[clang] [analyzer] Don't assume third iteration in loops (PR #119388)

[Balazs Benics via cfe-commits]

================
@@ -2801,8 +2799,35 @@ void ExprEngine::processBranch(const Stmt *Condition,
     if (StTrue && StFalse)
       assert(!isa<ObjCForCollectionStmt>(Condition));
 
-    if (StTrue)
-      Builder.generateNode(StTrue, true, PredN);
+    if (StTrue) {
+      // If we are processing a loop condition where two iterations have
+      // already been completed and the the false branch is also feasible, then
+      // don't assume a third iteration, because it is a redundant execution
+      // path (unlikely to be different from earlier loop exits) and can cause
+      // false positives if e.g. the loop iterates over a two-element structure
+      // with an opaque condition.
+      //
+      // The iteration count "2" is hardcoded because it's the natural limit:
+      // * the fact that the programmer wrote a loop (and not just an `if`)
+      //   implies that they thought that the loop body may be executed twice;
+      // * however, there are situations where the programmer knows that there
+      //   are at most two iterations, but writes a loop that appears to be
----------------
steakhal wrote:

```suggestion
      //   are at most two iterations but writes a loop that appears to be
```

https://github.com/llvm/llvm-project/pull/119388