[clang] [clang][analyzer] Support `fflush` in the StreamChecker (PR #74296)

[Balázs Kéri via cfe-commits]

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@@ -1191,6 +1199,82 @@ void StreamChecker::evalSetFeofFerror(const FnDescription *Desc,
   C.addTransition(State);
 }
 
+void StreamChecker::preFflush(const FnDescription *Desc, const CallEvent &Call,
+                              CheckerContext &C) const {
+  ProgramStateRef State = C.getState();
+  SVal StreamVal = getStreamArg(Desc, Call);
+  std::optional<DefinedSVal> Stream = StreamVal.getAs<DefinedSVal>();
+  if (!Stream)
+    return;
+
+  ConstraintManager::ProgramStatePair SP =
+      C.getConstraintManager().assumeDual(State, *Stream);
+  if (SP.first)
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balazske wrote:

It is better to check for `SP.first && !SP.second`. If a stream pointer is unknown (`SP.first` and `SP.second` is not null) there should be no `StreamState` for it, so the `ensureStreamOpened` does nothing useful.

https://github.com/llvm/llvm-project/pull/74296