Add precision to error message

Author: odersky

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

@@ -607,9 +607,10 @@ trait Checking {
     def fail(pat: Tree, pt: Type): Boolean = {
       var reportedPt = pt.dropAnnot(defn.UncheckedAnnot)
       if (!pat.tpe.isSingleton) reportedPt = reportedPt.widen
+      val problem = if (pat.tpe <:< pt) "is more specialized" else "does not match"
       val fix = if (isPatDef) "`: @unchecked` after" else "`case ` before"
       ctx.errorOrMigrationWarning(
-        ex"""pattern's type ${pat.tpe} is more specialized than the right hand side expression's type $reportedPt
+        ex"""pattern's type ${pat.tpe} $problem than the right hand side expression's type $reportedPt
             |
             |If the narrowing is intentional, this can be communicated by writing $fix the full pattern.${err.rewriteNotice}""",
         pat.sourcePos)

tests/neg-strict/unchecked-patterns.scala

@@ -9,6 +9,8 @@ object Test {
 
   val (_: Int | _: Any) = ??? : Any  // error
 
+  val 1 = 2  // error
+
   object Positive { def unapply(i: Int): Option[Int] = Some(i).filter(_ > 0) }
   object Always1 { def unapply(i: Int): Some[Int] = Some(i) }
   object Pair { def unapply(t: (Int, Int)): t.type = t }