Python: Add data-flow tests

Alas, all these demonstrate is that we already don't fully support the
desugared `yield from` form.

Author: tausbn

python/ql/test/library-tests/dataflow/coverage/test.py

@@ -257,6 +257,32 @@ def test_yield_from():
     SINK(next(g)) # $ MISSING:flow="SOURCE, l:-1 -> next()"
 
 
+# PEP 798: Unpacking in comprehensions.
+# These desugar to `yield from`, so flow depends on yield-from support (see above).
+def test_star_list_comp():
+    l = [[SOURCE]]
+    flat = [*x for x in l]
+    SINK(flat[0]) # $ MISSING:flow="SOURCE, l:-2 -> flat[0]"
+
+
+def test_star_set_comp():
+    l = [[SOURCE]]
+    flat = {*x for x in l}
+    SINK(flat.pop()) # $ MISSING:flow="SOURCE, l:-2 -> flat.pop()"
+
+
+def test_star_genexp():
+    l = [[SOURCE]]
+    g = (*x for x in l)
+    SINK(next(g)) # $ MISSING:flow="SOURCE, l:-2 -> next()"
+
+
+def test_star_dictcomp():
+    l = [{"key": SOURCE}]
+    merged = {**d for d in l}
+    SINK(merged["key"]) # $ MISSING:flow="SOURCE, l:-2 -> merged[..]"
+
+
 # a statement rather than an expression, but related to generators
 def test_for():
     for x in gen(SOURCE):