Exhaustive check in mixed compilation

src/compiler/scala/tools/nsc/typechecker/Namers.scala

@@ -795,6 +795,15 @@ trait Namers extends MethodSynthesis {
       tree.symbol = enterClassSymbol(tree)
       tree.symbol setInfo completerOf(tree)
 
+      // ensure completion before patmat (unit is not run for java, so add check directly to phase)
+      if (tree.symbol.isJava) {
+        val sym = tree.symbol
+        currentRun.typerPhase match {
+          case ph: typerFactory.TyperPhase => ph.addCheckAfterTyper(() => sym.initialize)
+          case _ =>
+        }
+      }
+
       if (mods.isCase) {
         val m = ensureCompanionObject(tree, caseModuleDef)
         m.moduleClass.updateAttachment(new ClassForCaseCompanionAttachment(tree))

test/files/neg/t12159d.check

@@ -1,7 +1,7 @@
-s.scala:5: error: illegal inheritance from sealed class H
-class S extends H {
-                ^
-s.scala:8: error: illegal inheritance from sealed trait I
-trait T extends I {
-                ^
-2 errors
+t_3.scala:7: warning: match may not be exhaustive.
+It would fail on the following input: W()
+    x match {
+    ^
+error: No warnings can be incurred under -Werror.
+1 warning
+1 error

test/files/neg/t12159d/X_3.java

@@ -0,0 +1,14 @@
+// javaVersion: 17+
+package p;
+
+sealed abstract public class X_3 {
+}
+
+final class W extends X_3 {
+}
+
+final class Y extends X_3 {
+}
+
+final class Z extends X_3 {
+}

test/files/neg/t12159d/t_3.scala

@@ -0,0 +1,12 @@
+// javaVersion: 17+
+// scalac: -Werror
+package p
+
+class C {
+  def f(x: X_3) =
+    x match {
+      case y: Y => y.toString
+      case z: Z => z.toString
+    }
+}
+