Harden checkMembersOK

We might provoke failures while checking whether all parts have a denotation.
Also, if this is the case, we should not propagate the bad type.

Author: odersky

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

@@ -1804,10 +1804,11 @@ object Types {
         param.derivedSingleDenotation(param, argInfo)
       }
       else {
-        assert(ctx.reporter.errorsReported,
-          i"""bad parameter reference $this at ${ctx.phase}
-            |the parameter is ${param.showLocated} but the prefix $prefix
-            |does not define any corresponding arguments.""")
+        if (!ctx.reporter.errorsReported)
+          throw new TypeError(
+            i"""bad parameter reference $this at ${ctx.phase}
+               |the parameter is ${param.showLocated} but the prefix $prefix
+               |does not define any corresponding arguments.""")
         NoDenotation
       }
     }

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

@@ -32,7 +32,7 @@ import config.Printers.typr
 import NameKinds.DefaultGetterName
 
 import collection.mutable
-import SymDenotations.NoCompleter
+import SymDenotations.{NoCompleter, NoDenotation}
 import dotty.tools.dotc.reporting.diagnostic.{ErrorMessageID, Message}
 import dotty.tools.dotc.reporting.diagnostic.messages._
 import dotty.tools.dotc.transform.ValueClasses._
@@ -731,20 +731,25 @@ trait Checking {
     case Nil =>
   }
 
-  /** Check that all named type that form part of this type have a denotation.
+  /** Check that all named types that form part of this type have a denotation.
    *  Called on inferred (result) types of ValDefs and DefDefs.
    *  This could fail for types where the member was originally available as part
    *  of the self type, yet is no longer visible once the `this` has been replaced
    *  by some other prefix. See neg/i3083.scala
    */
   def checkMembersOK(tp: Type, pos: Position)(implicit ctx: Context): Type = {
+    var ok = true
     val check: Type => Unit = {
-      case ref: NamedType if !ref.denot.exists =>
-        ctx.error(em"$ref is not defined in inferred type $tp", pos)
+      case ref: NamedType =>
+        val d = try ref.denot catch { case ex: TypeError => NoDenotation }
+        if (!d.exists) {
+          ctx.error(em"$ref is not defined in inferred type $tp", pos)
+          ok = false
+        }
       case _ =>
     }
     tp.foreachPart(check, stopAtStatic = true)
-    tp
+    if (ok) tp else UnspecifiedErrorType
   }
 
   /** Check that all non-synthetic references of the form `<ident>` or

tests/neg/parser-stability-7.scala

@@ -0,0 +1,8 @@
+class x0[x1] {
+def x2: x1
+}
+trait x3 extends x0 {
+class x2
+var x2 = 0     // error
+var x4 = x5 x2 // error
+}