[LLVMbugs] [Bug 21344] New: False positive garbage value reported in std::<random> param_type in distribution templates

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21344

            Bug ID: 21344
           Summary: False positive garbage value reported in std::<random>
                    param_type in distribution templates
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Static Analyzer
          Assignee: kremenek at apple.com
          Reporter: stevemac321 at live.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

To repro, navigate to 
/libcxx/test/numerics/rand/rand.dis/rand.dist.bern/rand.dist.bern.bin

Run:
scan-build -k -analyze-headers clang++ -std=c++1y -stdlib=libc++ -g3 -O0
-I/home/comet/libcxx/include eval_param.pass.cpp

Result:
libcxx/include/random:4024:29: warning: The right operand of '-' is a garbage
value
    double __u = __gen(__g) - __pr.__pr_;
                            ^ ~~~~~~~~~~

The code looks like this:

template<class _IntType>
template<class _URNG>
_IntType
binomial_distribution<_IntType>::operator()(_URNG& __g, const param_type& __pr)
{
    if (__pr.__t_ == 0 || __pr.__p_ == 0)
        return 0;
    if (__pr.__p_ == 1)
        return __pr.__t_;
    uniform_real_distribution<double> __gen;
    double __u = __gen(__g) - __pr.__pr_; // warning on this line

However, I think it is fair to say that param_type will at least be a
default-constructed IntType.  IntType is binomial_distribution's template
argument defaulted to int (class _IntType = int).  param_type has to be
assignable to numeric types:

explicit param_type(result_type __t = 1, double __p = 0.5);

So, I don't know how a garbage value could compile, or how one would test for
garbage in binomial_distribution's call operator (warning location)

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