Ycheck that companions do not disappear.

Author: DarkDimius

src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -56,6 +56,16 @@ class TreeChecker extends Phase with SymTransformer {
     registry(name) = sym
   }
 
+  def checkCompanion(symd: SymDenotation)(implicit ctx: Context): Unit = {
+    val cur = symd.linkedClass
+    val prev = ctx.atPhase(ctx.phase.prev) {
+      implicit  ctx =>
+        symd.symbol.linkedClass
+    }
+
+    if (prev.exists)
+      assert(cur.exists, i"companion disappeared from $symd")
+  }
 
   def transformSym(symd: SymDenotation)(implicit ctx: Context): SymDenotation = {
     val sym = symd.symbol
@@ -69,6 +79,8 @@ class TreeChecker extends Phase with SymTransformer {
       testDuplicate(sym, seenClasses, "class")
     }
 
+    checkCompanion(symd)
+
     symd
   }