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Fix PrepareForStrCat() for types that are AlphaNum constructible but not implicitly convertible.

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In the latest dev version of Eigen, `Eigen::half` is implicitly
convertible to float.  This makes `std::is_constructible<AlphaNum,Eigen::half>` true (using the
float constructor), but since `Eigen::half` is not implicitly convertible to `AlphaNum` directly,
this leads to the compile error:
```
./third_party/tensorflow/core/platform/errors.h:107:1: error: no matching function for call to 'PrepareForStrCat'
...

./third_party/tensorflow/core/platform/errors.h:54:33: note: candidate function not viable: no known conversion from 'Eigen::half' to 'const strings::AlphaNum' for 1st argument
inline const strings::AlphaNum& PrepareForStrCat(const strings::AlphaNum& a) {
                                ^
./third_party/tensorflow/core/platform/errors.h:49:1: note: candidate template ignored: requirement '!std::is_constructible_v<tensorflow::strings::AlphaNum, Eigen::half>' was not satisfied [with T = Eigen::half]
PrepareForStrCat(const T& t) {
^
```
The same error occurs for any type implicitly convertible to int/float/double/string/etc...

To fix this, we need to change the condition to `!is_convertible<T, AlphaNum>`, so that
if `T` *is* implicitly convertible to `AlphaNum`, it will use the `AlphaNum` version,
otherwise it will use the stream operator version.

See [MR !278](https://gitlab.com/libeigen/eigen/-/merge_requests/278).

PiperOrigin-RevId: 343357980
Change-Id: Ibad29a54105a70c473ca4c9b205fce8356d155ab
This commit is contained in:
A. Unique TensorFlower 2020-11-19 13:29:54 -08:00 committed by TensorFlower Gardener
parent ad792f5eaf
commit 21f5c12e3d
1 changed files with 1 additions and 1 deletions
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2
tensorflow/core/platform/errors.h
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@ -44,7 +44,7 @@ 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_constructible<strings::AlphaNum, T>::value,
typename std::enable_if<!std::is_convertible<T, strings::AlphaNum>::value,
std::string>::type
PrepareForStrCat(const T& t) {
std::stringstream ss;