Flag unloadable references as errors

#14783 shows an example where an implicit reference was generated that started in a type.
The backend then crashed since it could not emit instructions to load that reference. We
now detect such references when they are created and flag them as errors.

We need to separately fix the issue that created the bad reference in the first place.

Author: odersky

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

@@ -404,18 +404,21 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   }
 
   /** A tree representing the same reference as the given type */
-  def ref(tp: NamedType)(using Context): Tree =
+  def ref(tp: NamedType, needLoad: Boolean = true)(using Context): Tree =
     if (tp.isType) TypeTree(tp)
     else if (prefixIsElidable(tp)) Ident(tp)
     else if (tp.symbol.is(Module) && ctx.owner.isContainedIn(tp.symbol.moduleClass))
       followOuterLinks(This(tp.symbol.moduleClass.asClass))
     else if (tp.symbol hasAnnotation defn.ScalaStaticAnnot)
       Ident(tp)
-    else {
+    else
       val pre = tp.prefix
       if (pre.isSingleton) followOuterLinks(singleton(pre.dealias)).select(tp)
-      else Select(TypeTree(pre), tp)
-    }
+      else
+        val res = Select(TypeTree(pre), tp)
+        if needLoad && !res.symbol.isStatic then
+          throw new TypeError(em"cannot establish a reference to $res")
+        res
 
   def ref(sym: Symbol)(using Context): Tree =
     ref(NamedType(sym.owner.thisType, sym.name, sym.denot))

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -705,7 +705,7 @@ trait Applications extends Compatibility {
     def fail(msg: Message): Unit =
       ok = false
     def appPos: SrcPos = NoSourcePosition
-    @threadUnsafe lazy val normalizedFun:   Tree = ref(methRef)
+    @threadUnsafe lazy val normalizedFun: Tree = ref(methRef, needLoad = false)
     init()
   }