[FreeApophis]

Summary

• New opt-in rule λ1018 (Warning): SelectMany(x => x) and SelectMany(Identity) on a nested Option<Option<T>>, Result<Result<T>>, Either<L, Either<L, R>> or IEnumerable<IEnumerable<T>> is Flatten(). After λ1017 (Add analyzer λ1017: prefer the named function over the lambda #970) the lambda form becomes SelectMany(Identity), which makes the gap more visible, not less; this rule closes it.
• Also reported: a lambda that returns its parameter through an implicit reference conversion (IEnumerable<int[]>.SelectMany(x => x)), since Flatten accepts the receiver through the same covariance. Any type implementing IEnumerable<T> counts as receiver (List<int[]>).
• Not reported: a selector that does something, the result-selector overload and query syntax, the static call form Enumerable.SelectMany(xs, x => x), expression trees, and compilations without the matching Flatten extension.
• Code fix replaces the call with Flatten() and adds using Funcky.Monads; / using Funcky.Extensions; when the extension does not bind yet (same UsingDirectives helper as Add analyzer λ1012: prefer the …OrNone method over …OrDefault with a null check #973, here it also takes the file's own line ending for files without usings; I'll port that line to Add analyzer λ1012: prefer the …OrNone method over …OrDefault with a null check #973).
• Dogfooding: Sequence.Concat, CycleMaterialized, RepeatMaterialized and one test now call Flatten(); the four Flatten implementations themselves suppress the rule with a pragma.
• Docs: λ1018.md, rule table, SUMMARY, changelog, AnalyzerReleases.Unshipped.

Test plan

• 5 new tests in PreferFlattenTest: Option (lambda, block body, method group, chained), Result/Either, IEnumerable with using added and List<int[]> receiver, using added for a fully qualified monad type, the not-reported forms
• 81 analyzer tests pass, Roslyn 4.0 variant builds
• dotnet build Funcky.slnx -warnaserror --no-incremental green; Sequence, Flatten and Split tests pass on net10.0

🤖 Generated with Claude Code

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🟡 Changes recommended

Unsafe replacements and syntax-handling failures can change behavior, break compilation, or prevent fixes from applying.

6 open issues

• Preserve leading directives before inserting usings · New
• Avoid copying preprocessor directives into inserted usings · New
• Preserve explicit SelectMany result type in Flatten calls · New
• Track replacement nodes instead of reusing invalid spans · New
• Require successful binding to the intended Flatten extension · New
• Restrict SelectMany matching to supported methods · New

What changed in this PR

Adds λ1018 to recommend Flatten() for identity-based SelectMany calls, with a code fix, tests, documentation, and adoption in existing code.

Changes:

• Detects supported identity selectors and offers a Flatten() replacement.
• Adds namespace-import handling and analyzer tests.
• Documents the rule and updates existing flattening calls.

File	Description
Funcky/​Sequence/​Sequence.RepeatMaterialized.cs	Uses Flatten().
Funcky/​Sequence/​Sequence.CycleMaterialized.cs	Uses Flatten().
Funcky/​Sequence/​Sequence.Concat.cs	Uses Flatten().
Funcky/​Monads/​Result/​ResultExtensions.cs	Suppresses λ1018 inside Flatten.
Funcky/​Monads/​Option/​OptionExtensions.cs	Suppresses λ1018 inside Flatten.
Funcky/​Monads/​Either/​EitherExtensions.cs	Suppresses λ1018 inside Flatten.
Funcky/​Extensions/​EnumerableExtensions/​Flatten.cs	Suppresses λ1018 inside Flatten.
Funcky.Test/​Extensions/​EnumerableExtensions/​SplitTest.cs	Uses Flatten() in a property test.
Funcky.Analyzers/​Funcky.Analyzers/​Flatten/​PreferFlattenAnalyzer.cs	Implements λ1018 detection.
Funcky.Analyzers/​Funcky.Analyzers/​AnalyzerReleases.Unshipped.md	Registers the new rule.
Funcky.Analyzers/​Funcky.Analyzers.Test/​Flatten/​PreferFlattenTest.cs	Tests diagnostics and fixes.
Funcky.Analyzers/​Funcky.Analyzers.CodeFixes/​Flatten/​PreferFlattenCodeFix.cs	Implements the replacement fix.
Funcky.Analyzers/​Funcky.Analyzers.CodeFixes/​CodeAnalysisExtensions/​UsingDirectives.cs	Adds namespace imports.
Documentation/​src/​SUMMARY.md	Links the rule documentation.
Documentation/​src/​analyzer-rules/​λ1018.md	Explains the rule and examples.
Documentation/​src/​analyzer-rules/​analyzer-rules.md	Adds λ1018 to the rule table.
changelog.md	Announces the analyzer and fix.

_{Review effort: Balanced}