Put erased under -Yerased-terms flag

`erased` will not be included in Scala 3.0 but could be added in future version.
This feature will only be available with `-Yerased-terms`.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -166,6 +166,7 @@ class ScalaSettings extends Settings.SettingGroup {
   // Extremely experimental language features
   val YnoKindPolymorphism: Setting[Boolean] = BooleanSetting("-Yno-kind-polymorphism", "Enable kind polymorphism (see https://dotty.epfl.ch/docs/reference/kind-polymorphism.html). Potentially unsound.")
   val YexplicitNulls: Setting[Boolean] = BooleanSetting("-Yexplicit-nulls", "Make reference types non-nullable. Nullable types can be expressed with unions: e.g. String|Null.")
+  val YerasedTerms: Setting[Boolean] = BooleanSetting("-Yerased-terms", "Allows the use of erased terms.")
 
   /** Area-specific debug output */
   val YexplainLowlevel: Setting[Boolean] = BooleanSetting("-Yexplain-lowlevel", "When explaining type errors, show types at a lower level.")

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -221,6 +221,7 @@ class Definitions {
   @tu lazy val ScalaXmlPackageClass: Symbol = ctx.getPackageClassIfDefined("scala.xml")
 
   @tu lazy val CompiletimePackageObject: Symbol = ctx.requiredModule("scala.compiletime.package")
+    @tu lazy val Compiletime_erasedValue  : Symbol = CompiletimePackageObject.requiredMethod("erasedValue")
     @tu lazy val Compiletime_error        : Symbol = CompiletimePackageObject.requiredMethod(nme.error)
     @tu lazy val Compiletime_constValue   : Symbol = CompiletimePackageObject.requiredMethod("constValue")
     @tu lazy val Compiletime_constValueOpt: Symbol = CompiletimePackageObject.requiredMethod("constValueOpt")

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

@@ -457,7 +457,8 @@ object Checking {
         sym.setFlag(Private) // break the overriding relationship by making sym Private
       }
     if (sym.is(Erased))
-      checkApplicable(Erased, !sym.isOneOf(MutableOrLazy))
+      if ctx.settings.YerasedTerms.value || sym == defn.Compiletime_erasedValue then checkApplicable(Erased, !sym.isOneOf(MutableOrLazy))
+      else fail("cannot use `erased` without " + ctx.settings.YerasedTerms.name)
   }
 
   /** Check the type signature of the symbol `M` defined by `tree` does not refer

compiler/test/dotty/tools/dotc/BootstrappedOnlyCompilationTests.scala

@@ -118,6 +118,7 @@ class BootstrappedOnlyCompilationTests extends ParallelTesting {
     aggregateTests(
       compileFilesInDir("tests/run-macros", defaultOptions),
       compileFilesInDir("tests/run-custom-args/Yretain-trees", defaultOptions and "-Yretain-trees"),
+      compileFilesInDir("tests/run-custom-args/Yerased-terms", defaultOptions and "-Yerased-terms"),
     )
   }.checkRuns()
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -47,6 +47,7 @@ class CompilationTests extends ParallelTesting {
       compileFilesInDir("tests/pos-special/isInstanceOf", allowDeepSubtypes.and("-Xfatal-warnings")),
       compileFilesInDir("tests/new", defaultOptions),
       compileFilesInDir("tests/pos-scala2", scala2CompatMode),
+      compileFilesInDir("tests/pos-custom-args/erased", defaultOptions.and("-Yerased-terms")),
       compileFilesInDir("tests/pos", defaultOptions),
       compileFilesInDir("tests/pos-deep-subtype", allowDeepSubtypes),
       compileFile(
@@ -114,6 +115,7 @@ class CompilationTests extends ParallelTesting {
       compileFilesInDir("tests/neg-no-kind-polymorphism", defaultOptions and "-Yno-kind-polymorphism"),
       compileFilesInDir("tests/neg-custom-args/deprecation", defaultOptions.and("-Xfatal-warnings", "-deprecation")),
       compileFilesInDir("tests/neg-custom-args/fatal-warnings", defaultOptions.and("-Xfatal-warnings")),
+      compileFilesInDir("tests/neg-custom-args/erased", defaultOptions.and("-Yerased-terms")),
       compileFilesInDir("tests/neg-custom-args/allow-double-bindings", allowDoubleBindings),
       compileFilesInDir("tests/neg-custom-args/explicit-nulls", defaultOptions.and("-Yexplicit-nulls")),
       compileDir("tests/neg-custom-args/impl-conv", defaultOptions.and("-Xfatal-warnings", "-feature")),
@@ -165,6 +167,7 @@ class CompilationTests extends ParallelTesting {
       compileFile("tests/run-custom-args/i5256.scala", allowDeepSubtypes),
       compileFile("tests/run-custom-args/fors.scala", defaultOptions and "-strict"),
       compileFile("tests/run-custom-args/no-useless-forwarders.scala", defaultOptions and "-Xmixin-force-forwarders:false"),
+      compileFilesInDir("tests/run-custom-args/erased", defaultOptions.and("-Yerased-terms")),
       compileFilesInDir("tests/run-deep-subtype", allowDeepSubtypes),
       compileFilesInDir("tests/run", defaultOptions)
     ).checkRuns()