[clang-tools-extra] [clang-tidy] In C++17, callee is guaranteed to be sequenced before arguments. (PR #93623)

[Julian Schmidt via cfe-commits]

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@@ -95,9 +106,59 @@ bool ExprSequence::inSequence(const Stmt *Before, const Stmt *After) const {
       return true;
   }
 
+  SmallVector<const Stmt *, 1> BeforeParents = getParentStmts(Before, Context);
+
+  // Since C++17, the callee of a call expression is guaranteed to be sequenced
+  // before all of the arguments.
+  // We handle this as a special case rather than using the general
+  // `getSequenceSuccessor` logic above because the callee expression doesn't
+  // have an unambiguous successor; the order in which arguments are evaluated
+  // is indeterminate.
+  for (const Stmt *Parent : BeforeParents) {
+    // Special case: If the callee is a `MemberExpr` with a `DeclRefExpr` as its
+    // base, we consider it to be sequenced _after_ the arguments. This is
+    // because the variable referenced in the base will only actually be
+    // accessed when the call happens, i.e. once all of the arguments have been
+    // evaluated. This has no basis in the C++ standard, but it reflects actual
+    // behavior that is relevant to a use-after-move scenario:
+    //
+    // ```
+    // a.bar(consumeA(std::move(a));
+    // ```
+    //
+    // In this example, we end up accessing `a` after it has been moved from,
+    // even though nominally the callee `a.bar` is evaluated before the argument
+    // `consumeA(std::move(a))`. Note that this is not specific to C++17, so
+    // we implement this logic unconditionally.
+    if (const auto *call = dyn_cast<CXXMemberCallExpr>(Parent)) {
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5chmidti wrote:

`call` -> `Call`

https://github.com/llvm/llvm-project/pull/93623