Better error recovery

The test case is tests/neg/deferred-patdef.scala

Author: liufengyun

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3225,9 +3225,13 @@ object Parsers {
             case IdPattern(id, t) => t.isEmpty
             case _ => false
           }
-          if rhs.isEmpty && !isAllIds then
-            syntaxError(ExpectedTokenButFound(EQUALS, in.token), Span(in.lastOffset))
-          PatDef(mods, lhs, tpt, rhs)
+          val rhs2 =
+            if rhs.isEmpty && !isAllIds then
+              syntaxError(ExpectedTokenButFound(EQUALS, in.token), Span(in.lastOffset))
+              errorTermTree
+            else
+              rhs
+          PatDef(mods, lhs, tpt, rhs2)
       }
     }
 

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

@@ -696,7 +696,7 @@ trait Checking {
 
     def recur(pat: Tree, pt: Type): Boolean =
       !sourceVersion.isAtLeast(`3.1`) || // only for 3.1 for now since mitigations work only after this PR
-      pt.widen.hasAnnotation(defn.UncheckedAnnot) || {
+      pt.hasAnnotation(defn.UncheckedAnnot) || {
         patmatch.println(i"check irrefutable $pat: ${pat.tpe} against $pt")
         pat match {
           case Bind(_, pat1) =>

tests/neg-custom-args/infix.scala

@@ -58,7 +58,7 @@ def test() = {
   val Pair(_, _) = p
   val _ Pair _ = p   // error
   val _ `Pair` _ = p // OK
-  val (_ PP _): @unchecked = p     // OK
+  val (_ PP _) = p: @unchecked     // OK
 
   val q = Q(1, 2)
   val Q(_, _) = q