Implement individual erased parameters (#16507)

### Syntax

Erased parameters in a method / lambda comes with an `erased` modifier
before its name:
```scala
def erasedSecondParam(x: Int, erased y: Int): Int = x
type EraseSecondParam[T, U] = (T, erased U) => T

val esp: EraseSecondParam[Int, Int] = (x, erased y) => erasedSecondParam(x, y)
```
This is a breaking change, as previously erased methods / functions with
multiple parameters now only have its first parameter erased.

### Semantics

`[Impure][Contextual]ErasedFunctionN` traits are no longer available.
Instead, erased function values are denoted by refining the
`scala.runtime.ErasedFunction` trait:
```scala
type Int_EInt = (Int, erased Int) => Int
// is equivalent to
type Int_EInt2 = scala.runtime.ErasedFunction {
  def apply(x$0: Int, erased x$1: Int): Int
}
```

They are subsequently compiled (during Erasure) into
`[Contextual]FunctionM` where `M` is the number of non-erased
parameters.

### Erased Classes 

Any parameter that is an instance of an erased class is automatically
erased. This is different from before, where the parameters are erased
only if all parameters are instances of erased classes.

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1498,10 +1498,10 @@ object desugar {
       case vd: ValDef => vd
     }
 
-  def makeContextualFunction(formals: List[Tree], body: Tree, isErased: Boolean)(using Context): Function = {
-    val mods = if (isErased) Given | Erased else Given
+  def makeContextualFunction(formals: List[Tree], body: Tree, erasedParams: List[Boolean])(using Context): Function = {
+    val mods = Given
     val params = makeImplicitParameters(formals, mods)
-    FunctionWithMods(params, body, Modifiers(mods))
+    FunctionWithMods(params, body, Modifiers(mods), erasedParams)
   }
 
   private def derivedValDef(original: Tree, named: NameTree, tpt: Tree, rhs: Tree, mods: Modifiers)(using Context) = {
@@ -1834,6 +1834,7 @@ object desugar {
             cpy.ByNameTypeTree(parent)(annotate(tpnme.retainsByName, restpt))
           case _ =>
             annotate(tpnme.retains, parent)
+      case f: FunctionWithMods if f.hasErasedParams => makeFunctionWithValDefs(f, pt)
     }
     desugared.withSpan(tree.span)
   }
@@ -1909,6 +1910,28 @@ object desugar {
     TypeDef(tpnme.REFINE_CLASS, impl).withFlags(Trait)
   }
 
+  /** Ensure the given function tree use only ValDefs for parameters.
+   *  For example,
+   *      FunctionWithMods(List(TypeTree(A), TypeTree(B)), body, mods, erasedParams)
+   *  gets converted to
+   *      FunctionWithMods(List(ValDef(x$1, A), ValDef(x$2, B)), body, mods, erasedParams)
+   */
+  def makeFunctionWithValDefs(tree: Function, pt: Type)(using Context): Function = {
+    val Function(args, result) = tree
+    args match {
+      case (_ : ValDef) :: _ => tree // ValDef case can be easily handled
+      case _ if !ctx.mode.is(Mode.Type) => tree
+      case _ =>
+        val applyVParams = args.zipWithIndex.map {
+          case (p, n) => makeSyntheticParameter(n + 1, p)
+        }
+        tree match
+          case tree: FunctionWithMods =>
+            untpd.FunctionWithMods(applyVParams, tree.body, tree.mods, tree.erasedParams)
+          case _ => untpd.Function(applyVParams, result)
+    }
+  }
+
   /** Returns list of all pattern variables, possibly with their types,
    *  without duplicates
    */

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -960,7 +960,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
       && tree.isTerm
       && {
         val qualType = tree.qualifier.tpe
-        hasRefinement(qualType) && !qualType.derivesFrom(defn.PolyFunctionClass)
+        hasRefinement(qualType) && !defn.isRefinedFunctionType(qualType)
       }
     def loop(tree: Tree): Boolean = tree match
       case TypeApply(fun, _) =>

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -260,12 +260,12 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
           // If `isParamDependent == false`, the value of `previousParamRefs` is not used.
           if isParamDependent then mutable.ListBuffer[TermRef]() else (null: ListBuffer[TermRef] | Null).uncheckedNN
 
-        def valueParam(name: TermName, origInfo: Type): TermSymbol =
+        def valueParam(name: TermName, origInfo: Type, isErased: Boolean): TermSymbol =
           val maybeImplicit =
             if tp.isContextualMethod then Given
             else if tp.isImplicitMethod then Implicit
             else EmptyFlags
-          val maybeErased = if tp.isErasedMethod then Erased else EmptyFlags
+          val maybeErased = if isErased then Erased else EmptyFlags
 
           def makeSym(info: Type) = newSymbol(sym, name, TermParam | maybeImplicit | maybeErased, info, coord = sym.coord)
 
@@ -283,7 +283,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
               assert(vparams.hasSameLengthAs(tp.paramNames) && vparams.head.isTerm)
               (vparams.asInstanceOf[List[TermSymbol]], remaining1)
             case nil =>
-              (tp.paramNames.lazyZip(tp.paramInfos).map(valueParam), Nil)
+              (tp.paramNames.lazyZip(tp.paramInfos).lazyZip(tp.erasedParams).map(valueParam), Nil)
         val (rtp, paramss) = recur(tp.instantiate(vparams.map(_.termRef)), remaining1)
         (rtp, vparams :: paramss)
       case _ =>
@@ -1140,10 +1140,10 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
 
     def etaExpandCFT(using Context): Tree =
       def expand(target: Tree, tp: Type)(using Context): Tree = tp match
-        case defn.ContextFunctionType(argTypes, resType, isErased) =>
+        case defn.ContextFunctionType(argTypes, resType, _) =>
           val anonFun = newAnonFun(
             ctx.owner,
-            MethodType.companion(isContextual = true, isErased = isErased)(argTypes, resType),
+            MethodType.companion(isContextual = true)(argTypes, resType),
             coord = ctx.owner.coord)
           def lambdaBody(refss: List[List[Tree]]) =
             expand(target.select(nme.apply).appliedToArgss(refss), resType)(

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -76,9 +76,13 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     override def isType: Boolean = body.isType
   }
 
-  /** A function type or closure with `implicit`, `erased`, or `given` modifiers */
-  class FunctionWithMods(args: List[Tree], body: Tree, val mods: Modifiers)(implicit @constructorOnly src: SourceFile)
-    extends Function(args, body)
+  /** A function type or closure with `implicit` or `given` modifiers and information on which parameters are `erased` */
+  class FunctionWithMods(args: List[Tree], body: Tree, val mods: Modifiers, val erasedParams: List[Boolean])(implicit @constructorOnly src: SourceFile)
+    extends Function(args, body) {
+      assert(args.length == erasedParams.length)
+
+      def hasErasedParams = erasedParams.contains(true)
+    }
 
   /** A polymorphic function type */
   case class PolyFunction(targs: List[Tree], body: Tree)(implicit @constructorOnly src: SourceFile) extends Tree {

compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -146,7 +146,6 @@ extension (tp: Type)
       defn.FunctionType(
         fname.functionArity,
         isContextual = fname.isContextFunction,
-        isErased = fname.isErasedFunction,
         isImpure = true).appliedTo(args)
     case _ =>
       tp

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -336,8 +336,8 @@ class CheckCaptures extends Recheck, SymTransformer:
         mapArgUsing(_.forceBoxStatus(false))
       else if meth == defn.Caps_unsafeBoxFunArg then
         mapArgUsing {
-          case defn.FunctionOf(paramtpe :: Nil, restpe, isContectual, isErased) =>
-            defn.FunctionOf(paramtpe.forceBoxStatus(true) :: Nil, restpe, isContectual, isErased)
+          case defn.FunctionOf(paramtpe :: Nil, restpe, isContectual) =>
+            defn.FunctionOf(paramtpe.forceBoxStatus(true) :: Nil, restpe, isContectual)
         }
       else
         super.recheckApply(tree, pt) match
@@ -430,7 +430,7 @@ class CheckCaptures extends Recheck, SymTransformer:
       block match
         case closureDef(mdef) =>
           pt.dealias match
-            case defn.FunctionOf(ptformals, _, _, _)
+            case defn.FunctionOf(ptformals, _, _)
             if ptformals.nonEmpty && ptformals.forall(_.captureSet.isAlwaysEmpty) =>
               // Redo setup of the anonymous function so that formal parameters don't
               // get capture sets. This is important to avoid false widenings to `*`
@@ -598,18 +598,18 @@ class CheckCaptures extends Recheck, SymTransformer:
       //println(i"check conforms $actual1 <<< $expected1")
       super.checkConformsExpr(actual1, expected1, tree)
 
-    private def toDepFun(args: List[Type], resultType: Type, isContextual: Boolean, isErased: Boolean)(using Context): Type =
-      MethodType.companion(isContextual = isContextual, isErased = isErased)(args, resultType)
+    private def toDepFun(args: List[Type], resultType: Type, isContextual: Boolean)(using Context): Type =
+      MethodType.companion(isContextual = isContextual)(args, resultType)
         .toFunctionType(isJava = false, alwaysDependent = true)
 
     /** Turn `expected` into a dependent function when `actual` is dependent. */
     private def alignDependentFunction(expected: Type, actual: Type)(using Context): Type =
       def recur(expected: Type): Type = expected.dealias match
         case expected @ CapturingType(eparent, refs) =>
           CapturingType(recur(eparent), refs, boxed = expected.isBoxed)
-        case expected @ defn.FunctionOf(args, resultType, isContextual, isErased)
+        case expected @ defn.FunctionOf(args, resultType, isContextual)
           if defn.isNonRefinedFunction(expected) && defn.isFunctionType(actual) && !defn.isNonRefinedFunction(actual) =>
-          val expected1 = toDepFun(args, resultType, isContextual, isErased)
+          val expected1 = toDepFun(args, resultType, isContextual)
           expected1
         case _ =>
           expected
@@ -675,7 +675,7 @@ class CheckCaptures extends Recheck, SymTransformer:
 
         try
           val (eargs, eres) = expected.dealias.stripCapturing match
-            case defn.FunctionOf(eargs, eres, _, _) => (eargs, eres)
+            case defn.FunctionOf(eargs, eres, _) => (eargs, eres)
             case expected: MethodType => (expected.paramInfos, expected.resType)
             case expected @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionType(expected) => (rinfo.paramInfos, rinfo.resType)
             case _ => (aargs.map(_ => WildcardType), WildcardType)
@@ -739,7 +739,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             case actual @ AppliedType(tycon, args) if defn.isNonRefinedFunction(actual) =>
               adaptFun(actual, args.init, args.last, expected, covariant, insertBox,
                   (aargs1, ares1) => actual.derivedAppliedType(tycon, aargs1 :+ ares1))
-            case actual @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionType(actual) =>
+            case actual @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionOrPolyType(actual) =>
               // TODO Find a way to combine handling of generic and dependent function types (here and elsewhere)
               adaptFun(actual, rinfo.paramInfos, rinfo.resType, expected, covariant, insertBox,
                 (aargs1, ares1) =>
@@ -962,7 +962,7 @@ class CheckCaptures extends Recheck, SymTransformer:
             case CapturingType(parent, refs) =>
               healCaptureSet(refs)
               traverse(parent)
-            case tp @ RefinedType(parent, rname, rinfo: MethodType) if defn.isFunctionType(tp) =>
+            case tp @ RefinedType(parent, rname, rinfo: MethodType) if defn.isFunctionOrPolyType(tp) =>
               traverse(rinfo)
             case tp: TermLambda =>
               val saved = allowed

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -12,6 +12,7 @@ import transform.Recheck.*
 import CaptureSet.IdentityCaptRefMap
 import Synthetics.isExcluded
 import util.Property
+import dotty.tools.dotc.core.Annotations.Annotation
 
 /** A tree traverser that prepares a compilation unit to be capture checked.
  *  It does the following:
@@ -38,7 +39,6 @@ extends tpd.TreeTraverser:
   private def depFun(tycon: Type, argTypes: List[Type], resType: Type)(using Context): Type =
     MethodType.companion(
         isContextual = defn.isContextFunctionClass(tycon.classSymbol),
-        isErased = defn.isErasedFunctionClass(tycon.classSymbol)
       )(argTypes, resType)
       .toFunctionType(isJava = false, alwaysDependent = true)
 
@@ -54,7 +54,7 @@ extends tpd.TreeTraverser:
         val boxedRes = recur(res)
         if boxedRes eq res then tp
         else tp1.derivedAppliedType(tycon, args.init :+ boxedRes)
-      case tp1 @ RefinedType(_, _, rinfo) if defn.isFunctionType(tp1) =>
+      case tp1 @ RefinedType(_, _, rinfo: MethodType) if defn.isFunctionOrPolyType(tp1) =>
         val boxedRinfo = recur(rinfo)
         if boxedRinfo eq rinfo then tp
         else boxedRinfo.toFunctionType(isJava = false, alwaysDependent = true)
@@ -231,7 +231,7 @@ extends tpd.TreeTraverser:
                 tp.derivedAppliedType(tycon1, args1 :+ res1)
           else
             tp.derivedAppliedType(tycon1, args.mapConserve(arg => this(arg)))
-        case tp @ RefinedType(core, rname, rinfo) if defn.isFunctionType(tp) =>
+        case tp @ RefinedType(core, rname, rinfo: MethodType) if defn.isFunctionOrPolyType(tp) =>
           val rinfo1 = apply(rinfo)
           if rinfo1 ne rinfo then rinfo1.toFunctionType(isJava = false, alwaysDependent = true)
           else tp
@@ -260,7 +260,13 @@ extends tpd.TreeTraverser:
   private def expandThrowsAlias(tp: Type)(using Context) = tp match
     case AppliedType(tycon, res :: exc :: Nil) if tycon.typeSymbol == defn.throwsAlias =>
       // hard-coded expansion since $throws aliases in stdlib are defined with `?=>` rather than `?->`
-      defn.FunctionOf(defn.CanThrowClass.typeRef.appliedTo(exc) :: Nil, res, isContextual = true, isErased = true)
+      defn.FunctionOf(
+        AnnotatedType(
+          defn.CanThrowClass.typeRef.appliedTo(exc), 
+          Annotation(defn.ErasedParamAnnot, defn.CanThrowClass.span)) :: Nil,
+        res,
+        isContextual = true
+      )
     case _ => tp
 
   private def expandThrowsAliases(using Context) = new TypeMap:
@@ -323,7 +329,7 @@ extends tpd.TreeTraverser:
               args.last, CaptureSet.empty, currentCs ++ outerCs)
             tp.derivedAppliedType(tycon1, args1 :+ resType1)
         tp1.capturing(outerCs)
-      case tp @ RefinedType(parent, nme.apply, rinfo: MethodType) if defn.isFunctionType(tp) =>
+      case tp @ RefinedType(parent, nme.apply, rinfo: MethodType) if defn.isFunctionOrPolyType(tp) =>
         propagateDepFunctionResult(mapOver(tp), currentCs ++ outerCs)
           .capturing(outerCs)
       case _ =>