whenever value.ToString() differs from the source text.
• Round-trip tests through AddFrontmatter and CombineFrontmatter show each of these values unchanged: 1.10, 1.0, 01234, 0x1F, 51.507351, 1234567.89, 123456789012345678901234, .inf, +12.
• The Round-tripping strips quotes, so "true", "1.0", "007" and "null" come back as bool/number/null, and empty values become '' #137 tests in ScalarTypeRoundTripTests keep passing.

[matt-edmondson]

Triage

• Category: Bug (regression from [minor] Keep scalar types and nulls through a frontmatter round trip #174)
• Priority: High. Adding one unrelated key silently rewrites values nobody touched, such as version strings (1.10 becomes 1.1), zip codes and coordinates. Every document that is re-serialized is affected.
• Area: Frontmatter/YamlSerializer (WithAttemptingUnquotedStringTypeDeserialization and SerializeYamlObject)
• Duplicates / in progress: No duplicate or open PR found. Regression introduced by the fix for Round-tripping strips quotes, so "true", "1.0", "007" and "null" come back as bool/number/null, and empty values become '' #137.
• Next step: Keep the original scalar text whenever reformatting would change it, add round-trip tests for the listed values, and keep the Round-tripping strips quotes, so "true", "1.0", "007" and "null" come back as bool/number/null, and empty values become '' #137 tests passing.

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