Revert "Refactor compile-time assertion checks in c.h"

This reverts commit b7f64c64, which broke the fallback implementation for
C++.  We have discussed a couple of alternatives to reduce the number of
implementations for those asserts, but nothing allowing to reduce the
number of implementations down to three instead of four, so there is no
benefit in keeping this patch.

Thanks to Tom Lane for the discussion.

Discussion: https://postgr.es/m/20200313115033.GA183471@paquier.xyz

src/include/c.h

@@ -836,37 +836,43 @@ extern void ExceptionalCondition(const char *conditionName,
  * The macro StaticAssertDecl() is suitable for use at file scope (outside of
  * any function).
  *
- * On recent C++ compilers, we can use standard static_assert().
- *
  * Otherwise we fall back on a kluge that assumes the compiler will complain
  * about a negative width for a struct bit-field.  This will not include a
  * helpful error message, but it beats not getting an error at all.
  */
-#if !defined(__cplusplus) && defined(HAVE__STATIC_ASSERT)
-/* Default C implementation */
+#ifndef __cplusplus
+#ifdef HAVE__STATIC_ASSERT
 #define StaticAssertStmt(condition, errmessage) \
 	do { _Static_assert(condition, errmessage); } while(0)
 #define StaticAssertExpr(condition, errmessage) \
 	((void) ({ StaticAssertStmt(condition, errmessage); true; }))
 #define StaticAssertDecl(condition, errmessage) \
 	_Static_assert(condition, errmessage)
-#elif defined(__cplusplus) && __cpp_static_assert >= 200410
-/* Default C++ implementation */
+#else							/* !HAVE__STATIC_ASSERT */
+#define StaticAssertStmt(condition, errmessage) \
+	((void) sizeof(struct { int static_assert_failure : (condition) ? 1 : -1; }))
+#define StaticAssertExpr(condition, errmessage) \
+	StaticAssertStmt(condition, errmessage)
+#define StaticAssertDecl(condition, errmessage) \
+	extern void static_assert_func(int static_assert_failure[(condition) ? 1 : -1])
+#endif							/* HAVE__STATIC_ASSERT */
+#else							/* C++ */
+#if defined(__cpp_static_assert) && __cpp_static_assert >= 200410
 #define StaticAssertStmt(condition, errmessage) \
 	static_assert(condition, errmessage)
 #define StaticAssertExpr(condition, errmessage) \
 	({ static_assert(condition, errmessage); })
 #define StaticAssertDecl(condition, errmessage) \
 	static_assert(condition, errmessage)
-#else
-/* Fallback implementation for C and C++ */
+#else							/* !__cpp_static_assert */
 #define StaticAssertStmt(condition, errmessage) \
-	((void) sizeof(struct { int static_assert_failure : (condition) ? 1 : -1; }))
+	do { struct static_assert_struct { int static_assert_failure : (condition) ? 1 : -1; }; } while(0)
 #define StaticAssertExpr(condition, errmessage) \
-	StaticAssertStmt(condition, errmessage)
+	((void) ({ StaticAssertStmt(condition, errmessage); }))
 #define StaticAssertDecl(condition, errmessage) \
 	extern void static_assert_func(int static_assert_failure[(condition) ? 1 : -1])
-#endif
+#endif							/* __cpp_static_assert */
+#endif							/* C++ */
 
 
 /*