Use is_constructible instead of is_convertible.

Before this, all objects would follow the slow path (since nothing is
convertible to AlphaNum since it has a private copy constructor).

Before:
entry {
  name: "MicroBenchmarks.benchmark_defun_matmul_2_by_2_CPU"
  iters: 30000
  wall_time: 67.5895690918
  extras {
    key: "examples_per_sec"
    value {
      double_value: 14795.1823549
    }
  }
}

After:
entry {
  name: "MicroBenchmarks.benchmark_defun_matmul_2_by_2_CPU"
  iters: 30000
  wall_time: 61.0044002533
  extras {
    key: "examples_per_sec"
    value {
      double_value: 16392.2601623
    }
  }
}
PiperOrigin-RevId: 193247183

1 files changed, 1 insertions, 1 deletions

diff --git a/tensorflow/core/lib/core/errors.h b/tensorflow/core/lib/core/errors.h
index 1a0f4be2ea..51c09032df 100644
--- a/tensorflow/core/lib/core/errors.h
+++ b/tensorflow/core/lib/core/errors.h
@@ -42,7 +42,7 @@ namespace internal {
 // Eventually absl::strings will have native support for this and we will be
 // able to completely remove PrepareForStrCat().
 template <typename T>
-typename std::enable_if<!std::is_convertible<T, strings::AlphaNum>::value,
+typename std::enable_if<!std::is_constructible<strings::AlphaNum, T>::value,
                         string>::type
 PrepareForStrCat(const T& t) {
   std::stringstream ss;