Avoid NPE in simplify

Fixes #15272

There's no crash anymore, but #15272 still does not compile.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -145,7 +145,7 @@ object TypeOps:
         if normed.exists then normed else tp.map(simplify(_, theMap))
       case tp: TypeParamRef =>
         val tvar = ctx.typerState.constraint.typeVarOfParam(tp)
-        if (tvar.exists) tvar else tp
+        if tvar != null && tvar.exists then tvar else tp
       case  _: ThisType | _: BoundType =>
         tp
       case tp: AliasingBounds =>

tests/neg/i15272.scala

@@ -0,0 +1,11 @@
+    sealed trait EdgeN[+NT]
+    object EdgeN:
+      case class Head[+NT, +From <: NT, +To <: NT]    (from: From, to: To ) extends EdgeN[NT]
+      case class Cons[+NT, +From <: NT, +ToE <: EdgeN[NT]](from: From, to: ToE) extends EdgeN[NT]
+      final type InNodesTupleOf[NT, E <: EdgeN[NT]] <: Tuple = E match
+        case Cons[nt,from,toE] => from *: InNodesTupleOf[nt,toE]
+        case Head[nt,from ,to] => from *: EmptyTuple
+      def inNodesTuple[NT,E <: EdgeN[NT]](edge: E): InNodesTupleOf[NT,E] = edge match
+        case e: Cons[nt,from,toE] => e.from *: inNodesTuple[nt,toE](e.to) // error
+        case e: Head[nt,from,to] => e.from *: EmptyTuple
+    end EdgeN