Merge pull request #18738 from github/tausbn/python-fix-match-pruning-logic

Python: Don't prune any `MatchLiteralPattern`s

Author: yoff

python/extractor/semmle/python/passes/pruner.py

@@ -203,7 +203,8 @@ def __init__(self):
         self.nodes = set()
 
     def visit_MatchLiteralPattern(self, node):
-        # MatchLiteralPatterns _look_ like boolean tests, but are not.
+        # MatchLiteralPatterns _look_ like boolean tests in that they have both a true ("matched")
+        # and false ("didn't match") successor, but are not.
         # Thus, without this check, we would interpret
         #
         # match x:
@@ -212,8 +213,7 @@ def visit_MatchLiteralPattern(self, node):
         #
         # (and similarly for True) as if it was a boolean test. This would cause the true edge
         # (leading to pass) to be pruned later on.
-        if isinstance(node.literal, ast.Name) and node.literal.id in ('True', 'False'):
-            self.nodes.add(node.literal)
+        self.nodes.add(node.literal)
 
 class NonlocalVisitor(ASTVisitor):
     def __init__(self):

python/ql/lib/change-notes/2025-02-11-fix-match-literal-pruning.md

@@ -0,0 +1,5 @@
+---
+category: fix
+---
+
+- `MatchLiteralPattern`s such as `case None: ...` are now never pruned from the extracted source code. This fixes some situations where code was wrongly identified as unreachable.

python/ql/test/query-tests/Statements/unreachable/UnreachableCode.expected

@@ -4,6 +4,3 @@
 | test.py:21:5:21:38 | For | This statement is unreachable. |
 | test.py:28:9:28:21 | ExprStmt | This statement is unreachable. |
 | test.py:84:5:84:21 | ExceptStmt | This statement is unreachable. |
-| test.py:158:9:159:16 | Case | This statement is unreachable. |
-| test.py:162:13:162:16 | Pass | This statement is unreachable. |
-| test.py:167:13:167:16 | Pass | This statement is unreachable. |