[analyzer] Ignore [[clang::flag_enum]] enums in the EnumCastOutOfRange checker (#141232)

Resolves
https://github.com/llvm/llvm-project/issues/76208#issuecomment-2830854351

Quoting the docs of `[[clang::flag_enum]]`:
https://clang.llvm.org/docs/AttributeReference.html#flag-enum

> This attribute can be added to an enumerator to signal to the compiler that it
> is intended to be used as a flag type. This will cause the compiler to assume
> that the range of the type includes all of the values that you can get by
> manipulating bits of the enumerator when issuing warnings.

Ideally, we should still check the upper bounds but for simplicity let's not bother for now.

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/StaticAnalyzer/Checkers/EnumCastOutOfRangeChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/EnumCastOutOfRangeChecker.cpp
index 0fa2042..355e82e 100644
--- a/clang/lib/StaticAnalyzer/Checkers/EnumCastOutOfRangeChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/EnumCastOutOfRangeChecker.cpp
@@ -19,6 +19,7 @@
 //   enumeration value
 //===----------------------------------------------------------------------===//
 
+#include "clang/AST/Attr.h"
 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
 #include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
@@ -149,6 +150,10 @@ void EnumCastOutOfRangeChecker::checkPreStmt(const CastExpr *CE,
   // function to handle this.
   const EnumDecl *ED = T->castAs<EnumType>()->getDecl();
 
+  // [[clang::flag_enum]] annotated enums are by definition should be ignored.
+  if (ED->hasAttr<FlagEnumAttr>())
+    return;
+
   EnumValueVector DeclValues = getDeclValuesForEnum(ED);
 
   // If the declarator list is empty, bail out.