locale: fix compilation error in C++ mode on MSVC

[Bruno Haible]

When building a testdir with C++ tests on MSVC 14, I see this error:

source='../../gltests/test-wchar-c++3.cc' object='test-wchar-c++3.obj' 
libtool=no \
DEPDIR=.deps depmode=msvc7 /bin/sh ../../build-aux/depcomp \
/home/bruno/msvc/compile cl -nologo -DHAVE_CONFIG_H -DEXEEXT=\".exe\" -I. 
-I../../gltests -I..  -DGNULIB_STRICT_CHECKING=1 -DIN_GNULIB_TESTS=1 -I. 
-I../../gltests -I.. -I../../gltests/.. -I../gllib -I../../gltests/../gllib 
-D_WIN32_WINNT=_WIN32_WINNT_WINXP -I/usr/local/msvc64/include  -MD -c -o 
test-wchar-c++3.obj `cygpath -w '../../gltests/test-wchar-c++3.cc'`
test-wchar-c++3.cc
...
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\xlocinfo(127): 
error C2440: 'return': cannot convert from 'lconv *' to 'const rpl_lconv *'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\xlocinfo(127): 
note: Types pointed to are unrelated; conversion requires reinterpret_cast, 
C-style cast or function-style cast
...
make[4]: *** [Makefile:9886: test-wchar-c++3.obj] Error 2

The cause is a use of 'struct lconv' deep inside the C++ header files. The fix
is to not override 'struct lconv' and 'localeconv' on this platform. What the
override would do (see lib/localeconv.c lines 53..58) is to map the missing
struct members to those without 'int_' prefix. We can do the same thing through
C macros. This is not as good as the original override, because
  - it will also affect other uses of the identifiers int_p_cs_precedes,
    int_p_sign_posn, int_p_sep_by_space, int_n_cs_precedes, int_n_sign_posn,
        int_n_sep_by_space,
  - it will not work in code that uses 'struct lconv' for its own purposes
    (other than for localeconv() invocations).
But it should be good enough.


2019-12-11  Bruno Haible  <address@hidden>

        locale: Fix compilation error in C++ mode on MSVC.
        * m4/locale_h.m4 (gl_LOCALE_H): Don't set REPLACE_STRUCT_LCONV on MSVC.
        * lib/locale.in.h (int_p_cs_precedes, int_p_sign_posn,
        int_p_sep_by_space, int_n_cs_precedes, int_n_sign_posn,
        int_n_sep_by_space): Define as macros on MSVC.

diff --git a/m4/locale_h.m4 b/m4/locale_h.m4
index 380e40b..c0ec3fc 100644
--- a/m4/locale_h.m4
+++ b/m4/locale_h.m4
@@ -1,4 +1,4 @@
-# locale_h.m4 serial 22
+# locale_h.m4 serial 23
 dnl Copyright (C) 2007, 2009-2019 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -56,7 +56,20 @@ AC_DEFUN([gl_LOCALE_H],
        [gl_cv_sys_struct_lconv_ok=no])
     ])
   if test $gl_cv_sys_struct_lconv_ok = no; then
-    REPLACE_STRUCT_LCONV=1
+    dnl On native Windows with MSVC, merely define these member names as 
macros.
+    dnl This avoids trouble in C++ mode.
+    case "$host_os" in
+      mingw*)
+        AC_EGREP_CPP([Special], [
+#ifdef _MSC_VER
+ Special
+#endif
+          ],
+          [],
+          [REPLACE_STRUCT_LCONV=1])
+        ;;
+      *) REPLACE_STRUCT_LCONV=1 ;;
+    esac
   fi
 
   dnl <locale.h> is always overridden, because of GNULIB_POSIXCHECK.
diff --git a/lib/locale.in.h b/lib/locale.in.h
index 918d4dd..77b8b3b 100644
--- a/lib/locale.in.h
+++ b/lib/locale.in.h
@@ -64,6 +64,18 @@
 # define LC_MESSAGES 1729
 #endif
 
+/* On native Windows with MSVC, 'struct lconv' lacks the members int_p_* and
+   int_n_*.  Instead of overriding 'struct lconv', merely define these member
+   names as macros.  This avoids trouble in C++ mode.  */
+#if defined _MSC_VER
+# define int_p_cs_precedes   p_cs_precedes
+# define int_p_sign_posn     p_sign_posn
+# define int_p_sep_by_space  p_sep_by_space
+# define int_n_cs_precedes   n_cs_precedes
+# define int_n_sign_posn     n_sign_posn
+# define int_n_sep_by_space  n_sep_by_space
+#endif
+
 /* Bionic libc's 'struct lconv' is just a dummy.  */
 #if @REPLACE_STRUCT_LCONV@
 # define lconv rpl_lconv