forbid given patterns in val definitions.

Suggest that they use an alias
given instead.

Author: bishabosha

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

@@ -1093,6 +1093,24 @@ object desugar {
     case IdPattern(named, tpt) =>
       derivedValDef(original, named, tpt, rhs, mods)
     case _ =>
+      def givenPatternMessage(givenPat: Bind, tpt: Tree)(using Context): String =
+        val pat =
+          val givenTpt = em"${hl("given")} $tpt"
+          if givenPat.name == nme.WILDCARD then
+            givenTpt
+          else
+            em"${givenPat.name} @ $givenTpt"
+        em"""pattern `$pat` is not allowed here,
+            |please bind to a variable and use an alias given.""".stripMargin
+      end givenPatternMessage
+
+      def filterWildcardGivenBinding(givenPat: Bind, tpt: Tree): Boolean =
+        givenPat.name != nme.WILDCARD
+
+      def errorOnGivenBinding(bind: Bind, tpt: Tree)(using Context): Boolean =
+        report.error(givenPatternMessage(bind, tpt), bind)
+        false
+
       def isTuplePattern(arity: Int): Boolean = pat match {
         case Tuple(pats) if pats.size == arity =>
           pats.forall(isVarPattern)
@@ -1108,13 +1126,23 @@ object desugar {
       // - `pat` is a tuple of N variables or wildcard patterns like `(x1, x2, ..., xN)`
       val tupleOptimizable = forallResults(rhs, isMatchingTuple)
 
+      val inAliasGenerator = original match
+        case _: GenAlias => true
+        case _ => false
+
       val vars =
         if (tupleOptimizable) // include `_`
-          pat match {
-            case Tuple(pats) =>
-              pats.map { case id: Ident => id -> TypeTree() }
-          }
-        else getVariables(pat)  // no `_`
+          pat match
+            case Tuple(pats) => pats.map { case id: Ident => id -> TypeTree() }
+        else
+          getVariables(
+            tree = pat,
+            shouldAddGiven =
+              if inAliasGenerator then
+                filterWildcardGivenBinding
+              else
+                errorOnGivenBinding
+          ) // no `_`
 
       val ids = for ((named, _) <- vars) yield Ident(named.name)
       val caseDef = CaseDef(pat, EmptyTree, makeTuple(ids))
@@ -1800,16 +1828,21 @@ object desugar {
   /** Returns list of all pattern variables, possibly with their types,
    *  without duplicates
    */
-  private def getVariables(tree: Tree)(using Context): List[VarInfo] = {
+  private def getVariables(tree: Tree, shouldAddGiven: Context ?=> (Bind, Tree) => Boolean)(using Context): List[VarInfo] = {
     val buf = ListBuffer[VarInfo]()
     def seenName(name: Name) = buf exists (_._1.name == name)
     def add(named: NameTree, t: Tree): Unit =
       if (!seenName(named.name) && named.name.isTermName) buf += ((named, t))
     def collect(tree: Tree): Unit = tree match {
-      case Bind(nme.WILDCARD, tree1) =>
+      case tree @ Bind(nme.WILDCARD, tree1) =>
+        if tree.mods.is(Given) then
+          val Typed(_, tpt) = tree1: @unchecked
+          if shouldAddGiven(tree, tpt) then
+            add(tree, tpt)
         collect(tree1)
       case tree @ Bind(_, Typed(tree1, tpt)) =>
-        add(tree, tpt)
+        if !(tree.mods.is(Given) && !shouldAddGiven(tree, tpt)) then
+          add(tree, tpt)
         collect(tree1)
       case tree @ Bind(_, tree1) =>
         add(tree, TypeTree())
@@ -1827,7 +1860,7 @@ object desugar {
       case SeqLiteral(elems, _) =>
         elems foreach collect
       case Alternative(trees) =>
-        for (tree <- trees; (vble, _) <- getVariables(tree))
+        for (tree <- trees; (vble, _) <- getVariables(tree, shouldAddGiven))
           report.error(IllegalVariableInPatternAlternative(), vble.srcPos)
       case Annotated(arg, _) =>
         collect(arg)

tests/neg/given-pattern.scala

@@ -4,11 +4,11 @@ class Test {
   import scala.collection.immutable.{TreeSet, HashSet}
 
   def f2[T](x: Ordering[T]) = {
-    val (given Ordering[T]) = x
-    new TreeSet[T]    // error: no implicit ordering defined for T
+    val (given Ordering[T]) = x // error: given Ordering[T] not allowed here
+    new TreeSet[T] // error: no implicit ordering defined for T
   }
   def f3[T](x: Ordering[T]) = {
-    val given Ordering[T] = x
-    new TreeSet[T]    // error: no implicit ordering defined for T
+    val given Ordering[T] = x // error: given Ordering[T] not allowed here
+    new TreeSet[T] // error: no implicit ordering defined for T
   }
-}
+}

tests/neg/i11897.check

@@ -0,0 +1,45 @@
+-- Error: tests/neg/i11897.scala:11:10 ---------------------------------------------------------------------------------
+11 |  val (x, given A) = (1, A(23)) // error
+   |          ^^^^^^^
+   |          pattern `given A` is not allowed here,
+   |          please bind to a variable and use an alias given.
+-- Error: tests/neg/i11897.scala:12:10 ---------------------------------------------------------------------------------
+12 |  val (_, given B) = (true, B(false)) // error
+   |          ^^^^^^^
+   |          pattern `given B` is not allowed here,
+   |          please bind to a variable and use an alias given.
+-- Error: tests/neg/i11897.scala:13:8 ----------------------------------------------------------------------------------
+13 |  val D(given C) = D(C("c")) // error
+   |        ^^^^^^^
+   |        pattern `given C` is not allowed here,
+   |        please bind to a variable and use an alias given.
+-- Error: tests/neg/i11897.scala:14:11 ---------------------------------------------------------------------------------
+14 |  val F(y, given E) = F(47, E(93)) // error
+   |           ^^^^^^^
+   |           pattern `given E` is not allowed here,
+   |           please bind to a variable and use an alias given.
+-- Error: tests/neg/i11897.scala:15:11 ---------------------------------------------------------------------------------
+15 |  val H(z, q @ given G) = H(47, G(101)) // error
+   |           ^^^^^^^^^^^
+   |           pattern `q @ given G` is not allowed here,
+   |           please bind to a variable and use an alias given.
+-- Error: tests/neg/i11897.scala:16:18 ---------------------------------------------------------------------------------
+16 |  assert(summon[A] == A(23)) // error
+   |                  ^
+   |                  no implicit argument of type A was found for parameter x of method summon in object Predef
+-- Error: tests/neg/i11897.scala:17:18 ---------------------------------------------------------------------------------
+17 |  assert(summon[B] == B(false)) // error
+   |                  ^
+   |                  no implicit argument of type B was found for parameter x of method summon in object Predef
+-- Error: tests/neg/i11897.scala:18:18 ---------------------------------------------------------------------------------
+18 |  assert(summon[C] == C("c")) // error
+   |                  ^
+   |                  no implicit argument of type C was found for parameter x of method summon in object Predef
+-- Error: tests/neg/i11897.scala:19:18 ---------------------------------------------------------------------------------
+19 |  assert(summon[E] == E(93)) // error
+   |                  ^
+   |                  no implicit argument of type E was found for parameter x of method summon in object Predef
+-- Error: tests/neg/i11897.scala:20:18 ---------------------------------------------------------------------------------
+20 |  assert(summon[G] == G(101)) // error
+   |                  ^
+   |                  no implicit argument of type G was found for parameter x of method summon in object Predef

tests/neg/i11897.scala

@@ -0,0 +1,20 @@
+case class A(i: Int)
+case class B(b: Boolean)
+case class C(s: String)
+case class D(c: C)
+case class E(i: Int)
+case class F(i: Int, e: E)
+case class G(i: Int)
+case class H(i: Int, e: G)
+
+def Test =
+  val (x, given A) = (1, A(23)) // error
+  val (_, given B) = (true, B(false)) // error
+  val D(given C) = D(C("c")) // error
+  val F(y, given E) = F(47, E(93)) // error
+  val H(z, q @ given G) = H(47, G(101)) // error
+  assert(summon[A] == A(23)) // error
+  assert(summon[B] == B(false)) // error
+  assert(summon[C] == C("c")) // error
+  assert(summon[E] == E(93)) // error
+  assert(summon[G] == G(101)) // error

tests/run/i10178.scala

@@ -3,3 +3,42 @@
     x <- Option(23)
     given Int = x
   do assert(summon[Int] == 23)
+
+  for
+    y <- Option("ok")
+    q @ given String = y
+  do assert(summon[String] == "ok")
+
+  for
+    z <- Option("key" -> true)
+    (q @ given String, u @ given Boolean) = z
+  do
+    assert(summon[String] == "key")
+    assert(summon[Boolean] == true)
+
+  for
+    w <- Option("no" -> false)
+    (given String, given Boolean) = w
+  do
+    assert(summon[String] == "no")
+    assert(summon[Boolean] == false)
+
+  for
+    given Int <- Option(23)
+  do assert(summon[Int] == 23)
+
+  for
+    q @ given String <- Option("ok")
+  do assert(summon[String] == "ok")
+
+  for
+    (q @ given String, u @ given Boolean) <- Option("key" -> true)
+  do
+    assert(summon[String] == "key")
+    assert(summon[Boolean] == true)
+
+  for
+    (given String, given Boolean) <- Option("no" -> false)
+  do
+    assert(summon[String] == "no")
+    assert(summon[Boolean] == false)