https://github.com/wch/r-source/commit/489a6b8d330bb30da82329f1949f44a0f633f1e8

From 489a6b8d330bb30da82329f1949f44a0f633f1e8 Mon Sep 17 00:00:00 2001
From: ripley <ripley@00db46b3-68df-0310-9c12-caf00c1e9a41>
Date: Thu, 10 Apr 2025 05:13:13 +0000
Subject: [PATCH] workaround for building standalone nmath with recent
 compilers

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

diff --git a/src/nmath/mlutils.c b/src/nmath/mlutils.c
index 1b590f860a..8269e7d7f9 100644
--- a/src/nmath/mlutils.c
+++ b/src/nmath/mlutils.c
@@ -105,7 +105,20 @@ double R_pow_di(double x, int n)
     return pow;
 }
 
+/* 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>
-- 
2.49.1