I observed a crash in replace when hacking ElimOpaque the compiler.

It came down to the problem that a TermRef with a context function as underlying
type was added to the context as lower bound after some errors were already
reported. I could not track down the exact chain of effects. But anyway it's
better to be defensive and not let junk enter the context.

Author: odersky

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

@@ -484,6 +484,9 @@ trait ConstraintHandling {
    *  way isSubType is organized.
    */
   protected def addConstraint(param: TypeParamRef, bound: Type, fromBelow: Boolean)(using Context): Boolean =
+    if !bound.isValueTypeOrLambda then
+      assert(ctx.reporter.errorsReported)
+      return false
 
     /** When comparing lambdas we might get constraints such as
      *  `A <: X0` or `A = List[X0]` where `A` is a constrained parameter