feat: warn about TBranch name, alias name conflict. (#776)

Author: jpivarski

src/uproot/exceptions.py

@@ -5,10 +5,13 @@
 :doc:`uproot.exceptions.KeyInFileError`.
 """
 
-
 import uproot
 
 
+class NameConflictWarning(Warning):
+    pass
+
+
 class KeyInFileError(KeyError):
     """
     Exception raised by attempts to find ROOT objects in ``TDirectories``

src/uproot/language/python.py

@@ -10,6 +10,7 @@
 
 
 import ast
+import warnings
 
 import numpy
 
@@ -402,16 +403,27 @@ def compute_expressions(
             arrays (dict of arrays): Inputs to the computation.
             expression_context (list of (str, dict) tuples): Expression strings
                 and a dict of metadata about each.
-            keys (list of str): Names of branches or aliases (for aliases that
+            keys (set of str): Names of branches or aliases (for aliases that
                 refer to aliases).
-            aliases (list of str): Names of aliases.
+            aliases (dict of str \u2192 str): Names of aliases and their definitions.
             file_path (str): File path for error messages.
             object_path (str): Object path for error messages.
 
         Computes an array for each expression.
         """
         values = {}
 
+        if len(aliases) < len(keys):
+            shorter, longer = aliases, keys
+        else:
+            shorter, longer = keys, aliases
+        for x in shorter:
+            if x in longer:
+                warnings.warn(
+                    f"{x!r} is both an alias and a branch name",
+                    uproot.exceptions.NameConflictWarning,
+                )
+
         def getter(name):
             if name not in values:
                 values[name] = _expression_to_function(