[Bug math/16928] New: cacos wrong sign of zero in FE_DOWNWARD mode

["jsm28 at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16928

            Bug ID: 16928
           Summary: cacos wrong sign of zero in FE_DOWNWARD mode
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org

In FE_DOWNWARD mode, cacos of a value with real part +Inf and finite imaginary
part produces a result with real part -0 when the result should have real part
+0 (C99/C11 Annex G).  E.g., on x86_64 for float (but this applies for all
types):

Failure: Test: Real part of: cacos_downward (inf + 0 i)
Result:
 is:         -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
 should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
 difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
 ulp       :  0.0000
 max.ulp   :  2.0000
Failure: Test: Real part of: cacos_downward (inf - 0 i)
Result:
 is:         -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
 should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
 difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
 ulp       :  0.0000
 max.ulp   :  2.0000
Failure: Test: Real part of: cacos_downward (inf + 0.5 i)
Result:
 is:         -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
 should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
 difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
 ulp       :  0.0000
 max.ulp   :  2.0000
Failure: Test: Real part of: cacos_downward (inf - 0.5 i)
Result:
 is:         -0.00000000000000000000e+00  -0x0.00000000000000000000p+0
 should be:   0.00000000000000000000e+00   0x0.00000000000000000000p+0
 difference:  0.00000000000000000000e+00   0x0.00000000000000000000p+0
 ulp       :  0.0000
 max.ulp   :  2.0000

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