Disallow maybes with wildcard arguments

Author: odersky

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

@@ -129,11 +129,13 @@ object Checking {
       checkBounds(args, bounds, instantiate, tree.tpe, tpt)
 
     def checkWildcardApply(tp: Type): Unit = tp match {
-      case tp @ AppliedType(tycon, _) =>
+      case tp @ AppliedType(tycon, _) if tp.hasWildcardArg =>
         if tp.isUnreducibleWild then
           report.errorOrMigrationWarning(
             showInferred(UnreducibleApplication(tycon), tp, tpt),
             tree.srcPos, MigrationVersion.Scala2to3)
+        else if tp.typeSymbol == defn.MagicMaybeClass then
+          report.error(em"Maybe type may not contain wildcard arguments", tree.srcPos)
       case _ =>
     }
     def checkValidIfApply(using Context): Unit =

tests/pos/i14896-maybe.scala tests/neg/i14896-maybe.scalatests/pos/i14896-maybe.scala renamed to tests/neg/i14896-maybe.scala

@@ -1,4 +1,4 @@
 //> using options -Yexplicit-nulls
 import language.experimental.magic
-object Ex  { def unapply(p: Any): (? <: Int)? = null }
+object Ex  { def unapply(p: Any): (? <: Int)? = null } // error
 object Foo { val Ex(_) = null: @unchecked }

tests/neg/t8128-maybe.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/t8128-maybe.scala:6:23 -----------------------------------------------------------------------------
+6 |  def unapply(m: Any): Maybe[?, Unit] = Ok("")  // error
+  |                       ^^^^^^^^^^^^^^
+  |                       Maybe type may not contain wildcard arguments

tests/neg/t8128-maybe.scala

@@ -0,0 +1,8 @@
+//> using options -Yexplicit-nulls
+import language.experimental.magic
+import scala.magic.*
+import compiletime.Maybe
+object G {
+  def unapply(m: Any): Maybe[?, Unit] = Ok("")  // error
+}
+

tests/pos/extractor-types-maybe.scala

@@ -3,7 +3,7 @@ import language.experimental.magic
 import scala.magic.*
 import compiletime.Maybe
 object G {
-  def unapply(m: Any): Maybe[?, Unit] = Ok("")
+  def unapply(m: Any): Maybe[Any, Unit] = Ok("")
 }
 
 object H {