Cast to void in StaticAssertExpr, not its callers.

Seems a bit silly that many (in fact all, as of today) uses of
StaticAssertExpr would need to cast it to void to avoid warnings from
pickier compilers.  Let's just do the cast right in the macro, instead.

In passing, change StaticAssertExpr to StaticAssertStmt in one
place where that seems more apropos.

Discussion: https://postgr.es/m/16161.1518715186@sss.pgh.pa.us

src/backend/storage/lmgr/lwlock.c

@@ -380,10 +380,10 @@ LWLockShmemSize(void)
 void
 CreateLWLocks(void)
 {
-	StaticAssertExpr(LW_VAL_EXCLUSIVE > (uint32) MAX_BACKENDS,
+	StaticAssertStmt(LW_VAL_EXCLUSIVE > (uint32) MAX_BACKENDS,
 					 "MAX_BACKENDS too big for lwlock.c");
 
-	StaticAssertExpr(sizeof(LWLock) <= LWLOCK_MINIMAL_SIZE &&
+	StaticAssertStmt(sizeof(LWLock) <= LWLOCK_MINIMAL_SIZE &&
 					 sizeof(LWLock) <= LWLOCK_PADDED_SIZE,
 					 "Miscalculated LWLock padding");
 

src/include/c.h

@@ -779,7 +779,7 @@ extern void ExceptionalCondition(const char *conditionName,
 #define StaticAssertStmt(condition, errmessage) \
 	do { _Static_assert(condition, errmessage); } while(0)
 #define StaticAssertExpr(condition, errmessage) \
-	({ StaticAssertStmt(condition, errmessage); true; })
+	((void) ({ StaticAssertStmt(condition, errmessage); true; }))
 #else							/* !HAVE__STATIC_ASSERT */
 #define StaticAssertStmt(condition, errmessage) \
 	((void) sizeof(struct { int static_assert_failure : (condition) ? 1 : -1; }))
@@ -796,7 +796,7 @@ extern void ExceptionalCondition(const char *conditionName,
 #define StaticAssertStmt(condition, errmessage) \
 	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); }))
 #endif
 #endif							/* C++ */
 
@@ -817,14 +817,14 @@ extern void ExceptionalCondition(const char *conditionName,
 	StaticAssertStmt(__builtin_types_compatible_p(__typeof__(varname), typename), \
 	CppAsString(varname) " does not have type " CppAsString(typename))
 #define AssertVariableIsOfTypeMacro(varname, typename) \
-	((void) StaticAssertExpr(__builtin_types_compatible_p(__typeof__(varname), typename), \
+	(StaticAssertExpr(__builtin_types_compatible_p(__typeof__(varname), typename), \
 	 CppAsString(varname) " does not have type " CppAsString(typename)))
 #else							/* !HAVE__BUILTIN_TYPES_COMPATIBLE_P */
 #define AssertVariableIsOfType(varname, typename) \
 	StaticAssertStmt(sizeof(varname) == sizeof(typename), \
 	CppAsString(varname) " does not have type " CppAsString(typename))
 #define AssertVariableIsOfTypeMacro(varname, typename) \
-	((void) StaticAssertExpr(sizeof(varname) == sizeof(typename),		\
+	(StaticAssertExpr(sizeof(varname) == sizeof(typename), \
 	 CppAsString(varname) " does not have type " CppAsString(typename)))
 #endif							/* HAVE__BUILTIN_TYPES_COMPATIBLE_P */