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Detect unnamed structs

When a struct is declared without tag it is a unnamed struct. If
it is not declared a variable at the same time the struct is defined
is not posible use this struct, because it is not posible declare
a variable of this type.

--- a/decl.c

+++ b/decl.c

@@ -85,8 +85,11 @@

 static void

 new_struct(register struct ctype *tp)

 {

+	struct symbol *sym = NULL;

+

 	if (yytoken == IDEN) {

-		(namespace(NS_STRUCT, -1)->ctype = tp)->refcnt++;

+		sym = namespace(NS_STRUCT, -1);

+		(sym->ctype = tp)->refcnt++;

 		next();

 	}

 	if (nr_structs == NR_MAXSTRUCTS)

@@ -93,7 +96,9 @@

 		error("too much structs/unions/enum defined");

 	if (structp == &structbuf[NR_STRUCT_LEVEL])

 		error("too much nested structs/unions");

-	tp->c_struct = *structp++ = nr_structs;

+	tp->ns = *structp++ = nr_structs;

+	tp->forward = 1;

+	tp->sym = sym;

 }

 static struct ctype * spec(void);

@@ -114,7 +119,7 @@

 		error("storage specifier in a struct/union field declaration");

 	}

-	do {                      /* TODO: detect unnamed structs */

+	do {

 		declarator(base, ns);

 		tp = decl_type(base);

 		(cursym->ctype = tp)->refcnt++;

@@ -135,7 +140,7 @@

 		error("struct/union already defined");

 	do

-		struct_dcl(tp->c_struct);

+		struct_dcl(tp->ns);

 	while (!accept('}'));

 	tp->forward = 0;

@@ -336,6 +341,10 @@

 			    tp->c_register || tp->c_const || tp->c_volatile) {

 				warn(options.useless,

 				     "useless storage class specifier in empty declaration");

+			}

+			if (!tp->sym && type != ENUM) {

+				warn(options.useless,

+				     "unnamed struct/union that defines no instances");

 			}

 		} else {

 			warn(options.useless,

--- a/symbol.h

+++ b/symbol.h

@@ -35,7 +35,10 @@

 	bool forward : 1;

 	union {

 		unsigned len;

-		unsigned char c_struct;

+		struct {

+			struct symbol *sym;

+			unsigned char ns;

+		};

 	};

 	struct ctype *base;

 	unsigned char refcnt;