Typed-Parameter Lambdas with Inferred Return Type
3 sections
Overview
Add the lambda form (x: T) -> body: typed parameters with an inferred return type, no =. Unify it with the existing untyped simple_lambda so annotating a parameter no longer forces a return-type annotation. Retain (x: T) -> RetT = body for explicit return types. The change is parser-only; the new form lowers to the same AST node the untyped inferred-return form already produces.