Shared CFG: support for-else and while-else loops

Add two default predicates to AstSig:

  default AstNode getWhileElse(WhileStmt loop) { none() }
  default AstNode getForeachElse(ForeachStmt loop) { none() }

When defined, the explicit-step rules for While/Do and Foreach
route the loop's normal-completion exits through the else block
before reaching the after-loop node:

  - WhileStmt: after-false condition -> before-else -> after-while
    (instead of directly after-while).
  - ForeachStmt: after-collection [empty] and the LoopHeader exit
    are both routed through before-else -> after-foreach.

Python's Ast module overrides the predicates to return the
synthetic BlockStmt for the orelse slot, replacing the previous
customisations in Input::step. This eliminates parallel direct
successors emitted by the previous Python-side step additions
(verified: multipleSuccessors on a CPython database goes from
1340 to 0).

Java and C# CFG tests are unaffected.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/ql/lib/semmle/python/controlflow/internal/AstNodeImpl.qll

@@ -411,6 +411,16 @@ module Ast implements AstSig<Py::Location> {
    */
   AstNode getTryElse(TryStmt try) { result = try.getElse() }
 
+  /**
+   * Gets the `else` branch of `while` loop `loop`, if any.
+   */
+  AstNode getWhileElse(WhileStmt loop) { result = loop.getElse() }
+
+  /**
+   * Gets the `else` branch of `for` loop `loop`, if any.
+   */
+  AstNode getForeachElse(ForeachStmt loop) { result = loop.getElse() }
+
   /** An exception handler (`except` or `except*`). */
   class CatchClause extends Stmt {
     private Py::ExceptionHandler handler;
@@ -1182,29 +1192,6 @@ private module Input implements InputSig1, InputSig2 {
       n1.isAfter(assertStmt.getMsg()) and
       n2.isAdditional(assertStmt, assertThrowTag())
     )
-    or
-    // While/else: when the condition is false, flow to the else block
-    // (if present) before the after-while node.
-    exists(Ast::WhileStmt w, Ast::BlockStmt orelse | orelse = w.getElse() |
-      n1.isAfterFalse(w.getCondition()) and
-      n2.isBefore(orelse)
-      or
-      n1.isAfter(orelse) and
-      n2.isAfter(w)
-    )
-    or
-    // For/else: when the collection is empty or the loop completes
-    // normally, flow through the else block before the after-for node.
-    exists(Ast::ForeachStmt f, Ast::BlockStmt orelse | orelse = f.getElse() |
-      n1.isAfterValue(f.getCollection(), any(EmptinessSuccessor t | t.getValue() = true)) and
-      n2.isBefore(orelse)
-      or
-      n1.isAfter(f.getBody()) and
-      n2.isBefore(orelse)
-      or
-      n1.isAfter(orelse) and
-      n2.isAfter(f)
-    )
   }
 }
 

python/ql/test/library-tests/ControlFlow/evaluation-order/NewCfgBranchTimestamps.expected

@@ -54,8 +54,6 @@
 | test_loops.py:53:12:53:38 | After Compare | Timestamp 4 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:51:1:51:29 | Function test_while_else_break | test_while_else_break |
 | test_loops.py:53:12:53:38 | After Compare | Timestamp 13 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:51:1:51:29 | Function test_while_else_break | test_while_else_break |
 | test_loops.py:53:12:53:38 | After Compare | Timestamp 13 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:51:1:51:29 | Function test_while_else_break | test_while_else_break |
-| test_loops.py:53:12:53:38 | After Compare | Timestamp 16 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:51:1:51:29 | Function test_while_else_break | test_while_else_break |
-| test_loops.py:53:12:53:38 | After Compare | Timestamp 16 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:51:1:51:29 | Function test_while_else_break | test_while_else_break |
 | test_loops.py:54:13:54:40 | After Compare | Timestamp 7 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:51:1:51:29 | Function test_while_else_break | test_while_else_break |
 | test_loops.py:54:13:54:40 | After Compare | Timestamp 7 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:51:1:51:29 | Function test_while_else_break | test_while_else_break |
 | test_loops.py:54:13:54:40 | After Compare | Timestamp 16 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:51:1:51:29 | Function test_while_else_break | test_while_else_break |
@@ -158,8 +156,6 @@
 | test_loops.py:111:14:111:43 | After List | Timestamp 4 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
 | test_loops.py:111:14:111:43 | After List | Timestamp 8 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
 | test_loops.py:111:14:111:43 | After List | Timestamp 8 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
-| test_loops.py:111:14:111:43 | After List | Timestamp 11 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
-| test_loops.py:111:14:111:43 | After List | Timestamp 11 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
 | test_loops.py:112:13:112:38 | After Compare | Timestamp 7 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
 | test_loops.py:112:13:112:38 | After Compare | Timestamp 7 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
 | test_loops.py:112:13:112:38 | After Compare | Timestamp 11 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
@@ -168,8 +164,6 @@
 | test_loops.py:114:9:114:9 | x | Timestamp 4 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
 | test_loops.py:114:9:114:9 | x | Timestamp 8 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
 | test_loops.py:114:9:114:9 | x | Timestamp 8 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
-| test_loops.py:114:9:114:9 | x | Timestamp 11 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
-| test_loops.py:114:9:114:9 | x | Timestamp 11 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:110:1:110:27 | Function test_for_else_break | test_for_else_break |
 | test_loops.py:123:14:123:33 | After List | Timestamp 21 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:122:1:122:25 | Function test_nested_loops | test_nested_loops |
 | test_loops.py:123:14:123:33 | After List | Timestamp 21 on true/false branch is missing a dead() annotation on the true successor in $@ | test_loops.py:122:1:122:25 | Function test_nested_loops | test_nested_loops |
 | test_loops.py:124:18:124:47 | After List | Timestamp 3 on true/false branch is missing a dead() annotation on the false successor in $@ | test_loops.py:122:1:122:25 | Function test_nested_loops | test_nested_loops |