Java generic signatures can't refer to type members

- The PolyType case was dead code imported from Scala 2
- The HKTypeLambda case was correct for eta-expanded classes
  (e.g. in t7932 `Category[Tuple2]` should get as signature
  `Category<scala.Tuple2>`), but not for other type lambdas
  where it could lead to type members appearing in the signature.
  Fixed by splitting the case into an EtaExpansion case and
  a case for all other HKTypeLambdas (which are just replaced
  by a wildcard).

Fixes #15091.

Author: smarter

compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala

@@ -9,7 +9,7 @@ import core.Definitions
 import core.Flags._
 import core.Names.Name
 import core.Symbols._
-import core.TypeApplications.TypeParamInfo
+import core.TypeApplications.{EtaExpansion, TypeParamInfo}
 import core.TypeErasure.{erasedGlb, erasure, isGenericArrayElement}
 import core.Types._
 import core.classfile.ClassfileConstants
@@ -166,6 +166,7 @@ object GenericSignatures {
     // a type parameter or similar) must go through here or the signature is
     // likely to end up with Foo<T>.Empty where it needs Foo<T>.Empty$.
     def fullNameInSig(sym: Symbol): Unit = {
+      assert(sym.isClass)
       val name = atPhase(genBCodePhase) { sanitizeName(sym.fullName).replace('.', '/') }
       builder.append('L').nn.append(name)
     }
@@ -183,11 +184,10 @@ object GenericSignatures {
               boxedSig(bounds.lo)
             }
             else builder.append('*')
-          case PolyType(_, res) =>
-            builder.append('*') // scala/bug#7932
+          case EtaExpansion(tp) =>
+            argSig(tp)
           case _: HKTypeLambda =>
-            fullNameInSig(tp.typeSymbol)
-            builder.append(';')
+            builder.append('*')
           case _ =>
             boxedSig(tp)
         }

tests/run/t7932.check

@@ -1,9 +1,13 @@
 public Category C.category()
 public Category C.category1()
+public abstract Category<?> M2.category3()
+public abstract Category<java.lang.Object> M2.category2()
 public default Category<F> M1.category()
 public default Category<scala.Tuple2> M1.category1()
 public static Category M1.category$(M1)
 public static Category M1.category1$(M1)
+public abstract Category<?> M2.category3()
+public abstract Category<java.lang.Object> M2.category2()
 public default Category<F> M2.category()
 public default Category<scala.Tuple2> M2.category1()
 public static Category M2.category$(M2)

tests/run/t7932.scala

@@ -13,6 +13,10 @@ trait M1[F] {
 trait M2[F] { self: M1[F] =>
   override def category: Category[X] = null
   override def category1: Category[Tuple2] = null
+
+  type T[A, B]
+  def category2: Category[T]
+  def category3: Category[[A, B] =>> T[B, A]]
 }
 
 abstract class C extends M1[Float] with M2[Float]