Refuse trailing types parameters in extractors

Co-authored-by: Dimi Racordon <[email protected]>

Author: mbovel

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2421,13 +2421,15 @@ class ClassCannotExtendEnum(cls: Symbol, parent: Symbol)(using Context) extends
 }
 
 class NotAnExtractor(tree: untpd.Tree)(using Context) extends PatternMatchMsg(NotAnExtractorID) {
-  def msg(using Context) = i"$tree cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method"
+  def msg(using Context) = i"$tree cannot be used as an extractor in a pattern because it lacks an ${hl("unapply")} or ${hl("unapplySeq")} method with the appropriate signature"
   def explain(using Context) =
-    i"""|An ${hl("unapply")} method should be defined in an ${hl("object")} as follow:
+    i"""|An ${hl("unapply")} method should be in an ${hl("object")}, take a single explicit term parameter, and:
         |  - If it is just a test, return a ${hl("Boolean")}. For example ${hl("case even()")}
         |  - If it returns a single sub-value of type T, return an ${hl("Option[T]")}
         |  - If it returns several sub-values T1,...,Tn, group them in an optional tuple ${hl("Option[(T1,...,Tn)]")}
         |
+        |Additionnaly, ${hl("unapply")} or ${hl("unapplySeq")} methods can not take type parameters after their explicit term parameter.
+        |
         |Sometimes, the number of sub-values isn't fixed and we would like to return a sequence.
         |For this reason, you can also define patterns through ${hl("unapplySeq")} which returns ${hl("Option[Seq[T]]")}.
         |This mechanism is used for instance in pattern ${hl("case List(x1, ..., xn)")}"""

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

@@ -36,6 +36,7 @@ import annotation.threadUnsafe
 
 import scala.util.control.NonFatal
 import dotty.tools.dotc.inlines.Inlines
+import scala.annotation.tailrec
 
 object Applications {
   import tpd.*
@@ -1525,14 +1526,34 @@ trait Applications extends Compatibility {
     def trySelectUnapply(qual: untpd.Tree)(fallBack: (Tree, TyperState) => Tree): Tree = {
       // try first for non-overloaded, then for overloaded occurrences
       def tryWithName(name: TermName)(fallBack: (Tree, TyperState) => Tree)(using Context): Tree =
+        /** Returns `true` if there are type parameters after the last explicit
+         *  (non-implicit) term parameters list.
+         */
+        @tailrec
+        def hasTrailingTypeParams(paramss: List[List[Symbol]], acc: Boolean = false): Boolean =
+          paramss match
+            case Nil => acc
+            case params :: rest =>
+              val newAcc =
+                params match
+                  case param :: _ if param.isType => true
+                  case param :: _ if param.isTerm && !param.isOneOf(GivenOrImplicit) => false
+                  case _ => acc
+              hasTrailingTypeParams(paramss.tail, newAcc)
 
         def tryWithProto(qual: untpd.Tree, targs: List[Tree], pt: Type)(using Context) =
           val proto = UnapplyFunProto(pt, this)
           val unapp = untpd.Select(qual, name)
           val result =
             if targs.isEmpty then typedExpr(unapp, proto)
             else typedExpr(unapp, PolyProto(targs, proto)).appliedToTypeTrees(targs)
-          if !result.symbol.exists
+          if result.symbol.exists && hasTrailingTypeParams(result.symbol.paramSymss) then
+            // We don't accept `unapply` or `unapplySeq` methods with type
+            // parameters after the last explicit term parameter because we
+            // can't encode them: `UnApply` nodes cannot take type paremeters.
+            // See #22550 and associated test cases.
+            notAnExtractor(result)
+          else if !result.symbol.exists
              || result.symbol.name == name
              || ctx.reporter.hasErrors
           then result

tests/neg/22550.check

@@ -0,0 +1,12 @@
+-- [E127] Pattern Match Error: tests/neg/22550.scala:6:9 ---------------------------------------------------------------
+6 |    case Matches(x) => println(x) // error // error
+  |         ^^^^^^^
+  |Matches cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method with the appropriate signature
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E006] Not Found Error: tests/neg/22550.scala:6:31 ------------------------------------------------------------------
+6 |    case Matches(x) => println(x) // error // error
+  |                               ^
+  |                               Not found: x
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/22550.scala

@@ -0,0 +1,6 @@
+object Matches:
+  def unapply(y: Any)[T]: Option[Any] = None
+
+def main =
+  42 match
+    case Matches(x) => println(x) // error // error

tests/neg/22550b.check

@@ -0,0 +1,12 @@
+-- [E127] Pattern Match Error: tests/neg/22550b.scala:14:9 -------------------------------------------------------------
+14 |    case Matches[Unit](x) => println(x) // error // error
+   |         ^^^^^^^^^^^^
+   |Matches[Unit] cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method with the appropriate signature
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E006] Not Found Error: tests/neg/22550b.scala:14:37 ----------------------------------------------------------------
+14 |    case Matches[Unit](x) => println(x) // error // error
+   |                                     ^
+   |                                     Not found: x
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/22550b.scala

@@ -0,0 +1,14 @@
+case class Data[A]()
+
+trait TCl[A, B]
+
+object Matches:
+  def unapply[A](adt: Data[?])[B](using
+      ft: TCl[A, B]
+  ): Option[Data[A]] = None
+
+given TCl[Unit, String] = new TCl {}
+
+def main =
+  Data() match
+    case Matches[Unit](x) => println(x) // error // error

tests/neg/bad-unapplies.check

@@ -10,13 +10,13 @@
 -- [E127] Pattern Match Error: tests/neg/bad-unapplies.scala:23:9 ------------------------------------------------------
 23 |    case B("2") => // error (cannot be used as an extractor)
    |         ^
-   |         B cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method
+   |B cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method with the appropriate signature
    |
    | longer explanation available when compiling with `-explain`
 -- [E127] Pattern Match Error: tests/neg/bad-unapplies.scala:24:9 ------------------------------------------------------
 24 |    case D("2") => // error (cannot be used as an extractor)
    |         ^
-   |         D cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method
+   |D cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method with the appropriate signature
    |
    | longer explanation available when compiling with `-explain`
 -- [E050] Type Error: tests/neg/bad-unapplies.scala:25:9 ---------------------------------------------------------------

tests/neg/i18684.check

@@ -67,15 +67,17 @@
 -- [E127] Pattern Match Error: tests/neg/i18684.scala:12:6 -------------------------------------------------------------
 12 |  val inner(x) = 3 // error
    |      ^^^^^
-   |      Test.inner cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method
+   |Test.inner cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method with the appropriate signature
    |--------------------------------------------------------------------------------------------------------------------
    | Explanation (enabled by `-explain`)
    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-   | An unapply method should be defined in an object as follow:
+   | An unapply method should be in an object, take a single explicit term parameter, and:
    |   - If it is just a test, return a Boolean. For example case even()
    |   - If it returns a single sub-value of type T, return an Option[T]
    |   - If it returns several sub-values T1,...,Tn, group them in an optional tuple Option[(T1,...,Tn)]
    |
+   | Additionnaly, unapply or unapplySeq methods can not take type parameters after their explicit term parameter.
+   |
    | Sometimes, the number of sub-values isn't fixed and we would like to return a sequence.
    | For this reason, you can also define patterns through unapplySeq which returns Option[Seq[T]].
    | This mechanism is used for instance in pattern case List(x1, ..., xn)