This has been "won't fix" for over a month.

Is it open to avoid others reporting the same bug?  I'm (tentatively) closing it.

The following program seems to be enough to demonstrate the problem. I think it should write "val: 1, failbit: 0" but it writes "val: 0, failbit: 4" when using clang++ 3.7 and libc++.

If you put a space between the '1' and the 'a' in the stringstream, it works. But I think it should stop reading the double value after the 1 either way. Am I wrong? If the clang+libc++ behavior is standard conforming, we probably have a lot of work to do to fix the way Octave reads numbers, so I hope I'm not wrong...

#include <iostream>
#include <sstream>

int
main (void)
{
std::istringstream s1 ("1asdf");
double val = 0.0;
is >> val;
std::cerr << "val: " << val
<< ", failbit: " << (is.rdstate () & std::ios::failbit)
<< std::endl;
return 0;
}

I see the same results:

octave:1> sscanf("1asdf", "%f", 1)
ans = [](0x1)
octave:2> str2double("1i")
ans = NaN
octave:3> strread("1i", "%f")
ans = NaN

Oops, I might have been out of date with libc++.  I updated it and now I'm trying again.

On my Debian system, I used:

/home/jwe/src/octave/configure --prefix=/usr/local/octave/dev-clang --with-hdf5-includedir=/usr/include/hdf5/serial --with-hdf5-libdir=/usr/lib/x86_64-linux-gnu/hdf5/serial CC="clang-3.7" CXX="clang++-3.7 -stdlib=libc++" CFLAGS="-O0 -g" CXXFLAGS="-O0 -g" FFLAGS="-O0 -g"

and I'm seeing a build error that begins with

In file included from /home/jwe/src/octave/libinterp/operators/op-i64-i64.cc:27:
In file included from liboctave/operators/mx-i64nda-i8.h:5:
In file included from /home/jwe/src/octave/liboctave/array/int64NDArray.h:28:
In file included from /home/jwe/src/octave/liboctave/array/intNDArray.h:28:
In file included from /home/jwe/src/octave/liboctave/array/MArray.h:29:
In file included from /home/jwe/src/octave/liboctave/array/Array.h:34:
In file included from /usr/include/c++/v1/algorithm:625:
/usr/include/c++/v1/type_traits:1195:21: error: call to '__test' is ambiguous
   typedef decltype(__test(declval<_Tp>())) type;
                    ^~~~~~
/usr/include/c++/v1/type_traits:1208:18: note: in instantiation of template class 'std::__1::__numeric_type<octave_int<long> >' requested here
          bool = __numeric_type<_A1>::value &&
                 ^
/usr/include/c++/v1/cmath:959:14: note: in instantiation of default argument for '__promote<octave_int<long>, octave_int<long>, void>' required here
    typename __promote<_A1, _A2>::type
             ^~~~~~~~~~~~~~~~~~~
[...]

What configure command line did you use to build with clang and libc++?