[FreeApophis]

Summary

• New built-in rule λ0005 (Error): a null literal that is converted to Option<T>, Result<T> or Either<L, R> through the implicit conversion, or passed to Option.Some/Return, Result.Ok/Return, Result<T>.Error, Either<L, R>.Left/Right or Either<L>.Return. All of these throw an ArgumentNullException, so the code throws every time it runs. The literal may sit in a branch of a ?:, switch or ?? expression (same walker as λ0004 in Add built-in analyzer λ0004: default literal wrapped by an implicit conversion #966).
• The sneakiest form is option == null: null converts to Option<string> through the implicit operator, so the comparison compiles and throws instead of answering false (verified at run time against the current library).
• With nullable reference types disabled the compiler is silent about all of this; with them enabled it reports CS8625 for some forms. The rule is an error in both configurations.
• Code fix: replaces the literal with Option<T>.None, or the whole Option.Some(null) call. No fix for Result/Either, the intended error or left value is not knowable.
• Not reported: variables and calls that may be null (that is what Option.FromNullable is for), default, Option.FromNullable(null).
• Six tests in Funcky.Test that pass null on purpose to assert the exception suppress the rule with a pragma.
• Docs: λ0005.md, rule table, SUMMARY, changelog.

Shares the CompilationExtensions additions and the DiagnosticName.cs compile include with #966 (byte-identical, merges cleanly); the DiagnosticName constant and the Unshipped/changelog/table lines are adjacent-line conflicts.

Test plan

• 9 new tests in NullToMonadTest (conversion in assignment/return/argument/==/?:/switch/??, factory methods, Result/Either without fix, variables and unrelated conversions not reported); 86 analyzer tests pass
• dotnet build Funcky.slnx -warnaserror --no-incremental green (the rule found exactly the six intentional-null tests)
• Funcky.Test.Monads tests pass on net10.0

🤖 Generated with Claude Code

[Copilot]

🟡 Changes recommended

Lifted conversions can trigger false errors, and code fixes can produce invalid code or incorrect Option values.

3 open issues

• Stop fix discovery at explicit casts · New
• Bind fixes to the diagnostic's responsible conversion · New
• Exclude null-propagating lifted conversions from diagnostics · New

What changed in this PR

Adds built-in error rule λ0005 to detect null literals passed to null-rejecting monad conversions and factories.

Changes:

• Adds the analyzer and an Option-to-None code fix.
• Adds regression tests and suppressions for intentional-null tests.
• Documents and registers the new rule.

File	Description
Funcky.Test/​Monads/​ResultTest.cs	Suppresses intentional-null diagnostics.
Funcky.Test/​Monads/​OptionTest.cs	Suppresses intentional-null diagnostics.
Funcky.Test/​Monads/​OptionImplicitConversionTest.cs	Suppresses intentional-null diagnostics.
Funcky.Test/​Monads/​EitherTest.cs	Suppresses intentional-null diagnostics.
Funcky.Analyzers/​Funcky.BuiltinAnalyzers/​NullToMonadAnalyzer.cs	Implements λ0005 detection.
Funcky.Analyzers/​Funcky.BuiltinAnalyzers/​DiagnosticName.cs	Defines the shared diagnostic ID.
Funcky.Analyzers/​Funcky.BuiltinAnalyzers/​CompilationExtensions.cs	Adds monad type lookups.
Funcky.Analyzers/​Funcky.BuiltinAnalyzers/​AnalyzerReleases.Unshipped.md	Registers the new rule.
Funcky.Analyzers/​Funcky.BuiltinAnalyzers.CodeFixes/​NullToOptionNoneCodeFix.cs	Implements Option replacements.
Funcky.Analyzers/​Funcky.BuiltinAnalyzers.CodeFixes/​Funcky.BuiltinAnalyzers.CodeFixes.csproj	Includes shared diagnostic definitions.
Funcky.Analyzers/​Funcky.Analyzers.Test/​NullToMonadTest.cs	Tests detection and fixes.
Documentation/​src/​SUMMARY.md	Links the rule documentation.
Documentation/​src/​analyzer-rules/​λ0005.md	Explains the rule and remediation.
Documentation/​src/​analyzer-rules/​analyzer-rules.md	Lists λ0005.
changelog.md	Announces the analyzer and fix.

_{Review effort: Balanced}