<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/149610>149610</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Can't `#include <llvm/ADT/FloatingPointMode.h>` with `-std=c++23` with clang++ 21.1.0-rc1
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          chriselrod
      </td>
    </tr>
</table>

<pre>
    ```c++
#include <llvm/ADT/FloatingPointMode.h>

int main(){
  return 0;
}
```
I get
```
❯ clang++ llvminclude.cpp

~/Documents/progwork/cxx/test
❯ clang++ -std=c++23 llvminclude.cpp
In file included from llvminclude.cpp:1:
In file included from /usr/include/llvm/ADT/FloatingPointMode.h:19:
In file included from /usr/include/llvm/ADT/StringSwitch.h:16:
/usr/include/llvm/ADT/StringRef.h:864:22: error: 'enable_if' attribute expression never
      produces a constant expression
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                      ^
/usr/include/llvm/ADT/FloatingPointMode.h:193:14: note: in instantiation of function
      template specialization 'llvm::StringLiteral::StringLiteral<1UL>' requested here
  193 |       .Cases("", "ieee", DenormalMode::IEEE)
      |              ^
/usr/include/llvm/ADT/StringRef.h:864:49: note: read of variable 'Str' whose value is not
      known
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                                                 ^
/usr/include/llvm/ADT/StringRef.h:860:42: note: declared here
  860 |     constexpr StringLiteral(const char (&Str)[N])
      |                                          ^
/usr/include/llvm/ADT/StringRef.h:864:22: error: 'enable_if' attribute expression never
      produces a constant expression
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                      ^
/usr/include/llvm/ADT/FloatingPointMode.h:193:18: note: in instantiation of function
      template specialization 'llvm::StringLiteral::StringLiteral<5UL>' requested here
  193 |       .Cases("", "ieee", DenormalMode::IEEE)
      |                  ^
/usr/include/llvm/ADT/StringRef.h:864:49: note: read of variable 'Str' whose value is not
      known
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                                                 ^
/usr/include/llvm/ADT/StringRef.h:860:42: note: declared here
  860 |     constexpr= StringLiteral(const char (&Str)[N])
      |                                          ^
/usr/include/llvm/ADT/StringRef.h:864:22: error: 'enable_if' attribute expression never
      produces a constant expression
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                      ^
/usr/include/llvm/ADT/FloatingPointMode.h:194:13: note: in instantiation of function
      template specialization 'llvm::StringLiteral::StringLiteral<14UL>' requested here
  194 |       .Case("preserve-sign", DenormalMode::PreserveSign)
      |             ^
/usr/include/llvm/ADT/StringRef.h:864:49: note: read of variable 'Str' whose value is not
      known
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                                                 ^
/usr/include/llvm/ADT/StringRef.h:860:42: note: declared here
  860 |     constexpr StringLiteral(const char (&Str)[N])
      |                                          ^
/usr/include/llvm/ADT/StringRef.h:864:22: error: 'enable_if' attribute expression never
      produces a constant expression
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                      ^
/usr/include/llvm/ADT/FloatingPointMode.h:196:13: note: in instantiation of function
      template specialization 'llvm::StringLiteral::StringLiteral<8UL>' requested here
  196 |       .Case("dynamic", DenormalMode::Dynamic)
      |             ^
/usr/include/llvm/ADT/StringRef.h:864:49: note: read of variable 'Str' whose value is not
      known
  864 |         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
      |                                                 ^
/usr/include/llvm/ADT/StringRef.h:860:42: note: declared here
  860 |     constexpr StringLiteral(const char (&Str)[N])
      |                                          ^
4 errors generated.
```

This works with clang++ 20.
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJzsWE1v4zYT_jXjCxFDGsr6OOigWBYQYN_F4s32bNDS2GIjky5J2dke-tsLSk7WSeNs2g0QFDVByDY5nI9nZh7IFNbKjSLKYXYNs3Iietdqk9etkZY6o5vJSjffcoiDcdaA134GBSCXqu76hhjwedftt4BVUX4FrKpOCyfV5ouWyv1PNzRtgS_8maCQyrGtkAowBcwg8aoYM-R6o1gAfFCdlP75YBOC4oZtyD1bgwVCVkJRsboTajM6xrwjR7-m9W43Gv0DsCp13W9JOQtY7YzeHLS5A6zq-3vAypF1Z1VeWdcAL4-hI3_Bxo1ia9kRO642bG309i9yvAiBF2fFAaveGsDquAxY_RjWIsx-RuetM1Jtbg_S1e2oLh7Vve3g_2k9nErjCHiBCLxgZIw2_gtgQkqsOlrKNWDChHNGrnpHjO53hqyVWjFFezJDDfixM7rpa7JMsFor64RyJ8KDWBpHDJI5exjL5aPeoabSE5vpcrnqZeekWlpnOvJVd-sMYMaAl8BL9pldsRBw_ujBqeonA2aLN8ByLkfcPz1GTGlH_lMqJscIpXAeCb1m617V7iFQPxxtd51wxOyOaik6-fsoC5gMZnkBvBhT8Uk6MqJ7cWke_vLJdyAmzNBvPVlHDWvJ0GAozPhJ2NO5sGQHKHGYcwaIkoiOv0pS2mxF56Mbrd0sFgvfzGcxfBt2L5VUlJ1iZkg0Hqe9MNIn2eMw5DNhh1ZbYnvR9cSk9Sce3blT-vCxtfPK-GfQBB4aPIWmoboT5jStaRw8ejQ0k28k9rQyMB12WN0KwwYI4jFGmF1_hln5albfP6wLifyYRNIPI5HZx5LIz5XVhUjel0i83xcu-fdyiYcn5B_3QhK9TibRczIZucQDT2ZPV_5fy1kq-XKUuh2EzlfdhU3euecuryX_RSqJP5ZK0teZJH6ZSZpvSmxlfZZDyof9C31c6ONpWNFIA5ZtSJERjprp86upoPjaSssO2txZdpCufXKhhMF00uS8yXgmJpSHyQzTJIuCdNLms1WNiElDIuWCAh6nPEvXaR1HXDRBRhOZY4CzIAlTxFkcpdOgCesoTGJer7MwEBlEAW2F7KYe66k2m4m0tqc8jLI4DCadWFFnh_s-xKNXCLNyYnJ_4GrVbyxEQSets99VOOk6yudCASaO-Sj_9vVfHIxIQBw8v1F73HuCUjgNp8GVqcNJb7q8dW5nfW9iBVhtpGv71bTW2-9FNbi_M_pXqp0vOh-1BayOge9z_DMAAP__RsYC1g">