build: Fix -Wconversion warnings for fpclassify et al

closes #8274

- Instead of #undef and re-#define, define "xfoo" wrappers to avoid
  include-order sensitivity.
- The warnings are bogus, caused by bad interaction between glibc and
  clang 6+.
  - https://bugs.llvm.org/show_bug.cgi?id=35268
  - https://bugs.llvm.org/show_bug.cgi?id=39738
  - https://bugzilla.redhat.com/show_bug.cgi?id=1472437

CMakeLists.txt

@@ -258,6 +258,19 @@ int main(void)
 }
 " HAVE_BUILTIN_ADD_OVERFLOW)
 
+set(CMAKE_REQUIRED_LIBRARIES m)
+check_c_source_compiles("
+#include <math.h>
+int main(void)
+{
+  double d = 1.0;
+  int ok = (FP_NORMAL == __fpclassify(d) && !__isnan(d) && !__isinf(d));
+  return ok ? 0 : 1;
+}
+" HAVE___FPCLASSIFY)
+unset(CMAKE_REQUIRED_LIBRARIES)
+
+
 if(MSVC)
   # XXX: /W4 gives too many warnings. #3241
   add_definitions(/W3 -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE)

config/config.h.in

@@ -71,5 +71,6 @@
 
 #cmakedefine HAVE_EXECINFO_BACKTRACE
 #cmakedefine HAVE_BUILTIN_ADD_OVERFLOW
+#cmakedefine HAVE___FPCLASSIFY
 
 #endif  // AUTO_CONFIG_H

src/nvim/eval/encode.c

@@ -28,11 +28,6 @@
 #include "nvim/lib/kvec.h"
 #include "nvim/eval/typval_encode.h"
 
-#ifdef __MINGW32__
-# undef fpclassify
-# define fpclassify __fpclassify
-#endif
-
 #define ga_concat(a, b) ga_concat(a, (char_u *)b)
 #define utf_ptr2char(b) utf_ptr2char((char_u *)b)
 #define utf_ptr2len(b) ((size_t)utf_ptr2len((char_u *)b))
@@ -327,7 +322,7 @@ int encode_read_from_list(ListReaderState *const state, char *const buf,
 #define TYPVAL_ENCODE_CONV_FLOAT(tv, flt) \
     do { \
       const float_T flt_ = (flt); \
-      switch (fpclassify(flt_)) { \
+      switch (xfpclassify(flt_)) { \
         case FP_NAN: { \
           ga_concat(gap, (char_u *) "str2float('nan')"); \
           break; \
@@ -531,7 +526,7 @@ int encode_read_from_list(ListReaderState *const state, char *const buf,
 #define TYPVAL_ENCODE_CONV_FLOAT(tv, flt) \
     do { \
       const float_T flt_ = (flt); \
-      switch (fpclassify(flt_)) { \
+      switch (xfpclassify(flt_)) { \
         case FP_NAN: { \
           EMSG(_("E474: Unable to represent NaN value in JSON")); \
           return FAIL; \

src/nvim/macros.h

@@ -1,6 +1,10 @@
 #ifndef NVIM_MACROS_H
 #define NVIM_MACROS_H
 
+#include <math.h>
+
+#include "auto/config.h"
+
 // EXTERN is only defined in main.c. That's where global variables are
 // actually defined and initialized.
 #ifndef EXTERN
@@ -19,6 +23,23 @@
 # define MAX(X, Y) ((X) > (Y) ? (X) : (Y))
 #endif
 
+#if defined(__clang__) && __clang__ == 1 && __clang_major__ >= 6 \
+    && defined(HAVE___FPCLASSIFY)
+// Workaround glibc + Clang 6+ bug. #8274
+// https://bugzilla.redhat.com/show_bug.cgi?id=1472437
+# define xfpclassify __fpclassify
+# define xisnan __isnan
+# define xisinf __isinf
+#elif defined(__MINGW32__)
+// Workaround mingw warning. #7863
+# define xfpclassify __fpclassify
+# define xisnan _isnan
+#else
+# define xfpclassify fpclassify
+# define xisnan isnan
+# define xisinf isinf
+#endif
+
 /// String with length
 ///
 /// For use in functions which accept (char *s, size_t len) pair in arguments.

src/nvim/strings.c

@@ -50,16 +50,7 @@
 #include "nvim/os/shell.h"
 #include "nvim/eval/encode.h"
 
-#ifdef __MINGW32__
-# undef fpclassify
-# define fpclassify __fpclassify
-# undef isnan
-# define isnan _isnan
-#endif
-
-/*
- * Copy "string" into newly allocated memory.
- */
+/// Copy "string" into newly allocated memory.
 char_u *vim_strsave(const char_u *string)
   FUNC_ATTR_NONNULL_RET FUNC_ATTR_MALLOC FUNC_ATTR_NONNULL_ALL
 {
@@ -1214,14 +1205,14 @@ int vim_vsnprintf(char *str, size_t str_m, const char *fmt, va_list ap,
               remove_trailing_zeroes = true;
             }
 
-            if (isinf(f)
+            if (xisinf(f)
                 || (strchr("fF", fmt_spec) != NULL && abs_f > 1.0e307)) {
               xstrlcpy(tmp, infinity_str(f > 0.0, fmt_spec,
                                          force_sign, space_for_positive),
                        sizeof(tmp));
               str_arg_l = strlen(tmp);
               zero_padding = 0;
-            } else if (isnan(f)) {
+            } else if (xisnan(f)) {
               // Not a number: nan or NAN
               memmove(tmp, ASCII_ISUPPER(fmt_spec) ? "NAN" : "nan", 4);
               str_arg_l = 3;