ToUppercaseFirstChar/ToLowercaseFirstChar do nothing when the input starts with whitespace (" hello" → "hello")

[matt-edmondson]

What's wrong

ToUppercaseFirstChar and ToLowercaseFirstChar (CaseConverter/CaseConverter.cs ~L129–155) change the case of input[0] first, and only then call CollapseSpaces(...).Trim():

return CollapseSpaces(char.ToUpperInvariant(input[0]) + input[1..]).Trim();

If the string starts with whitespace, the whitespace is what gets case-mapped. The trim then removes it, and the first letter is returned unchanged. Both methods also collapse runs of internal spaces, which their summaries ("with the first character converted…") don't mention.

Failure scenario (reproduced)

• " hello".ToUppercaseFirstChar() → "hello" (expected "Hello")
• "\thello".ToUppercaseFirstChar() → "hello"
• " Hello".ToLowercaseFirstChar() → "Hello" (expected "hello")
• "a b".ToUppercaseFirstChar() → "A b" (internal spacing changed without saying so)

The caller does get a trimmed string back, so the result looks processed, but the case change never happened.

This is not covered by #74 (first letters outside the BMP). The open PR for #74 rewrites these methods around MapFirstCodePoint, but it keeps the map-then-trim order, so this bug is still there after that PR.

Suggested fix

Trim, or skip leading whitespace, before mapping the first code point. Then either remove CollapseSpaces from these two public helpers or document it in their XML summaries.

Acceptance criteria

• " hello".ToUppercaseFirstChar() == "Hello" and " Hello".ToLowercaseFirstChar() == "hello", with tests.
• The internal-space behaviour is either kept and documented, or removed and tested.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. These are public helpers, and for input with leading whitespace they return a trimmed string whose first letter was never re-cased. The fix is straightforward.
• Area / suggested assignee: CaseConverter/CaseConverter.cs (ToUppercaseFirstChar / ToLowercaseFirstChar), @matt-edmondson.
• Duplicates: None. The issue already explains why ToCamelCase, ToLowercaseFirstChar and ToUppercaseFirstChar leave a first letter outside the BMP unchanged #74 does not cover this.
• In progress: Partly overlaps open PR Case-map an astral first letter in ToCamelCase and the first-char helpers #76 (ToCamelCase, ToLowercaseFirstChar and ToUppercaseFirstChar leave a first letter outside the BMP unchanged #74), which rewrites these same methods around MapFirstCodePoint but keeps the map-then-trim order. The simplest route is to fold the trim-first change into Case-map an astral first letter in ToCamelCase and the first-char helpers #76, or land this fix right after it, so the two don't conflict.

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