Fix ambiguous std::isnan call on xnn_float16 in vbinary tester

[anagnorisis2peripeteia]

Defect: the x86_64 test build does not compile

test/vbinary-microkernel-tester.cc:66 fails to build on x86_64:

test/vbinary-microkernel-tester.cc:66:11: error: call to 'isnan' is ambiguous
   66 |       if (std::isnan(y_ref[i])) {
/usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/cmath:1182:3: note: candidate function
 1182 |   isnan(float __x)
/usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/cmath:1190:3: note: candidate function
 1190 |   isnan(double __x)
/usr/lib/gcc/x86_64-linux-gnu/15/../../../../include/c++/15/cmath:1195:3: note: candidate function
 1195 |   isnan(long double __x)

Root cause

src/xnnpack/math.h:528-532 enables native _Float16 on x86 when F16C is present:

#if (defined(__i386__) || defined(__x86_64__)) && defined(__SSE2__) && \
    defined(__FLT16_MAX__) && defined(__F16C__) &&                     \
    ((__clang_major__ >= 15 && !defined(_MSC_VER)) || (XNN_GNUC_ACTUAL >= 12))
#define XNN_HAVE_FLOAT16 1
#endif
...
#if XNN_HAVE_FLOAT16
typedef _Float16 xnn_float16;

std::isnan has no _Float16 overload, so the argument converts to float, double and long double by equally-ranked floating conversions. Overload resolution cannot pick one.

Minimal reproduction (libstdc++ 15):

_Float16 x = 1.0f; std::isnan(x);        // error: call to 'isnan' is ambiguous
_Float16 x = 1.0f; std::isnan(float(x)); // compiles

Impact

This is the only compile error across a 797-target x86_64 build, so it blocks every test binary that links vbinary-microkernel-tester. ninja -k 0 reports exactly one distinct error.

Fix

Cast to float — the same conversion the surrounding lines already use (static_cast<float>(y_ref[i]) appears three lines below). One line.

Why ARM64 never saw it

On ARM64, xnn_float16 is the struct form (math.h:552-566) with a single operator float(), which is unambiguous. Only the native-_Float16 path is affected.

Verification

• Before: clang++ -std=gnu++17 -march=native -c test/vbinary-microkernel-tester.cc on x86_64 → error: call to 'isnan' is ambiguous
• After: the same command compiles cleanly, no diagnostics.
• ARM64 vbinary-microkernel-tester target still builds.

1 file, +5/-1. git diff --check clean; no line exceeds 120 columns.