Fix #11118: check irrefutable pattern def in 3.0

After #10793, we can perform the check in 3.0

Author: liufengyun

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

@@ -700,7 +700,6 @@ trait Checking {
     def check(pat: Tree, pt: Type): Boolean = (pt <:< pat.tpe) || fail(pat, pt)
 
     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.hasAnnotation(defn.UncheckedAnnot) || {
         patmatch.println(i"check irrefutable $pat: ${pat.tpe} against $pt")
         pat match {

tests/neg/i11118.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i11118.scala:2:6 -----------------------------------------------------------------------------------
+2 |  val (a,b) = (1,2,3) // error
+  |      ^
+  |      this case is unreachable since type (Int, Int, Int) is not a subclass of class Tuple2

tests/neg/i11118.scala

@@ -0,0 +1,3 @@
+class Foo {
+  val (a,b) = (1,2,3) // error
+}