Add "maybe" language import

Author: odersky

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

@@ -45,6 +45,7 @@ object Feature:
   val magic = experimental("magic")
   val inlineTraits = experimental("inlineTraits")
   val specializedTraits = experimental("specializedTraits")
+  val maybe = experimental("maybe")
 
   val nonViralExperimentalFeatures: Set[TermName] =
     Set(captureChecking, separationChecking, safe, magic)
@@ -86,6 +87,7 @@ object Feature:
     (magic, "Enable extensions for working with coding agents"),
     (inlineTraits, "Allow inline traits"),
     (specializedTraits, "Allow specialized traits"),
+    (maybe, "Allow T? and T ? E types"),
   )
 
   /** Features that are now standard; the language import / -language choice is
@@ -193,10 +195,13 @@ object Feature:
     || enabledBySetting(specializedTraits)
     || ctx.compilationUnit.knowsInlineTraits
 
+  def maybeEnabled(using Context) =
+    enabled(maybe) || magicEnabled
+
   /** Is pureFunctions enabled for this compilation unit? */
   def pureFunsEnabled(using Context) =
     enabledBySetting(pureFunctions)
-    || ctx.compilationUnit.knowsPureFuns
+    || ctx.originalCompilationUnit.knowsPureFuns
     || ccEnabled
 
   /** Is capture checking enabled by a command-line setting? */

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

@@ -2116,7 +2116,7 @@ object Parsers {
           if in.token == LBRACE
           then makeRetaining(t, captureSet(), tpnme.retains)
           else makeRetaining(t, Nil, tpnme.retainsCap)
-      else if in.isIdent(nme.?) && Feature.magicEnabled && isPostfixQmark(isType = true) then
+      else if in.isIdent(nme.?) && Feature.maybeEnabled && isPostfixQmark(isType = true) then
         atSpan(t.span.start):
           PostfixOp(t, typeIdent())
       else
@@ -2678,7 +2678,7 @@ object Parsers {
 
     def expr1(location: Location = Location.Elsewhere): Tree = in.token match
       case IF =>
-        ifExpr(in.offset, If, canOmitThen = Feature.magicEnabled && location == Location.InBlock)
+        ifExpr(in.offset, If, canOmitThen = Feature.maybeEnabled && location == Location.InBlock)
       case WHILE =>
         atSpan(in.skipToken()) {
           val cond = condExpr(DO)
@@ -3127,7 +3127,7 @@ object Parsers {
         case USCORE =>
           atSpan(startOffset(t), in.skipToken()) { PostfixOp(t, Ident(nme.WILDCARD)) }
         case _ =>
-          if in.isIdent(nme.?) && Feature.magicEnabled && isPostfixQmark(isType = false) then
+          if in.isIdent(nme.?) && Feature.maybeEnabled && isPostfixQmark(isType = false) then
             atSpan(t.span.start):
               PostfixOp(t, termIdent())
           else if in.isColon && location == Location.InParens && followingIsLambdaParams() then

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -894,7 +894,7 @@ object PatternMatcher {
             false
 
         override def apply(plan: SeqPlan): Plan = {
-          if Feature.magicEnabled then
+          if Feature.maybeEnabled then
             plan.head = apply(plan.head)
             plan.tail = apply(plan.tail)
             plan.head match

library/src/scala/language.scala

@@ -270,7 +270,7 @@ object language {
     object erasedDefinitions
 
     /** Experimental support for specialized traits
-     * 
+     *
      * @see [[docs/_docs/internals/specialized-traits.md]]
      */
     @compileTimeOnly("`specializedTraits` can only be used at compile time in import statements")
@@ -425,7 +425,11 @@ object language {
     @compileTimeOnly("`magic` can only be used at compile time in import statements")
     object magic
 
-    
+    /** Experimental support for maybe types `T?` and result types `T ? E`.
+     */
+    @compileTimeOnly("`maybe` can only be used at compile time in import statements")
+    object maybe
+
     /** Experimental support for inline traits
       */
     @compileTimeOnly("`inlineTraits` can only be used at compile time in import statements")

tests/pos/i16342-maybe.scala

@@ -0,0 +1,20 @@
+//> using options -Yexplicit-nulls
+import language.experimental.maybe
+type Opaque = Base & Tag
+
+type Base = Any {
+  type Hack
+}
+
+trait Tag
+
+object Opaque {
+  def apply(value: String): Opaque = value.asInstanceOf[Opaque]
+
+  def unapply(userId: Opaque): String? = userId.value
+  def unappy2(userId: Base & Tag): Option[String] = Option(userId).map(_.value)
+}
+
+final implicit class Ops(private val userId: Opaque) extends AnyVal {
+  def value: String = userId.asInstanceOf[String]
+}