[PATCH] assert: Support types without operator== (int) [BZ #21972]

[fweimer@redhat.com (Florian Weimer)]

2017-08-18  Florian Weimer  <fweimer@redhat.com>

	[BZ #21972]
	* assert/assert.h (assert): Use static_cast (bool) for C++ in the
	pedantic warning check, to avoid a requirement for operator== (int).
	* assert/Makefile (tests): Add tst-assert-c++, tst-assert-g++.
	(CFLAGS-tst-assert-c++.o): Compile in C++11 mode.
	(CFLAGS-tst-assert-g++.o): Compile in GnU C++11 mode.
	(LDLIBS-tst-assert-c++, LDLIBS-tst-assert-g++): Link with libstdc++.
	* assert/tst-assert-c++.cc, assert/tst-assert-g++.cc: New files.

diff --git a/assert/Makefile b/assert/Makefile
index 1c3be9b01f..9ec1be81a9 100644
--- a/assert/Makefile
+++ b/assert/Makefile
@@ -25,6 +25,15 @@ include ../Makeconfig
 headers	:= assert.h
 
 routines := assert assert-perr __assert
-tests := test-assert test-assert-perr
+tests := test-assert test-assert-perr tst-assert-c++ tst-assert-g++
 
 include ../Rules
+
+ifeq ($(have-cxx-thread_local),yes)
+CFLAGS-tst-assert-c++.o = -std=c++11
+LDLIBS-tst-assert-c++ = -lstdc++
+CFLAGS-tst-assert-g++.o = -std=gnu++11
+LDLIBS-tst-assert-g++ = -lstdc++
+else
+tests-unsupported += tst-assert-c++ tst-assert-g++
+endif
diff --git a/assert/assert.h b/assert/assert.h
index 6801cfeb10..4fea59e222 100644
--- a/assert/assert.h
+++ b/assert/assert.h
@@ -97,13 +97,25 @@ __END_DECLS
    required to support function pointers and bit fields in this
    context, and to suppress the evaluation of variable length
    arrays.  */
-#  define assert(expr)							\
+#  ifndef __cplusplus
+#   define assert(expr)							\
   ((void) sizeof ((expr) == 0), __extension__ ({			\
       if (expr)								\
         ; /* empty */							\
       else								\
         __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION);	\
     }))
+#  else	 /* __cplusplus */
+/* If EXPR is a type without a matching operrator== (int), the C
+   definition above does not work.  */
+#   define assert(expr)							\
+  ((void) sizeof (static_cast<bool> (expr)), __extension__ ({		\
+      if (expr)								\
+        ; /* empty */							\
+      else								\
+        __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION);	\
+    }))
+#  endif
 # endif
 
 # ifdef	__USE_GNU
diff --git a/assert/tst-assert-c++.cc b/assert/tst-assert-c++.cc
new file mode 100644
index 0000000000..c001bfff49
--- /dev/null
+++ b/assert/tst-assert-c++.cc
@@ -0,0 +1,72 @@
+/* Tests for interactions between C++ and assert.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <assert.h>
+
+/* The C++ standard requires that if the assert argument is a constant
+   subexpression, then the assert itself is one, too.  */
+constexpr int
+check_constexpr ()
+{
+  return (assert (true), 1);
+}
+
+/* Objects of this class can be contextually converted to bool, but
+   cannot be compared to int.  */
+struct no_int
+{
+  no_int () = default;
+  no_int (const no_int &) = delete;
+
+  explicit operator bool () const
+  {
+    return true;
+  }
+};
+
+/* This class tests that operator== is not used by assert.  */
+struct bool_and_int
+{
+  bool_and_int () = default;
+  bool_and_int (const no_int &) = delete;
+
+  explicit operator bool () const
+  {
+    return true;
+  }
+
+  template <class T> bool operator== (T) const; /* No definition.  */
+};
+
+static int
+do_test ()
+{
+  {
+    no_int value;
+    assert (value);
+  }
+
+  {
+    bool_and_int value;
+    assert (value);
+  }
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/assert/tst-assert-g++.cc b/assert/tst-assert-g++.cc
new file mode 100644
index 0000000000..8c06402825
--- /dev/null
+++ b/assert/tst-assert-g++.cc
@@ -0,0 +1,19 @@
+/* Tests for interactions between C++ and assert.  GNU C++11 version.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <tst-assert-c++.cc>