Add scala.Dynamic language feature check.

Author: nicolasstucki

src/dotty/tools/dotc/core/StdNames.scala

@@ -383,6 +383,7 @@ object StdNames {
     val delayedInit: N          = "delayedInit"
     val delayedInitArg: N       = "delayedInit$body"
     val drop: N                 = "drop"
+    val dynamics: N             = "dynamics"
     val dummyApply: N           = "<dummy-apply>"
     val elem: N                 = "elem"
     val emptyValDef: N          = "emptyValDef"

src/dotty/tools/dotc/reporting/Reporter.scala

@@ -68,6 +68,23 @@ trait Reporting { this: Context =>
   def featureWarning(msg: => String, pos: SourcePosition = NoSourcePosition): Unit =
     reporter.report(new FeatureWarning(msg, pos))
 
+  def featureWarning(pos: SourcePosition, feature: String, featureDescription: String, required: Boolean): Unit = {
+    val req     = if (required) "needs to" else "should"
+    val fqname  = "dotty.language." + feature
+
+    val explain = {
+      s"""|
+          |This can be achieved by adding the import clause 'import $fqname'
+          |or by setting the compiler option -language:$feature.
+          |See the Scala docs for value $fqname for a discussion
+          |why the feature $req be explicitly enabled.""".stripMargin
+    }
+
+    val msg = s"$featureDescription $req be enabled\nby making the implicit value $fqname visible.$explain"
+    if (required) error(msg, pos)
+    else featureWarning(msg, pos)
+  }
+
   def warning(msg: => String, pos: SourcePosition = NoSourcePosition): Unit =
     reporter.report(new Warning(msg, pos))
 

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

@@ -1098,7 +1098,12 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       .withType(dummy.nonMemberTermRef)
     checkVariance(impl1)
     if (!cls.is(AbstractOrTrait) && !ctx.isAfterTyper) checkRealizableBounds(cls.typeRef, cdef.pos)
-    assignType(cpy.TypeDef(cdef)(name, impl1, Nil), cls)
+    val cdef1 = assignType(cpy.TypeDef(cdef)(name, impl1, Nil), cls)
+    if (cdef1.tpe <:< defn.DynamicType && !ctx.featureEnabled(defn.LanguageModuleClass, nme.dynamics)) {
+      val isRequired = parents1.exists(_.tpe =:= defn.DynamicType)
+      ctx.featureWarning(cdef.pos, nme.dynamics.toString, "extension of type scala.Dynamic", isRequired)
+    }
+    cdef1
 
     // todo later: check that
     //  1. If class is non-abstract, it is instantiatable:

tests/neg/dynamicNoImport.scala

@@ -0,0 +1,3 @@
+class Foo extends scala.Dynamic // error
+trait Bar extends scala.Dynamic // error
+object Baz extends scala.Dynamic // error

tests/pos/dynamicExtendsNoImport.scala

@@ -0,0 +1,7 @@
+
+package foo {
+  import dotty.language.dynamics
+  class Bar extends scala.Dynamic
+}
+
+class Baz extends foo.Bar

tests/run/dynamicDynamicTests.scala

@@ -1,3 +1,4 @@
+import dotty.language.dynamics
 
 class Foo extends scala.Dynamic {
   def selectDynamic(name: String): String = "selectDynamic(" + name + ")"