Refine handling of pattern binders for large tuples

Author: odersky

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

@@ -2434,7 +2434,13 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                                          // wrt to operand order for `&`, we include the explicit subtype test here.
                                          // See also #5649.
             then body1.tpe
-            else pt & body1.tpe
+            else body1.tpe match
+              case btpe: TypeRef
+              if btpe.symbol == defn.TupleXXLClass && pt.tupleElementTypes.isDefined =>
+                // leave the original tuple type; don't mix with & TupleXXL which would only obscure things
+                pt
+              case _ =>
+                pt & body1.tpe
           val sym = newPatternBoundSymbol(name, symTp, tree.span)
           if (pt == defn.ImplicitScrutineeTypeRef || tree.mods.is(Given)) sym.setFlag(Given)
           if (ctx.mode.is(Mode.InPatternAlternative))