Invent given pattern name in for comprehension

Author: som-snytt

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

@@ -1653,18 +1653,19 @@ object desugar {
        *  that refers to the bound variable for the pattern. Wildcard Binds are
        *  also replaced by Binds with fresh names.
        */
-      def makeIdPat(pat: Tree): (Tree, Ident) = pat match {
-        case bind @ Bind(name, pat1) =>
-          if name == nme.WILDCARD then
-            val name = UniqueName.fresh()
-            (cpy.Bind(pat)(name, pat1).withMods(bind.mods), Ident(name))
-          else (pat, Ident(name))
+      def makeIdPat(pat: Tree): (Tree, Ident) = pat match
+        case pat @ Bind(nme.WILDCARD, body) =>
+          val name =
+            body match
+            case Typed(Ident(nme.WILDCARD), tpt) if pat.mods.is(Given) => inventGivenName(tpt)
+            case _ => UniqueName.fresh()
+          (cpy.Bind(pat)(name, body).withMods(pat.mods), Ident(name))
+        case Bind(name, _) => (pat, Ident(name))
         case id: Ident if isVarPattern(id) && id.name != nme.WILDCARD => (id, id)
         case Typed(id: Ident, _) if isVarPattern(id) && id.name != nme.WILDCARD => (pat, id)
         case _ =>
           val name = UniqueName.fresh()
           (Bind(name, pat), Ident(name))
-      }
 
       /** Make a pattern filter:
        *    rhs.withFilter { case pat => true case _ => false }

tests/pos/i23119.scala

@@ -0,0 +1,29 @@
+//> using options -Wunused:patvars -Werror
+
+def make: IndexedSeq[FalsePositive] =
+  for {
+    i <- 1 to 2
+    given Int = i
+    fp = FalsePositive()
+  } yield fp
+
+def broken =
+  for
+    i <- List(42)
+    (x, y) = "hello" -> "world"
+  yield
+    s"$x, $y" * i
+
+def alt: IndexedSeq[FalsePositive] =
+  given String = "hi"
+  for
+    given Int <- 1 to 2
+    j: Int = summon[Int] // simple assign because irrefutable
+    _ = j + 1
+    k :: Nil = j :: Nil : @unchecked // pattern in one var
+    fp = FalsePositive(using k)
+  yield fp
+
+class FalsePositive(using Int):
+  def usage(): Unit =
+    println(summon[Int])