Add failing assign test retype.py, with explanation

tests/failing_tests/assign/retype.py

@@ -0,0 +1,39 @@
+from pythonbpf import bpf, map, section, bpfglobal, compile
+from ctypes import c_void_p, c_int64, c_uint64
+from pythonbpf.maps import HashMap
+
+
+# NOTE: This example tries to reinterpret the variable `x` to a different type.
+# We do not allow this for now, as stack allocations are typed and have to be
+# done in the first basic block. Allowing re-interpretation would require
+# re-allocation of stack space (possibly in a new basic block), which is not
+# supported in eBPF yet.
+# We can allow bitcasts in cases where the width of the types is the same in
+# the future. But for now, we do not allow any re-interpretation of variables.
+
+@bpf
+@map
+def last() -> HashMap:
+    return HashMap(key=c_uint64, value=c_uint64, max_entries=3)
+
+
+@bpf
+@section("tracepoint/syscalls/sys_enter_execve")
+def hello_world(ctx: c_void_p) -> c_int64:
+    last.update(0, 1)
+    x = last.lookup(0)
+    x = 20
+    if x == 2:
+        print("Hello, World!")
+    else:
+        print("Goodbye, World!")
+    return
+
+
+@bpf
+@bpfglobal
+def LICENSE() -> str:
+    return "GPL"
+
+
+compile()