check qualifier is stable in pattern select

Author: liufengyun

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

@@ -75,6 +75,15 @@ object RefChecks {
     }
   }
 
+  /** Check that a stable identifier pattern is indeed stable (SLS 8.1.5)
+   */
+  private def checkStableIdentPattern(tree: Tree)(implicit ctx: Context) = tree match {
+    case _: Select | _: Ident if !isWildcardArg(tree)  =>
+      if (!tree.tpe.isStable)
+        ctx.error(s"stable identifier required, but ${tree.show} found", tree.pos)
+    case _ =>
+  }
+
   /** The this-type of `cls` which should be used when looking at the types of
    *  inherited members. If `cls` has a non-trivial self type, this returns a skolem
    *  with the class type instead of the `this-type` of the class as usual.
@@ -884,6 +893,26 @@ class RefChecks extends MiniPhase { thisPhase =>
     currentLevel.enterReference(sym, tree.pos)
     tree
   }
+
+  override def transformUnApply(tree: UnApply)(implicit ctx: Context): Tree = {
+    tree.patterns.foreach(checkStableIdentPattern(_))
+    tree
+  }
+
+  override def transformAlternative(tree: Alternative)(implicit ctx: Context): Tree = {
+    tree.trees.foreach(checkStableIdentPattern(_))
+    tree
+  }
+
+  override def transformBind(tree: Bind)(implicit ctx: Context): Tree = {
+     checkStableIdentPattern(tree.body)
+    tree
+  }
+
+  override def transformCaseDef(tree: CaseDef)(implicit ctx: Context) = {
+    checkStableIdentPattern(tree.pat)
+    tree
+  }
 }
 
 /* todo: rewrite and re-enable

tests/neg/i1846.scala

@@ -11,7 +11,6 @@ object Test {
     x match { case { "42" }.toInt   => () } // error
     x match { case { "42".toInt }   => () } // error
     x match { case Y                => () } // error
-    x match { case Y.toInt          => () } // ok
     x match { case { Y.toInt }      => () } // error
     x match { case { Y }.toInt      => () } // error
     x match { case Y.toString       => () } // error

tests/neg/i3812.scala

@@ -1,14 +1,17 @@
 object Test {
   def main(args: Array[String]): Unit = {
+    case class Box(v: Int)
+
     val x = 42
     val Y = "42"
+    var Z1 = Box(4)
+    val Z2 = Box(4)
 
     x match { case { 42 }           => () } // error
     x match { case { "42".toInt }   => () } // error
     x match { case { "42" }.toInt   => () } // error
     x match { case { "42".toInt }   => () } // error
     x match { case { Y.toInt }      => () } // error
     x match { case { Y }.toInt      => () } // error
-    x match { case Y.toInt          => () } // ok
   }
 }

tests/neg/i3812b.scala

@@ -0,0 +1,26 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    case class Box(v: Int)
+
+    val x = 42
+    var Y1 = 42
+    val Y2 = "42"
+    var Z1 = Box(4)
+    val Z2 = Box(4)
+
+    x match { case Y1                => () } // error
+    x match { case Y2.toInt          => () } // error
+
+    x match { case Z1.v              => () } // error
+    x match { case Z2.v              => () } // ok
+
+    Some(x) match { case Some(Z1.v)  => () } // error
+    Some(x) match { case Some(Z2.v)  => () } // ok
+
+    Some(x) match { case Some(4) | Some(Z1.v) => () } // error
+    Some(x) match { case a @ Some(Z1.v)       => () } // error
+
+    Some(x) match { case Some(4) | Some(Z2.v) => () } // ok
+    Some(x) match { case a @ Some(Z2.v)       => () } // ok
+  }
+}