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

[Balázs Kéri via cfe-commits]

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@@ -1191,6 +1199,84 @@ 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>();
+  SymbolRef StreamSym = StreamVal.getAsSymbol();
+  if (!Stream || !StreamSym)
+    return;
+
+  ProgramStateRef StateNotNull, StateNull;
+  std::tie(StateNotNull, StateNull) =
+      C.getConstraintManager().assumeDual(State, *Stream);
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balazske wrote:

My opinion is now that here we should not add any transitions (except the possible errors). Otherwise there will be two branches with a known NULL and non-NULL stream, even if the stream was unknown before the call. This causes unwanted "noisiness" of the checker.

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