workaround for building standalone nmath with recent compilers

git-svn-id: https://svn.r-project.org/R/trunk@88128 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

doc/NEWS.Rd

@@ -67,6 +67,18 @@
   }
 
 }
+
+\section{\Rlogo CHANGES IN R 4.5.0 patched}{
+  \subsection{BUG FIXES}{
+    \itemize{
+      \item Standalone \code{nmath} can be built again with early-2025
+      versions of \command{clang}-based compilers such as LLVM
+      \command{clang}\sspace{}20, Apple \command{clang}\sspace{}17 and Intel
+      \command{icx}\sspace{}2025.0.
+    }
+  }
+}
+
 \section{\Rlogo CHANGES IN R 4.5.0}{
   %% \subsection{SIGNIFICANT USER-VISIBLE CHANGES}{
   %%   \itemize{

src/nmath/mlutils.c

@@ -105,8 +105,20 @@ double R_pow_di(double x, int n)
     return pow;
 }
 
-double NA_REAL = ML_NAN; // Intel icx will not compile this.
-                         // In R it is computed at runtime.
+/* It is not clear why these are being defined in standalone nmath:
+ * but that they are is stated in the R-admin manual.
+ *
+ * In R NA_AREAL is a specific NaN computed during initialization.
+ */
+#if defined(__clang__) && defined(NAN)
+// C99 (optionally) has NAN, which is a float but will coerce to double.
+double NA_REAL = NAN;
+#else
+// ML_NAN is defined as (0.0/0.0) in nmath.h
+// Fails to compile in Intel ics 2025.0, Apple clang 17, LLVM clang 20
+double NA_REAL = ML_NAN;
+#endif
+
 double R_PosInf = ML_POSINF, R_NegInf = ML_NEGINF;
 
 #include <stdio.h>