Fix ostream for complex numbers with fixed field width

[cantonios]

Summary

The ostream operator << is currently broken for std::complex with specified field widths.

For example,

const std::complex<float> x(10, 34);
std::cout << std::setfill('_') << std::setw(10) << x << std::endl;

should output

___(10,34)

for a total width of 10, but currently does:

_________(10,34)

for a total width of width-1+7 = 16.

This patch a partial reversion of c3478eff7a65d6a77b34e756eabc7176a2b422e8 (reviewed at https://reviews.llvm.org/D71214), restoring the correct behavior. Also added a test to libcxx/test/std/numerics/complex.number/complex.ops/stream_output.pass.cpp to catch this error in the future.

Tested:

$ llvm-lit libcxx/test/std/numerics/complex.number

[dexonsmith]

@cantonios, thanks for working on this. I'll let @ldionne or others review in detail, but I have two high-level notes:

• Given that this is a partial revert of c3478eff7a65d6a77b34e756eabc7176a2b422e8 (reviewed at https://reviews.llvm.org/D71214) I think the commit message should mention that commit.
• To avoid the same issue in the future, I think this needs a test! Probably somewhere in libcxx/test/std/numerics/complex.number makes sense.

[ldionne]

Ugh, sorry I hadn't seen that before. The patch LGTM but I agree we should add a test -- @cantonios can you add one?

[cantonios]

Updated with test.

[cantonios]

In D78816#2013207, @dexonsmith wrote:

@cantonios, thanks for working on this. I'll let @ldionne or others review in detail, but I have two high-level notes:

• Given that this is a partial revert of c3478eff7a65d6a77b34e756eabc7176a2b422e8 (reviewed at https://reviews.llvm.org/D71214) I think the commit message should mention that commit.
• To avoid the same issue in the future, I think this needs a test! Probably somewhere in libcxx/test/std/numerics/complex.number makes sense.

Done.

In D78816#2013597, @ldionne wrote:

Ugh, sorry I hadn't seen that before. The patch LGTM but I agree we should add a test -- @cantonios can you add one?

Done.

[ldionne]

Do you need help committing this? If so, please specify the Git information to commit under as "First Last <[e-mail]>". Thanks!

[cantonios]

Yes, thanks, I don't have commit access.

"Antonio Sanchez <[e-mail]>"

[ldionne]

Thanks! Committed as

commit aa0b991dcf0510793a42a5b01aaf06a6c0892bcf
Author: Antonio Sanchez <[e-mail]>
Date:   Thu May 7 13:49:12 2020 -0400

    [libc++] Fix ostream for complex numbers with fixed field width

    The ostream operator<< is currently broken for std::complex with
    specified field widths.

    This patch a partial revert of c3478eff7a65 (reviewed as D71214),
    restoring the correct behavior.

    Differential Revision: https://reviews.llvm.org/D78816

libcxx/include/complex

 #include <stdexcept>
 #include <cmath>
 #include <iosfwd>
+#include <sstream>
 #include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)

 basic_ostream<_CharT, _Traits>&
 operator<<(basic_ostream<_CharT, _Traits>& __os, const complex<_Tp>& __x)
 {
-    return __os << '(' << __x.real() << ',' << __x.imag() << ')';
+    basic_ostringstream<_CharT, _Traits> __s;
+    __s.flags(__os.flags());
+    __s.imbue(__os.getloc());
+    __s.precision(__os.precision());
+    __s << '(' << __x.real() << ',' << __x.imag() << ')';
+    return __os << __s.str();
 }
 
 #if _LIBCPP_STD_VER > 11

libcxx/test/std/numerics/complex.number/complex.ops/stream_output.pass.cpp

 
 int main(int, char**)
 {
-    std::complex<double> c(1, 2);
-    std::ostringstream os;
-    os << c;
-    assert(os.str() == "(1,2)");
+    {
+        std::complex<double> c(1, 2);
+        std::ostringstream os;
+        os << c;
+        assert(os.str() == "(1,2)");
+    }
+    {
+        std::complex<double> c(1, 2);
+        std::ostringstream os;
+        os.width(8);
+        os.fill('_');
+        os << c;
+        assert(os.str() == "___(1,2)");
+    }
 
   return 0;
 }