[llvm-bugs] [Bug 40299] New: Static Analyzer emits false-positive warnings for reference-count code

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Sat Jan 12 20:14:02 PST 2019


            Bug ID: 40299
           Summary: Static Analyzer emits false-positive warnings for
                    reference-count code
           Product: clang
           Version: trunk
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: Static Analyzer
          Assignee: dcoughlin at apple.com
          Reporter: jaf at meyersound.com
                CC: dcoughlin at apple.com, llvm-bugs at lists.llvm.org

Created attachment 21317
  --> https://bugs.llvm.org/attachment.cgi?id=21317&action=edit
An MCVE that demonstrates/reproduces the fault described in this case.

Running the Static Analyzer (built from a checkout of today's SVN trunk, llvm
SVN revision 351016) on the attached toy C++ program yields the following
false-positive warnings:

$ scan-build clang++ simple_refcount_demo.cpp -o simple_refcount_demo -isysroot
scan-build: Using '/Users/jaf/build_clang/_build/bin/clang-8' for static
simple_refcount_demo.cpp:43:54: warning: Use of memory after it is freed
       const MyRefCountedObject * GetObject() const {return _obj;}
simple_refcount_demo.cpp:52:8: warning: Potential memory leak
2 warnings generated.
scan-build: 2 bugs found.

These warnings are false-positives because the simple reference-counting
(implemented in the MyRef and MyRefCountedObject classes in the toy program)
ensures that no memory is leaked and that the allocated MyRefCountedObject
remains valid until it can no longer be accessed.

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