Merge pull request #10856 from dotty-staging/fix-#10848

Fix #10848: Enable `erased given` under `-Yerased-terms`

Author: nicolasstucki

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

@@ -434,7 +434,7 @@ object Checking {
       if sym.isAllOf(flag1 | flag2) then fail(i"illegal combination of modifiers: `${flag1.flagsString}` and `${flag2.flagsString}` for: $sym")
     def checkApplicable(flag: FlagSet, ok: Boolean) =
       if (!ok && !sym.is(Synthetic))
-        fail(i"modifier `${flag.flagsString}` is not allowed for this definition")
+        fail(ModifierNotAllowedForDefinition(Erased))
 
     if (sym.is(Inline) &&
           (  sym.is(ParamAccessor) && sym.owner.isClass
@@ -500,7 +500,7 @@ object Checking {
         sym.setFlag(Private) // break the overriding relationship by making sym Private
       }
     if (sym.is(Erased))
-      checkApplicable(Erased, !sym.isOneOf(MutableOrLazy))
+      checkApplicable(Erased, !sym.isOneOf(MutableOrLazy, butNot = Given))
   }
 
   /** Check the type signature of the symbol `M` defined by `tree` does not refer

docs/docs/reference/metaprogramming/erased-terms.md

@@ -8,7 +8,7 @@ title: "Erased Terms"
 Let's describe the motivation behind erased terms with an example. In the
 following we show a simple state machine which can be in a state `On` or `Off`.
 The machine can change state from `Off` to `On` with `turnedOn` only if it is
-currently `Off`. This last constraint is captured with the `IsOff[S]` implicit
+currently `Off`. This last constraint is captured with the `IsOff[S]` contextual
 evidence which only exists for `IsOff[Off]`. For example, not allowing calling
 `turnedOn` on in an `On` state as we would require an evidence of type
 `IsOff[On]` that will not be found.
@@ -21,11 +21,11 @@ final class Off extends State
 @implicitNotFound("State must be Off")
 class IsOff[S <: State]
 object IsOff {
-  implicit def isOff: IsOff[Off] = new IsOff[Off]
+  given isOff: IsOff[Off] = new IsOff[Off]
 }
 
 class Machine[S <: State] {
-  def turnedOn(implicit ev: IsOff[S]): Machine[On] = new Machine[On]
+  def turnedOn(using IsOff[S]): Machine[On] = new Machine[On]
 }
 
 val m = new Machine[Off]

tests/pos-custom-args/erased/i10848a.scala

@@ -0,0 +1,5 @@
+class IsOn[T]
+type On
+object IsOn {
+  erased given IsOn[On] = new IsOn[On]
+}

tests/pos-custom-args/erased/i10848b.scala

@@ -0,0 +1,4 @@
+class Foo:
+  erased given Int = 1
+  def foo(using erased x: Int): Unit = ()
+  foo

tests/pos-custom-args/phantom-Eq.scala

@@ -17,16 +17,15 @@ object EqUtil {
   type PhantomEq[-L, -R]
   type PhantomEqEq[T] = PhantomEq[T, T]
 
-  implicit class EqualsDeco[T](val x: T) extends AnyVal {
-    def ===[U] (y: U) (using erased ce: PhantomEq[T, U]) = x.equals(y)
-  }
+  extension [T](x: T)
+    def ===[U](y: U)(using erased PhantomEq[T, U]) = x.equals(y)
 
-  implicit erased def eqString: PhantomEqEq[String] = ???
-  implicit erased def eqInt: PhantomEqEq[Int]       = ???
-  implicit erased def eqDouble: PhantomEqEq[Double] = ???
+  erased given eqString: PhantomEqEq[String] = ???
+  erased given eqInt: PhantomEqEq[Int]       = ???
+  erased given eqDouble: PhantomEqEq[Double] = ???
 
-  implicit erased def eqByteNum: PhantomEq[Byte, Number] = ???
-  implicit erased def eqNumByte: PhantomEq[Number, Byte] = ???
+  erased given eqByteNum: PhantomEq[Byte, Number] = ???
+  erased given eqNumByte: PhantomEq[Number, Byte] = ???
 
-  implicit erased def eqSeq[T, U] (using erased eq: PhantomEq[T, U]): PhantomEq[Seq[T], Seq[U]] = ???
+  erased given eqSeq[T, U](using erased PhantomEq[T, U]): PhantomEq[Seq[T], Seq[U]] = ???
 }

tests/pos-custom-args/phantom-Eq2/Phantom-Eq_1.scala

@@ -5,17 +5,14 @@ object EqUtil {
   final class PhantomEq[-L, -R] private[EqUtil]()
   type PhantomEqEq[T] = PhantomEq[T, T]
 
-  implicit class EqualsDeco[T](val x: T) extends AnyVal {
-    def ===[U] (y: U) (using erased ce: PhantomEq[T, U]) = x.equals(y)
-  }
+  extension [T](x: T)
+    def ===[U] (y: U) (using erased PhantomEq[T, U]) = x.equals(y)
 
-  implicit erased def eqString: PhantomEqEq[String] = new PhantomEq[String, String]
-  implicit erased def eqInt: PhantomEqEq[Int]       = new PhantomEq[Int, Int]
-  implicit erased def eqDouble: PhantomEqEq[Double] = new PhantomEq[Double, Double]
-
-  implicit erased def eqByteNum: PhantomEq[Byte, Number] = new PhantomEq[Byte, Number]
-  implicit erased def eqNumByte: PhantomEq[Number, Byte] = new PhantomEq[Number, Byte]
-
-  implicit erased def eqSeq[T, U] (using erased eq: PhantomEq[T, U]): PhantomEq[Seq[T], Seq[U]] =
+  erased given eqString: PhantomEqEq[String] = new PhantomEq[String, String]
+  erased given eqInt: PhantomEqEq[Int]       = new PhantomEq[Int, Int]
+  erased given eqDouble: PhantomEqEq[Double] = new PhantomEq[Double, Double]
+  erased given eqByteNum: PhantomEq[Byte, Number] = new PhantomEq[Byte, Number]
+  erased given eqNumByte: PhantomEq[Number, Byte] = new PhantomEq[Number, Byte]
+  erased given eqSeq[T, U] (using erased eq: PhantomEq[T, U]): PhantomEq[Seq[T], Seq[U]] =
     new PhantomEq[Seq[T], Seq[U]]
 }

tests/pos-custom-args/phantom-Evidence.scala

@@ -24,5 +24,5 @@ object WithNormalState {
 
 object Utils {
   type =::=[From, To]
-  implicit erased def tpEquals[A]: A =::= A = ???
+  erased given tpEquals[A]: A =::= A = ???
 }