Refactor compile-time assertion checks in c.h

This commit refactors and simplifies the definitions of StaticAssertStmt,
StaticAssertExpr and StaticAssertDecl.  By unifying the C and C++
fallback implementations, this reduces the number of different
implementations from four to three.

Author: Michael Paquier
Reviewed-by: Georgios Kokolatos, Tom Lane
Discussion: https://postgr.es/m/20200204081503.GF2287@paquier.xyz

Author: michaelpq

src/include/c.h

@@ -836,43 +836,37 @@ 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.
  */
-#ifndef __cplusplus
-#ifdef HAVE__STATIC_ASSERT
+#if !defined(__cplusplus) && defined(HAVE__STATIC_ASSERT)
+/* Default C implementation */
 #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)
-#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
+#elif defined(__cplusplus) && __cpp_static_assert >= 200410
+/* Default C++ implementation */
 #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							/* !__cpp_static_assert */
+#else
+/* Fallback implementation for C and C++ */
 #define StaticAssertStmt(condition, errmessage) \
-	do { struct static_assert_struct { int static_assert_failure : (condition) ? 1 : -1; }; } while(0)
+	((void) sizeof(struct { int static_assert_failure : (condition) ? 1 : -1; }))
 #define StaticAssertExpr(condition, errmessage) \
-	((void) ({ StaticAssertStmt(condition, errmessage); }))
+	StaticAssertStmt(condition, errmessage)
 #define StaticAssertDecl(condition, errmessage) \
 	extern void static_assert_func(int static_assert_failure[(condition) ? 1 : -1])
-#endif							/* __cpp_static_assert */
-#endif							/* C++ */
+#endif
 
 
 /*