Python: use comprehension function argument

For a comprehension `[x for x in l]
- `l` is now a legal argument (in DataFlowPublic)
- `l` is the argument of the comprehension function (in DataFlowDispatch)
- the parameter of the comprehension function is being read rather than `l` (in IterableUnpacking)
Thus the read that used to cross callable boundaries is now split into a arg-param edge and a read from that param.

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowDispatch.qll

@@ -1491,7 +1491,10 @@ class ComprehensionCall extends ExtractedDataFlowCall, TComprehensionCall {
 
   override DataFlowCallable getCallable() { result.(DataFlowFunction).getScope() = target }
 
-  override ArgumentNode getArgument(ArgumentPosition apos) { none() }
+  override ArgumentNode getArgument(ArgumentPosition apos) {
+    result.asExpr() = c.getIterable() and
+    apos.isPositional(0)
+  }
 
   override Location getLocation() { result = c.getLocation() }
 }

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -350,6 +350,9 @@ class ExtractedArgumentNode extends ArgumentNode {
     or
     // and self arguments
     this.asCfgNode() = any(CallNode c).getFunction().(AttrNode).getObject()
+    or
+    // for comprehensions, we allow the synthetic `iterable` argument
+    this.asExpr() = any(Comp c).getIterable()
   }
 
   final override predicate argumentOf(DataFlowCall call, ArgumentPosition pos) {

python/ql/lib/semmle/python/dataflow/new/internal/IterableUnpacking.qll

@@ -187,7 +187,7 @@ class ForTarget extends ControlFlowNode {
     )
     or
     exists(Comp comp |
-      source = comp.getIterable() and
+      source = comp.getFunction().getArg(0) and
       this.getNode() = comp.getNthInnerLoop(0).getTarget()
     )
   }