Fix #9740: harden type checking for pattern match on objects

We enforce that when pattern match on an object, the object should be
a subtype of the scrutinee type.

Reasons for doing so:

- such code patterns usually implies hidden errors in the code
- it's always safe/sound to reject the code

We could check whether `equals` is overridden in the object, but

- it complicates the protocol
- overriding `equals` of object is also a bad practice
- there is no sign that the slightly improved completeness is useful

Author: liufengyun

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

@@ -3780,11 +3780,20 @@ class Typer extends Namer
         withMode(Mode.GadtConstraintInference) {
           TypeComparer.constrainPatternType(tree.tpe, pt)
         }
-        val cmp =
-          untpd.Apply(
-            untpd.Select(untpd.TypedSplice(tree), nme.EQ),
-            untpd.TypedSplice(dummyTreeOfType(pt)))
-        typedExpr(cmp, defn.BooleanType)
+
+        if tree.symbol.is(Module) && !(tree.tpe <:< pt) then
+          // We could check whether `equals` is overriden.
+          // Reasons for not doing so:
+          // - it complicates the protocol
+          // - such code patterns usually implies hidden errors in the code
+          // - it's safe/sound to reject the code
+          report.error(TypeMismatch(tree.tpe, pt, "\npattern type is incompatible with expected type"), tree.srcPos)
+        else
+          val cmp =
+            untpd.Apply(
+              untpd.Select(untpd.TypedSplice(tree), nme.EQ),
+              untpd.TypedSplice(dummyTreeOfType(pt)))
+          typedExpr(cmp, defn.BooleanType)
       case _ =>
 
   private def checkStatementPurity(tree: tpd.Tree)(original: untpd.Tree, exprOwner: Symbol)(using Context): Unit =

tests/neg/i9740.scala

@@ -0,0 +1,16 @@
+abstract class RecoveryCompleted
+object RecoveryCompleted extends RecoveryCompleted
+
+abstract class TypedRecoveryCompleted
+object TypedRecoveryCompleted extends TypedRecoveryCompleted
+
+class Test {
+  TypedRecoveryCompleted match {
+    case RecoveryCompleted => println("Recovery completed")            // error
+    case TypedRecoveryCompleted => println("Typed recovery completed")
+  }
+
+  def foo(x: TypedRecoveryCompleted) = x match
+    case RecoveryCompleted =>             // error
+    case TypedRecoveryCompleted =>
+}