Kotlin: fix type variable erasure inside default function values

Previously because extractClassInstance didn't use the declaration stack, we wouldn't notice that it was legal to refer to its type variable in the context of extracting a specialised method <-> method source-decl edge. This led to erasing the types of the source-decl, so that e.g. Map.put(...) would have signature (Object, Object) not (K, V)
as it should.

java/ql/test/kotlin/library-tests/parameter-defaults/PrintAst.expected

@@ -1165,3 +1165,59 @@ test.kt:
 #  161|             -1: [VarAccess] p0
 #  161|             0: [VarAccess] p1
 #  161|             1: [VarAccess] p2
+#  165|   15: [Class,GenericType,ParameterizedType] TestGenericUsedWithinDefaultValue
+#-----|     -2: (Generic Parameters)
+#  165|       0: [TypeVariable] T
+#  165|     1: [Constructor] TestGenericUsedWithinDefaultValue
+#  165|       5: [BlockStmt] { ... }
+#  165|         0: [SuperConstructorInvocationStmt] super(...)
+#  165|         1: [BlockStmt] { ... }
+#  171|     2: [Method] f
+#  171|       3: [TypeAccess] Unit
+#-----|       4: (Parameters)
+#  171|         0: [Parameter] x
+#  171|           0: [TypeAccess] int
+#  171|         1: [Parameter] y
+#  171|           0: [TypeAccess] String
+#  171|       5: [BlockStmt] { ... }
+#  171|     3: [Method] f$default
+#  171|       3: [TypeAccess] Unit
+#-----|       4: (Parameters)
+#  171|         0: [Parameter] p0
+#  171|           0: [TypeAccess] TestGenericUsedWithinDefaultValue<>
+#  171|         1: [Parameter] p1
+#  171|           0: [TypeAccess] int
+#  171|         2: [Parameter] p2
+#  171|           0: [TypeAccess] String
+#  171|         3: [Parameter] p3
+#  171|           0: [TypeAccess] int
+#  171|         4: [Parameter] p4
+#  171|           0: [TypeAccess] Object
+#  171|       5: [BlockStmt] { ... }
+#  171|         0: [IfStmt] if (...)
+#  171|           0: [EQExpr] ... == ...
+#  171|             0: [AndBitwiseExpr] ... & ...
+#  171|               0: [IntegerLiteral] 2
+#  171|               1: [VarAccess] p3
+#  171|             1: [IntegerLiteral] 0
+#  171|           1: [ExprStmt] <Expr>;
+#  171|             0: [AssignExpr] ...=...
+#  171|               0: [VarAccess] p2
+#  171|               1: [MethodAccess] ident(...)
+#  171|                 -1: [ClassInstanceExpr] new TestGenericUsedWithinDefaultValue<String>(...)
+#  171|                   -3: [TypeAccess] TestGenericUsedWithinDefaultValue<String>
+#  171|                     0: [TypeAccess] String
+#  171|                 0: [StringLiteral] Hello world
+#  171|         1: [ReturnStmt] return ...
+#  171|           0: [MethodAccess] f(...)
+#  171|             -1: [VarAccess] p0
+#  171|             0: [VarAccess] p1
+#  171|             1: [VarAccess] p2
+#  173|     4: [Method] ident
+#  173|       3: [TypeAccess] T
+#-----|       4: (Parameters)
+#  173|         0: [Parameter] t
+#  173|           0: [TypeAccess] T
+#  173|       5: [BlockStmt] { ... }
+#  173|         0: [ReturnStmt] return ...
+#  173|           0: [VarAccess] t

java/ql/test/kotlin/library-tests/parameter-defaults/erasure.ql

@@ -1,9 +1,15 @@
 import java
 
+class InstantiatedType extends ParameterizedType {
+  InstantiatedType() { typeArgs(_, _, this) }
+}
+
 // This checks that all type parameter references are erased in the context of a $default function.
 predicate containsTypeVariables(Type t) {
-  t != t.getErasure() and
-  not t.getErasure().(GenericType).getRawType() = t
+  t instanceof TypeVariable or
+  containsTypeVariables(t.(InstantiatedType).getATypeArgument()) or
+  containsTypeVariables(t.(NestedType).getEnclosingType()) or
+  containsTypeVariables(t.(Wildcard).getATypeBound().getType())
 }
 
 from Expr e

java/ql/test/kotlin/library-tests/parameter-defaults/test.kt

@@ -161,3 +161,15 @@ class VisibilityTests {
   private fun i(x: Int, y: Int = 0) = x + y
 
 }
+
+class TestGenericUsedWithinDefaultValue<T> {
+
+  // This tests parameter erasure works properly: we should notice that here the type variable T
+  // isn't used in the specialisation TestGenericUsedWithinDefaultValue<String>, but it can be
+  // cited in contexts like "the signature of the source declaration of 'TestGenericUsedWithinDefaultValue<String>.f(String)' is 'f(T)'",
+  // not 'f(Object)' as we might mistakenly conclude if we're inappropriately erasing 'T'.
+  fun f(x: Int, y: String = TestGenericUsedWithinDefaultValue<String>().ident("Hello world")) { }
+
+  fun ident(t: T) = t
+
+}