Add precision to error message

Author: odersky

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

@@ -609,9 +609,10 @@ trait Checking {
       else {
         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

@@ -7,4 +7,6 @@ object Test {
   val 1 *: cs = 1 *: ()              // error
 
   val (_: Int | _: Any) = ??? : Any  // error
+
+  val 1 = 2  // error
 }