SLC: Add andThen to Function2..Function22 and FunctionXXL

hepin1989 · May 18, 2026, 7:40am

Add `andThen` to `Function2..Function22` and `FunctionXXL`

main ← He-Pin:add-function-andthen

opened 01:46PM - 17 May 26 UTC

## Summary Mirror `java.util.function.BiFunction.andThen` for Scala's higher-ar…ity function types. Port of scala/scala#11121. - Adds `def andThen[A](g: R => A): (T1, .., Tn) => A` to each of `Function2..Function22` (default trait method, `@unspecialized`). - Adds `def andThen(g: Object => Object): FunctionXXL^{this, g}` to `scala.runtime.FunctionXXL`. The capture-polymorphic return type is required because the SAM body captures `this` and `g`. - Drops the `Function1[(K, V1), Unit]` mixin from the private `accum` class inside `HashMap.iadd`: once `Function2` carries a default `andThen`, mixing in both produces a clash between the two trait-default implementations. The iterator loop is inlined to a `while` to replace the Function1 dispatch. - Overrides `andThen` on `dotty.tools.dotc.core.SymDenotations.LazyType`, which extends both `Function1[Symbol, LazyType]` and `Function2[(TermSymbol, ClassSymbol), LazyType]`, to disambiguate the inherited members. Composition is not meaningful for a symbol completer, so the override throws `UnsupportedOperationException`. - Patches the extracted Scala 2 library sources in `ScalaLibraryPlugin` so the `scala2-library` project (whose compiled binaries replace ours for specialized synthetics) also carries `andThen`. The Scala 2 backport will land in a separate PR; this in-tree source patch keeps the present change self-contained until it does. - Adds MiMa `ReversedMissingMethodProblem` filters for the 22 new default methods. ## Test plan - [x] `testCompilation function-andthen` passes (`tests/run/function-andthen.scala` covers `Function2`, `Function3`, `Function22`, type-changing chains, evaluation order, fresh-instance return, interaction with `tupled` / `curried`, and `FunctionXXL` at arity 23). - [x] `scala2-library` recompiles with patched sources; `Function2.class` exposes `andThen`. - [x] `scala-library-nonbootstrapped` rebuilds cleanly with the new binaries. ## Notes A dedicated Scala 2.13 backport will follow. The plugin-side source patch is the bridge that keeps this PR runnable in isolation until that lands; once it does, the patch can be removed.

the andThen and compose mehtod is quite helpful.