Test that TypeTrees are not in an unexpected tree

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -1638,6 +1638,7 @@ object Trees {
 
     abstract class TreeTraverser extends TreeAccumulator[Unit] {
       def traverse(tree: Tree)(using Context): Unit
+      def traverse(trees: List[Tree])(using Context) = apply((), trees)
       def apply(x: Unit, tree: Tree)(using Context): Unit = traverse(tree)
       protected def traverseChildren(tree: Tree)(using Context): Unit = foldOver((), tree)
     }

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

@@ -130,6 +130,7 @@ class TreeChecker extends Phase with SymTransformer {
       assert(ctx.typerState.constraint.domainLambdas.isEmpty,
         i"non-empty constraint at end of $fusedPhase: ${ctx.typerState.constraint}, ownedVars = ${ctx.typerState.ownedVars.toList}%, %")
       assertSelectWrapsNew(ctx.compilationUnit.tpdTree)
+      TreeNodeChecker.traverse(ctx.compilationUnit.tpdTree)
     }
 
     val checkingCtx = ctx
@@ -646,4 +647,25 @@ object TreeChecker {
       tp
     }
   }.apply(tp0)
+
+  /** Run some additional checks on the nodes of the trees.  Specifically:
+   *
+   *    - TypeTree can only appear in TypeApply args, New, Typed tpt, Closure
+   *      tpt, SeqLiteral elemtpt, ValDef tpt, DefDef tpt, and TypeDef rhs.
+   */
+  object TreeNodeChecker extends untpd.TreeTraverser:
+    import untpd._
+    def traverse(tree: Tree)(using Context) = tree match
+      case t: TypeTree                             => assert(assertion = false, i"TypeTree not expected: $t")
+      case t @ TypeApply(fun, _targs)              => traverse(fun)
+      case t @ New(_tpt)                           =>
+      case t @ Typed(expr, _tpt)                   => traverse(expr)
+      case t @ Closure(env, meth, _tpt)            => traverse(env); traverse(meth)
+      case t @ SeqLiteral(elems, _elemtpt)         => traverse(elems)
+      case t @ ValDef(_, _tpt, _)                  => traverse(t.rhs)
+      case t @ DefDef(_, paramss, _tpt, _)         => for params <- paramss do traverse(params); traverse(t.rhs)
+      case t @ TypeDef(_, _rhs)                    =>
+      case t @ Template(constr, parents, self, _)  => traverse(constr); traverse(parents); traverse(self); traverse(t.body)
+      case t                                       => traverseChildren(t)
+    end traverse
 }