Survive NoType when erasing method result types

Fixes #15377

Author: odersky

compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala

@@ -138,7 +138,7 @@ trait BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
    * `refedInnerClasses` may contain duplicates, need not contain the enclosing inner classes of
    * each inner class it lists (those are looked up and included).
    *
-   * This method serializes in the InnerClasses JVM attribute in an appropriate order, 
+   * This method serializes in the InnerClasses JVM attribute in an appropriate order,
    * not necessarily that given by `refedInnerClasses`.
    *
    * can-multi-thread

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

@@ -638,6 +638,11 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
       eraseResult(tp.resultType) match {
         case rt: MethodType =>
           tp.derivedLambdaType(names ++ rt.paramNames, formals ++ rt.paramInfos, rt.resultType)
+        case NoType =>
+          // Can happen if we smuggle in a Nothing in the qualifier. Normally we prevent that
+          // in Checking.checkMembersOK, but compiler-generated code can bypass this test.
+          // See i15377.scala for a test case.
+          NoType
         case rt =>
           tp.derivedLambdaType(names, formals, rt)
       }

tests/neg/i15377.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i15377.scala:11:6 ----------------------------------------------------------------------------------
+11 |  def func(a: A) = a // error
+   |  ^^^^^^^^^^^^^^^^^^
+   |  a.A is not defined in inferred type a.A

tests/neg/i15377.scala

@@ -0,0 +1,11 @@
+
+trait WithPath:
+  type A
+
+trait Proto:
+  type A <: WithPath
+  def func(a: A): a.A
+
+object Impl extends Proto:
+  type A = Nothing
+  def func(a: A) = a // error

tests/pos/i15377.scala

@@ -0,0 +1,7 @@
+trait Tst:
+  val a: Int
+
+trait Q[A <: Tst]:
+  def mk(a: A): a.a.type = a.a
+
+object Q extends Q[Nothing]