https://issues.dlang.org/show_bug.cgi?id=23632

          Issue ID: 23632
           Summary: [ImportC] undefined identifier `_Float128`
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: lance@lancebachmeier.com

Here's the full code of mathcalls-helper-functions.h:

/* Classify given number.  */
__MATHDECL_ALIAS (int, __fpclassify,, (_Mdouble_ __value), fpclassify)
     __attribute__ ((__const__));

/* Test for negative number.  */
__MATHDECL_ALIAS (int, __signbit,, (_Mdouble_ __value), signbit)
     __attribute__ ((__const__));

/* Return 0 if VALUE is finite or NaN, +1 if it
   is +Infinity, -1 if it is -Infinity.  */
__MATHDECL_ALIAS (int, __isinf,, (_Mdouble_ __value), isinf)
  __attribute__ ((__const__));

/* Return nonzero if VALUE is finite and not NaN.  Used by isfinite macro.  */
__MATHDECL_ALIAS (int, __finite,, (_Mdouble_ __value), finite)
  __attribute__ ((__const__));

/* Return nonzero if VALUE is not a number.  */
__MATHDECL_ALIAS (int, __isnan,, (_Mdouble_ __value), isnan)
  __attribute__ ((__const__));

/* Test equality.  */
__MATHDECL_ALIAS (int, __iseqsig,, (_Mdouble_ __x, _Mdouble_ __y), iseqsig);

/* Test for signaling NaN.  */
__MATHDECL_ALIAS (int, __issignaling,, (_Mdouble_ __value), issignaling)
     __attribute__ ((__const__));

And here's the error message:

In file included from <command-line>:
/home/gunther/place/dlang/dmd/generated/linux/debug/64/../../../../druntime/import/importc.h:80:8:
warning: undefining "__has_builtin"
   80 | #undef __has_builtin
      |        ^~~~~~~~~~~~~
/usr/include/x86_64-linux-gnu/bits/mathcalls-helper-functions.h(21): Error:
undefined identifier `_Float128`
/usr/include/x86_64-linux-gnu/bits/mathcalls-helper-functions.h(25): Error:
undefined identifier `_Float128`
/usr/include/x86_64-linux-gnu/bits/mathcalls-helper-functions.h(30): Error:
undefined identifier `_Float128`
/usr/include/x86_64-linux-gnu/bits/mathcalls-helper-functions.h(34): Error:
undefined identifier `_Float128`
/usr/include/x86_64-linux-gnu/bits/mathcalls-helper-functions.h(38): Error:
undefined identifier `_Float128`
/usr/include/x86_64-linux-gnu/bits/mathcalls-helper-functions.h(41): Error:
undefined identifier `_Float128`
/usr/include/x86_64-linux-gnu/bits/mathcalls-helper-functions.h(41): Error:
undefined identifier `_Float128`
/usr/include/x86_64-linux-gnu/bits/mathcalls-helper-functions.h(45): Error:
undefined identifier `_Float128`

The lines corresponding to the error message are all

__attribute__ ((__const__));

--