Proposal: lambda-improvements.md
Branch: features/lambdas

Championed issue (explicit return type, attributes): dotnet/csharplang#4934
Championed issue (direct invocation): dotnet/csharplang#4748

Remaining work items

• using explicit return type in inference (PR Method type inference should make inference from explicit lambda return type #56534)
• ref return types should parse (PR Account for a ref returning lambda as a local variable initializer. #56814)
• add LangVer hint to diagnostics that depend on LangVer (issue Improve diagnostic for conversion of lambda expression to base type with -langversion:9 and earlier #56391)
• conversions (to object and interfaces)
• function types in method type and best type inference

Attributes

Parsing

• static lambdas
• async lambdas
• With parameter types
• Without parameter types
• With parameter ref specifiers
• Without parameter ref specifiers
• Nonparenthesized parameter list (should be a binding error)
• Conditional array accesses
• Object/collection initializers
• After ternaries
• Switch expression arms
• Operator precedence
• Disallow on lambda converted to expression tree

Semantics

• Nullability enforcements on lambda returns when there is an explicit return type and/or attributes (PR Enforce nullability contract in lambda body #56590)
  • DisallowNull
  • NotNull/NotNullWhen
  • AllowNull
  • MaybeNull/MaybeNullWhen
  • MemberNotNull/MemberNotNullWhen
  • DoesNotReturn/DoesNotReturnIf
• Special attributes
  • UnmanagedCallersOnlyAttribute
  • ObsoleteAttribute/ExperimentalAttribute/DeprecatedAttribute
  • SkipLocalsInitAttribute
  • ExcludeFromCodeCoverageAttribute
  • Security attributes
  • CallerFilePathAttribute
  • CallerLineNumberAttribute
  • CallerMemberNameAttribute
  • ConditionalAttribute
  • MethodImplAttribute
  • PreserveSigAttribute
  • MarshalAsAttribute
  • DllImportAttribute (error)
  • EnumeratorCancellationAttribute
• GetTypeInfo (see LambdaTests.LambdaAttributes_AttributeSemanticModel())
• GetSymbolInfo
• SyntaxNormalizer

IDE/Public API

• Attribute type completion inside brackets
• Automatic close bracket insertion
• Smart complete statement
• Speculation inside attribute brackets
  • New attributes
  • Argument positions
• Debugging: DebuggerStepThroughAttribute
• DebuggerDisplay
• Find all refs on attribute types
• IDE syntax formatter
• EnC
• Convert to local function/method refactorings

Explicit return type

Syntax

• async async(async async) => async
• async MyMethod() => null; on the top level parses as a local function statement with a return type of async
• Other async contexts
• static lambdas
• void return type
• disallow var return type (reserved)
• ref/ref readonly return
  • Errors on void return
• Nested lambdas
• After is
• switch expression arms
• Ternary arms
• Operator precedence
• Type-tuple patterns in switch expression arms continue to be parsed as type-tuple patterns
• Before and after range operators
• Everywhere a method can be used

Semantics

• Mismatched return statement types and declared return type
  • Nullable-only
  • Inconvertible types
• Target-typing for contained return statements
• Type inference
  • LDM: Should the explicit return type contribute to type inference? (Yes) (see TypeInference_ExplicitReturnType_01, ...)
• Pathological inference case short-circuiting (see OverloadResolutionPerfTests.NestedLambdas_WithParameterAndReturnTypes())
• GetTypeInfo (see LambdaTests.LambdaReturnType_SemanticModel())
• GetSymbolInfo (see LambdaTests.LambdaReturnType_SemanticModel())
• SyntaxNormalizer (issue SyntaxNormalizer does not include space between lambda expression return type and parenthesized parameter list #59653)

IDE/Public API

• FindAllRefs finds return type
• Convert to local function/method
• Intellisense shows types (including after things like static or async)
• IDE formatter

Inferred (natural) delegate type

Syntax

• None

Semantics

• revert conditional binding patch from 16.11 (ie. revert Infer delegate types with -langversion:preview only #53241)
• Implicit and explicit conversions of lambda expression to:
  • System.MulticastDelegate and base types and interfaces
  • System.Linq.Expressions.Expression<TDelegate>
  • System.Linq.Expressions.Expression, System.Linq.Expressions.LambdaExpression
• Implicit and explicit conversions of method group to
  • System.MulticastDelegate and base types and interfaces
• Directly invoke lambda expressions
• Breaking change tests
• Type inference based on lambdas
  • Covariant on return natural type for value returns (see also issue Type inference with method groups does not support variance for signatures with ref parameters #55909)
  • Contravariant on parameter natural type for value parameters (see also issue Type inference with method groups does not support variance for signatures with ref parameters #55909)
  • Invariant for ref returns/parameters
  • Variance inversion handled correctly
• var
  • direct lambda/method group assignment
  • variant assignment through ternaries
• dicard: _ = () => {}; (disallowed)
• Method group with multiple candidates
  • instance and static methods with same/different signature invoked with implicit/explicit receiver or explicit type qualifier
  • instance and extension methods with same/different signature invoked with implicit/explicit receiver
  • overridden method
  • hidden method
  • accessible and inaccessible overloads
  • overloads that differ by parameter types/ref or return type/ref
  • overloads that differ by object/dynamic, tuple element names, nullability, etc.
  • overloads with different arity, invoked with implicit/explicit type arguments
  • overloads with different type parameter constraints
• Missing Action or Func types
• Missing System.Delegate type
• modopt and modreq do not affect delegate type
• Delegate parameter types or return type that cannot be used as type arguments
• Delegate type includes inferred nullability
• Synthesized types are shared
• Handling of parameter names (see VB)
• Delegate type is only synthesized when required (not when converting to explicit type)
• ErrorCode.ERR_CannotInferDelegateType only reported when delegate type is required
• Pattern-based language constructs
  • Query expression on lambda expression/method group with extension Select on delegate type
  • Extension GetAwaiter
  • Extension GetEnumerator
  • Extension GetAsyncEnumerator
  • Extension deconstruct
• Address of on single-method method groups correctly infers function pointer type
• Interaction with discard parameters
• Lambdas that just throw
• GetTypeInfo on lambdas and method groups (issue GetTypeInfo() should return the inferred delegate type for method groups #52874)
  • Does .Type return null or the Action/Func/synthesized delegate type? How does ASP.NET source generator obtain the lambda signature? (see here)
• GetSymbolInfo on lambdas and method groups (issue GetSymbolInfo() should return the resolved method from a method group with inferred delegate type #52870)
• IOperation nodes adjusted to give natural types
• ClassifyConversion (see DelegateTypeTests.ClassifyConversionFromExpression())
• ToDisplayString on synthesized delegate types

IDE/Public API

• QuickInfo
• FindAllReferences to Action/Func type
• Maybe FindAllReferences on the delegate shows synthesized?
• Remove unnecessary cast
• Introduce local
• To/from var (issue Use implicit/explicit type doesn't work with lambda function types #58404)

Open issues

• LDM: potential breaking change with overload resolution (issue Overload resolution and inferred lambda / method group delegate type: potential breaking change csharplang#4674)
• LDM: LambdaExpression