uchar: Fix compilation errors in C++ mode on OpenBSD

[Bruno Haible]

On OpenBSD 6.7, a testdir with C++ tests fails to compile:

c++ -DHAVE_CONFIG_H -DEXEEXT=\"\" -I. -I../../gltests -I..  
-DGNULIB_STRICT_CHECKING=1 -DIN_GNULIB_TESTS=1 -I. -I../../gltests -I.. 
-I../../gltests/.. -I../gllib -I../../gltests/../gllib 
-I/home/bruno/prefix-cc/include -Wall  -g -O2 -MT test-uchar-c++.o -MD -MP -MF 
$depbase.Tpo -c -o test-uchar-c++.o ../../gltests/test-uchar-c++.cc &&\
mv -f $depbase.Tpo $depbase.Po
In file included from ../../gltests/test-uchar-c++.cc:22:
../gllib/uchar.h:372:24: error: cannot combine with previous 'type-name' 
declaration specifier
typedef uint_least16_t char16_t;
                       ^
../gllib/uchar.h:386:24: error: cannot combine with previous 'type-name' 
declaration specifier
typedef uint_least32_t char32_t;
                       ^
2 errors generated.

The reason is that the clang C++ compiler, at least in this configuration
but probably also in other configurations, defines the types 'char16_t'
and 'char32_t' without any includes.

This patch fixes the problem.


2020-08-19  Bruno Haible  <bruno@clisp.org>

        uchar: Fix compilation errors in C++ mode on OpenBSD.
        * lib/uchar.in.h (char16_t, char32_t): Don't define in C++ mode if
        CXX_HAS_UCHAR_TYPES is 1.
        * m4/uchar.m4 (gl_UCHAR_H): Determine whether the C++ compiler
        predefines char16_t and char32_t. Substitute CXX_HAS_UCHAR_TYPES.
        * modules/uchar (Makefile.am): Substitute CXX_HAS_UCHAR_TYPES.

diff --git a/lib/uchar.in.h b/lib/uchar.in.h
index b77116d..12f9935 100644
--- a/lib/uchar.in.h
+++ b/lib/uchar.in.h
@@ -40,7 +40,7 @@
 /* The definitions of _GL_FUNCDECL_RPL etc. are copied here.  */
 
 
-#if !@HAVE_UCHAR_H@
+#if !(@HAVE_UCHAR_H@ || (defined __cplusplus && @CXX_HAS_UCHAR_TYPES@))
 
 /* A 16-bit variant of wchar_t.
    Note: This type does *NOT* denote UTF-16 units.  (Only on platforms
@@ -54,7 +54,7 @@ typedef uint_least16_t gl_char16_t;
 
 #endif
 
-#if !@HAVE_UCHAR_H@
+#if !(@HAVE_UCHAR_H@ || (defined __cplusplus && @CXX_HAS_UCHAR_TYPES@))
 
 /* A 32-bit variant of wchar_t.
    Note: This type does *NOT* denote UTF-32 code points.  (Only on platforms
diff --git a/m4/uchar.m4 b/m4/uchar.m4
index 0875caa..a573f5d 100644
--- a/m4/uchar.m4
+++ b/m4/uchar.m4
@@ -1,4 +1,4 @@
-# uchar.m4 serial 14
+# uchar.m4 serial 15
 dnl Copyright (C) 2019-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -22,6 +22,31 @@ AC_DEFUN_ONCE([gl_UCHAR_H],
   gl_TYPE_CHAR16_T
   gl_TYPE_CHAR32_T
 
+  dnl In C++ mode, clang defines 'char16_t' and 'char32_t' as built-in types
+  dnl on some platforms (e.g. OpenBSD 6.7).
+  CXX_HAS_UCHAR_TYPES=0
+  if test $HAVE_UCHAR_H = 0; then
+    if test "$CXX" != no; then
+      AC_CACHE_CHECK([whether the C++ compiler predefines the <uchar.h> types],
+        [gl_cv_cxx_has_uchar_types],
+        [dnl We can't use AC_LANG_PUSH([C++]) and AC_LANG_POP([C++]) here, due 
to
+         dnl an autoconf bug <https://savannah.gnu.org/support/?110294>.
+         echo 'char16_t a; char32_t b;' > conftest.cpp
+         gl_command="$CXX $CXXFLAGS $CPPFLAGS -c conftest.cpp"
+         if AC_TRY_EVAL([gl_command]); then
+           gl_cv_cxx_has_uchar_types=yes
+         else
+           gl_cv_cxx_has_uchar_types=no
+         fi
+         rm -fr conftest*
+        ])
+      if test $gl_cv_cxx_has_uchar_types = yes; then
+        CXX_HAS_UCHAR_TYPES=1
+      fi
+    fi
+  fi
+  AC_SUBST([CXX_HAS_UCHAR_TYPES])
+
   dnl Test whether a 'char32_t' can hold more characters than a 'wchar_t'.
   gl_STDINT_BITSIZEOF([wchar_t], [gl_STDINT_INCLUDES])
   if test $BITSIZEOF_WCHAR_T -lt 32; then
diff --git a/modules/uchar b/modules/uchar
index 7215b36..7cddcad 100644
--- a/modules/uchar
+++ b/modules/uchar
@@ -27,6 +27,7 @@ uchar.h: uchar.in.h $(top_builddir)/config.status $(CXXDEFS_H)
              -e 's|@''PRAGMA_SYSTEM_HEADER''@|@PRAGMA_SYSTEM_HEADER@|g' \
              -e 's|@''PRAGMA_COLUMNS''@|@PRAGMA_COLUMNS@|g' \
              -e 's|@''NEXT_UCHAR_H''@|$(NEXT_UCHAR_H)|g' \
+             -e 's|@''CXX_HAS_UCHAR_TYPES''@|$(CXX_HAS_UCHAR_TYPES)|g' \
              -e 's|@''SMALL_WCHAR_T''@|$(SMALL_WCHAR_T)|g' \
              -e 
's|@''GNULIB_OVERRIDES_CHAR16_T''@|$(GNULIB_OVERRIDES_CHAR16_T)|g' \
              -e 
's|@''GNULIB_OVERRIDES_CHAR32_T''@|$(GNULIB_OVERRIDES_CHAR32_T)|g' \