Use the flag as a way to override the compiler default value

Author: allanrenucci

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

@@ -499,7 +499,7 @@ object desugar {
         } :+ defaultCase
         val body = Match(paramRef, patternMatchCases)
         DefDef(nme.productElementName, Nil, List(List(methodParam)), javaDotLangDot(tpnme.String), body)
-          .withFlags(if (ctx.settings.YnewLibrary.value) Override | Synthetic else Synthetic)
+          .withFlags(if (defn.isNewCollections) Override | Synthetic else Synthetic)
       }
 
       if (isCaseClass)

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

@@ -95,7 +95,7 @@ class ScalaSettings extends Settings.SettingGroup {
 
   val Yscala2Unpickler: Setting[String] = StringSetting("-Yscala2-unpickler", "", "Control where we may get Scala 2 symbols from. This is either \"always\", \"never\", or a classpath.", "always")
   // TODO: Remove once we drop support for 2.12 standard library
-  val YnewLibrary: Setting[Boolean] = BooleanSetting("-Ynew-library", "Inform the compiler that we are using the 2.13 standard library.")
+  val YnewCollections: Setting[Boolean] = BooleanSetting("-Ynew-collections", "Inform the compiler that we are using the 2.13 collection library (even if the 2.12 library is on the classpath).")
 
   val YnoImports: Setting[Boolean] = BooleanSetting("-Yno-imports", "Compile without importing scala.*, java.lang.*, or Predef.")
   val YnoInline: Setting[Boolean] = BooleanSetting("-Yno-inline", "Suppress inlining.")

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

@@ -396,7 +396,8 @@ class Definitions {
     def newArrayMethod(implicit ctx: Context): TermSymbol = DottyArraysModule.requiredMethod("newArray")
 
   // TODO: Remove once we drop support for 2.12 standard library
-  private def isNewCollections = ctx.settings.YnewLibrary.value
+  lazy val isNewCollections: Boolean = ctx.settings.YnewCollections.value ||
+    ctx.base.staticRef("scala.collection.IterableOnce".toTypeName).exists
 
   def getWrapVarargsArrayModule: Symbol = if (isNewCollections) ScalaRuntimeModule else ScalaPredefModule
 

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

@@ -48,7 +48,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("tests/pos-special/completeFromSource/Test2.scala", defaultOptions.and("-sourcepath", "tests/pos-special")) +
     compileFile("tests/pos-special/completeFromSource/Test3.scala", defaultOptions.and("-sourcepath", "tests/pos-special", "-scansource")) +
     compileFile("tests/pos-special/completeFromSource/nested/Test4.scala", defaultOptions.and("-sourcepath", "tests/pos-special", "-scansource")) +
-    compileFile("tests/pos-special/repeatedArgs213.scala", defaultOptions.and("-Ynew-library")) +
+    compileFile("tests/pos-special/repeatedArgs213.scala", defaultOptions.and("-Ynew-collections")) +
     compileFilesInDir("tests/pos-special/fatal-warnings", defaultOptions.and("-Xfatal-warnings", "-feature")) +
     compileList(
       "compileMixed",
@@ -164,7 +164,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("tests/neg-custom-args/i3627.scala", allowDeepSubtypes) +
     compileFile("tests/neg-custom-args/matchtype-loop.scala", allowDeepSubtypes) +
     compileFile("tests/neg-custom-args/completeFromSource/nested/Test1.scala", defaultOptions.and("-sourcepath", "tests/neg-custom-args", "-scansource")) +
-    compileFile("tests/neg-custom-args/repeatedArgs213.scala", defaultOptions.and("-Ynew-library"))
+    compileFile("tests/neg-custom-args/repeatedArgs213.scala", defaultOptions.and("-Ynew-collections"))
   }.checkExpectedErrors()
 
   // Run tests -----------------------------------------------------------------