Prioritise tree type over proto type when typing Binds

Author: jchyb

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

@@ -2875,7 +2875,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 // leave the original tuple type; don't mix with & TupleXXL which would only obscure things
                 pt
               case _ =>
-                pt & body1.tpe
+                body1.tpe & pt
           val sym = newPatternBoundSymbol(name, symTp, tree.span)
           if (pt == defn.ImplicitScrutineeTypeRef || tree.mods.is(Given)) sym.setFlag(Given)
           if (ctx.mode.is(Mode.InPatternAlternative))

tests/pos/i24038a.scala

@@ -0,0 +1,31 @@
+final class MBufferLong:
+  final def +=(elem: Long): this.type = ???
+
+type M[Tup <: Tuple] <: Tuple = Tup match
+  case EmptyTuple => EmptyTuple
+  case h *: t     => BufferOf[h] *: M[t]
+
+type M2[T <: Tuple] <: Tuple = (T, M[T]) match
+  case (h *: t, a *: b)         => BufferOf[h] *: M2[t]
+  case (EmptyTuple, EmptyTuple) => EmptyTuple
+  case (_, EmptyTuple)          => EmptyTuple
+  case (EmptyTuple, _)          => EmptyTuple
+
+type BufferOf[T] = T match
+  case Long              => MBufferLong
+
+inline def append[T](t: T, buffer: BufferOf[T]): BufferOf[T] =
+  inline (t, buffer) match
+    case (x: Long, y: BufferOf[Long])     => y.+=(x)
+  buffer
+
+transparent inline def appendBuffers[T <: Tuple](t: T, buffers: M[T]): M2[T] = {
+  inline (t, buffers) match
+    case abcd: ((h *: t), bh *: bt) =>
+      val (hh *: tt, bh *: bt) = abcd
+      val x: BufferOf[h] = append[h](hh, bh.asInstanceOf[BufferOf[h]])
+      x *: appendBuffers[t](tt, bt.asInstanceOf[M[t]])
+    case _: (EmptyTuple, EmptyTuple) => EmptyTuple
+    case _: (_, EmptyTuple)          => EmptyTuple
+    case _: (EmptyTuple, _)          => EmptyTuple
+}

tests/pos/i24038b.scala

@@ -0,0 +1,12 @@
+final class MBufferLong
+
+type M[Tup <: Tuple] <: Tuple = Tup match
+  case EmptyTuple => EmptyTuple
+  case h *: t     => MBufferLong *: M[t]
+
+def appendBuffers[T <: Tuple](t: T, buffers: M[T]): Unit = {
+  (t, buffers) match
+    case abcd: (h *: t, bh *: bt) =>
+      val (hh *: tt, bh *: bt) = abcd
+      summon[hh.type <:< h]
+}