Support polymorphic functions with erased parameters

This adds support for
```scala
[T1, ..., Tn] => ([erased] x1: X1, ..., [erased] xm: Xm) => R
```

Polymorphic function types with erased parameters are represented as
using a refinement on `PolyFunction`. `ErasedFunction` is not needed.
```scala
PolyFunction {
  def apply[T1, ..., Tn]([erased] x1: X1, ..., [erased] xm: Xm): R
}
```

Author: nicolasstucki

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

@@ -1100,19 +1100,21 @@ object desugar {
    */
   def makePolyFunctionType(tree: PolyFunction)(using Context): RefinedTypeTree =
     val PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun @ untpd.Function(vparamTypes, res)) = tree: @unchecked
-    val funFlags = fun match
+    val paramFlags = fun match
       case fun: FunctionWithMods =>
-        fun.mods.flags
-      case _ => EmptyFlags
+        // TODO: make use of this in the desugaring when pureFuns is enabled.
+        // val isImpure = funFlags.is(Impure)
 
-    // TODO: make use of this in the desugaring when pureFuns is enabled.
-    // val isImpure = funFlags.is(Impure)
+        // Function flags to be propagated to each parameter in the desugared method type.
+        val givenFlag = fun.mods.flags.toTermFlags & Given
+        fun.erasedParams.map(isErased => if isErased then givenFlag | Erased else givenFlag)
+      case _ =>
+        vparamTypes.map(_ => EmptyFlags)
 
-    // Function flags to be propagated to each parameter in the desugared method type.
-    val paramFlags = funFlags.toTermFlags & Given
-    val vparams = vparamTypes.zipWithIndex.map:
-      case (p: ValDef, _) => p.withAddedFlags(paramFlags)
-      case (p, n) => makeSyntheticParameter(n + 1, p).withAddedFlags(paramFlags)
+    val vparams = vparamTypes.lazyZip(paramFlags).zipWithIndex.map {
+      case ((p: ValDef, paramFlags), n) => p.withAddedFlags(paramFlags)
+      case ((p, paramFlags), n) => makeSyntheticParameter(n + 1, p).withAddedFlags(paramFlags)
+    }.toList
 
     RefinedTypeTree(ref(defn.PolyFunctionType), List(
        DefDef(nme.apply, tparams :: vparams :: Nil, res, EmptyTree).withFlags(Synthetic)

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1426,23 +1426,6 @@ object Parsers {
       case _ => None
     }
 
-    private def checkFunctionNotErased(f: Function, context: String) =
-      def fail(span: Span) =
-        syntaxError(em"Implementation restriction: erased parameters are not supported in $context", span)
-      // erased parameter in type
-      val hasErasedParam = f match
-        case f: FunctionWithMods => f.hasErasedParams
-        case _ => false
-      if hasErasedParam then
-        fail(f.span)
-      // erased parameter in term
-      val hasErasedMods = f.args.collectFirst {
-        case v: ValDef if v.mods.is(Flags.Erased) => v
-      }
-      hasErasedMods match
-        case Some(param) => fail(param.span)
-        case _ =>
-
     /** CaptureRef  ::=  ident | `this`
      */
     def captureRef(): Tree =
@@ -1592,7 +1575,6 @@ object Parsers {
             atSpan(start, arrowOffset) {
               getFunction(body) match {
                 case Some(f) =>
-                  checkFunctionNotErased(f, "poly function")
                   PolyFunction(tparams, body)
                 case None =>
                   syntaxError(em"Implementation restriction: polymorphic function types must have a value parameter", arrowOffset)
@@ -2159,7 +2141,6 @@ object Parsers {
           atSpan(start, arrowOffset) {
             getFunction(body) match
               case Some(f) =>
-                checkFunctionNotErased(f, "poly function")
                 PolyFunction(tparams, f)
               case None =>
                 syntaxError(em"Implementation restriction: polymorphic function literals must have a value parameter", arrowOffset)

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2604,17 +2604,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       report.error(em"Cannot return repeated parameter type ${sym.info.finalResultType}", sym.srcPos)
     if !sym.is(Module) && !sym.isConstructor && sym.info.finalResultType.isErasedClass then
       sym.setFlag(Erased)
-    if
-      sym.info.isInstanceOf[PolyType] &&
-      ((sym.name eq nme.ANON_FUN) ||
-       (sym.name eq nme.apply) && sym.owner.derivesFrom(defn.PolyFunctionClass))
-    then
-      mdef match
-        case DefDef(_, _ :: vparams :: Nil, _, _) =>
-          vparams.foreach: vparam =>
-            if vparam.symbol.is(Erased) then
-              report.error(em"Implementation restriction: erased classes are not allowed in a poly function definition", vparam.srcPos)
-        case _ =>
 
   def typedTypeDef(tdef: untpd.TypeDef, sym: Symbol)(using Context): Tree = {
     val TypeDef(name, rhs) = tdef

tests/neg-custom-args/erased/poly-functions.scala

@@ -0,0 +1,8 @@
+-- Error: tests/neg/polymorphic-erased-functions.scala:3:33 ------------------------------------------------------------
+3 |def t1 = [T] => (erased t: T) => t // error
+  |                                 ^
+  |                                 parameter t is declared as `erased`, but is in fact used
+-- Error: tests/neg/polymorphic-erased-functions.scala:4:42 ------------------------------------------------------------
+4 |def t2 = [T, U] => (t: T, erased u: U) => u // error
+  |                                          ^
+  |                                          parameter u is declared as `erased`, but is in fact used

tests/neg/polymorphic-erased-functions.check

@@ -0,0 +1,4 @@
+import language.experimental.erasedDefinitions
+
+def t1 = [T] => (erased t: T) => t // error
+def t2 = [T, U] => (t: T, erased u: U) => u // error

tests/neg/polymorphic-erased-functions.scala

@@ -0,0 +1,15 @@
+import language.experimental.erasedDefinitions
+
+object Test:
+  type T1 = [X] => (erased x: X, y: Int) => Int
+  type T2 = [X] => (x: X, erased y: Int) => X
+
+  val t1 = [X] => (erased x: X, y: Int) => y
+  val t2 = [X] => (x: X, erased y: Int) => x
+
+  erased class A
+
+  type T3 = [X] => (x: A, y: X) => X
+
+  val t3 = [X] => (x: A, y: X) => y
+

tests/pos/poly-erased-functions.scala

@@ -0,0 +1,22 @@
+import language.experimental.erasedDefinitions
+
+object Test extends App {
+
+  // Types
+  type F1 = [T] => (erased T) => Int
+  type F2 = [T, U] => (T, erased U) => T
+
+  // Terms
+  val t1 = [T] => (erased t: T) => 3
+  assert(t1(List(1, 2, 3)) == 3)
+  val t1a: F1 = t1
+  val t1b: F1 = [T] => (erased t) => 3
+  assert(t1b(List(1, 2, 3)) == 3)
+
+  val t2 = [T, U] => (t: T, erased u: U) => t
+  assert(t2(1, "abc") == 1)
+  val t2a: F2 = t2
+  val t2b: F2 = [T, U] => (t, erased u) => t
+  assert(t2b(1, "abc") == 1)
+
+}