fix #3200: bind var for singleton pattern get scrutinee type

Related issues:

https://issues.scala-lang.org/browse/SI-1503
#1463

Author: liufengyun

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

@@ -1320,7 +1320,12 @@ class Typer extends Namer
       case _ =>
         if (tree.name == nme.WILDCARD) body1
         else {
-          val sym = ctx.newPatternBoundSymbol(tree.name, body1.tpe.underlyingIfRepeated(isJava = false), tree.pos)
+          // for a singleton pattern like `x @ Nil`, `x` should get the type from the scrutinee
+          // see tests/neg/3200b.scala and SI-1503
+          val symTp =
+            if (body1.tpe.isInstanceOf[TermRef]) pt1
+            else body1.tpe.underlyingIfRepeated(isJava = false)
+          val sym = ctx.newPatternBoundSymbol(tree.name, symTp, tree.pos)
           if (ctx.mode.is(Mode.InPatternAlternative))
             ctx.error(i"Illegal variable ${sym.name} in pattern alternative", tree.pos)
           assignType(cpy.Bind(tree)(tree.name, body1), sym)

tests/neg/i3200.scala

@@ -0,0 +1,6 @@
+object Test {
+  case object Bob { override def equals(other: Any) = true }
+  def main(args: Array[String]): Unit = {
+    val m : Bob.type = (5: Any) match { case x @ Bob => x }  // error
+  }
+}

tests/neg/i3200b.scala

@@ -0,0 +1,5 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    val a : Nil.type = (Vector(): Any) match { case n @ Nil => n } // error
+  }
+}