Python: Don't allow getParameter(-1) for BoundMethodValue

As per discussion in the PR

Author: RasmusWL

python/ql/src/semmle/python/objects/Callables.qll

@@ -438,10 +438,18 @@ class BoundMethodObjectInternal extends CallableObjectInternal, TBoundMethod {
         PointsTo::pointsTo(result.getFunction(), ctx, this, _)
     }
 
-    override NameNode getParameter(int n) { result = this.getFunction().getParameter(n + 1) }
+    /** Gets the parameter node that will be used for `self`. */
+    NameNode getSelfParameter() { result = this.getFunction().getParameter(0) }
+
+    override NameNode getParameter(int n) {
+        result = this.getFunction().getParameter(n + 1) and
+        // don't return the parameter for `self` at `n = -1`
+        n >= 0
+    }
 
     override NameNode getParameterByName(string name) {
-        result = this.getFunction().getParameterByName(name)
+        result = this.getFunction().getParameterByName(name) and
+        not result = this.getSelfParameter()
     }
 
     override predicate neverReturns() { this.getFunction().neverReturns() }

python/ql/src/semmle/python/objects/ObjectAPI.qll

@@ -454,6 +454,9 @@ class BoundMethodValue extends CallableValue {
      * The value for `o` in `o.func`.
      */
     Value getSelf() { result = this.(BoundMethodObjectInternal).getSelf() }
+
+    /** Gets the parameter node that will be used for `self`. */
+    NameNode getSelfParameter() { result = this.(BoundMethodObjectInternal).getSelfParameter() }
 }
 
 /**

python/ql/test/library-tests/PointsTo/calls/getParameter.expected

@@ -5,9 +5,6 @@
 | Function f | 1 | ControlFlowNode for arg1 |
 | Function f | 2 | ControlFlowNode for arg2 |
 | Method(Function C.n, C()) | 0 | ControlFlowNode for arg1 |
-| Method(Function C.n, C()) | -1 | ControlFlowNode for self |
 | Method(Function C.n, class C) | 0 | ControlFlowNode for arg1 |
-| Method(Function C.n, class C) | -1 | ControlFlowNode for self |
 | Method(Function f, C()) | 0 | ControlFlowNode for arg1 |
 | Method(Function f, C()) | 1 | ControlFlowNode for arg2 |
-| Method(Function f, C()) | -1 | ControlFlowNode for arg0 |

python/ql/test/library-tests/PointsTo/calls/getParameterByName.expected

@@ -5,9 +5,6 @@
 | Function f | arg1 | ControlFlowNode for arg1 |
 | Function f | arg2 | ControlFlowNode for arg2 |
 | Method(Function C.n, C()) | arg1 | ControlFlowNode for arg1 |
-| Method(Function C.n, C()) | self | ControlFlowNode for self |
 | Method(Function C.n, class C) | arg1 | ControlFlowNode for arg1 |
-| Method(Function C.n, class C) | self | ControlFlowNode for self |
-| Method(Function f, C()) | arg0 | ControlFlowNode for arg0 |
 | Method(Function f, C()) | arg1 | ControlFlowNode for arg1 |
 | Method(Function f, C()) | arg2 | ControlFlowNode for arg2 |