[clang] [analyzer] Refine invalidation caused by `fread` (PR #93408)

[Donát Nagy via cfe-commits]

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@@ -717,18 +717,71 @@ const ExplodedNode *StreamChecker::getAcquisitionSite(const ExplodedNode *N,
   return nullptr;
 }
 
+/// Invalidate only the requested elements instead of the whole buffer.
+/// This is basically a refinement of the more generic 'escapeArgs' or
+/// the plain old 'invalidateRegions'.
+/// This only works if the \p StartIndex and \p Count are concrete or
+/// perfectly-constrained.
+static ProgramStateRef
+escapeByStartIndexAndCount(ProgramStateRef State, CheckerContext &C,
+                           const CallEvent &Call, const MemRegion *Buffer,
+                           QualType ElemType, SVal StartIndex, SVal Count) {
+  if (!llvm::isa_and_nonnull<SubRegion>(Buffer))
+    return State;
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NagyDonat wrote:

I'd slightly prefer a `dyn_cast_or_null` instead of this `isa` check that's eventually followed by a bare `cast`.

https://github.com/llvm/llvm-project/pull/93408