Add support for nullable enums in expressive properties

[koenbeuk]

fixes #59

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

Adds generator + integration coverage for expanding expressive properties involving nullable enum method calls (e.g., Trend?.ToString() and nullable-enum extension methods) so these can be expressed as expandable expression trees.

Changes:

• Update enum method expansion emission to lift per-arm enum constants to Nullable<TEnum> when the receiver is nullable.
• Add an integration test covering ToString() on a nullable enum within an [Expressive] property.
• Add generator verification tests + verified outputs for nullable-enum ToString() and nullable-enum extension method expansion.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/ExpressiveSharp.IntegrationTests/Tests/EnumComparisonTests.cs	Adds runtime test and expressive property for ToString() on nullable enum.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/EnumTests.cs	Adds generator tests for nullable-enum ToString() and nullable-enum extension method expansion.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/EnumTests.ExpandToStringOnNullableEnum.verified.txt	Adds golden output for nullable-enum ToString() expansion.
tests/ExpressiveSharp.Generator.Tests/ExpressiveGenerator/EnumTests.ExpandExtensionMethodOnNullableEnum.verified.txt	Adds golden output for nullable-enum extension method expansion.
src/ExpressiveSharp.Generator/Emitter/ExpressionTreeEmitter.cs	Lifts enum constants to nullable receiver type during enum method expansion.

Comments suppressed due to low confidence (1)

src/ExpressiveSharp.Generator/Emitter/ExpressionTreeEmitter.cs:823

• For nullable enum receivers, the null wrapper currently returns defaultVar when the receiver is null. This changes semantics for methods that have a defined null behavior, e.g. Nullable<TEnum>.ToString() (returns "" when HasValue is false) and extension methods like the new Trend?.Describe() example (should return "n/a" on null, but the expansion yields null). Instead of returning defaultVar for the null branch, compute the call result for a null receiver/argument (using a Constant(null, typeof(Nullable<TEnum>)) as the receiver or first argument, matching static vs instance) and return that for the null branch so the expansion preserves runtime behavior.

        // For Nullable<TEnum>, wrap in: receiver == null ? default : chain
        if (isNullable)
        {
            var nullConst = NextVar();
            AppendLine($"var {nullConst} = {Expr}.Constant(null, typeof({receiverTypeFqn}));");

            var nullCheck = NextVar();
            AppendLine($"var {nullCheck} = {Expr}.Equal({receiverVar}, {nullConst});");

            var wrappedVar = NextVar();
            AppendLine($"var {wrappedVar} = {Expr}.Condition({nullCheck}, {defaultVar}, {currentVar}, typeof({returnTypeFqn}));");
            currentVar = wrappedVar;
        }

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