https://issues.dlang.org/show_bug.cgi?id=23869

          Issue ID: 23869
           Summary: ImportC: undefined identifier `isfinite`
           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

This code

if (!R_FINITE(x)) return x;

expands to

if (!__builtin_isfinite (x)) return x;

It is fixed by adding this to the .c file:

#define __builtin_isfinite finite

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