write-only variable _

R=ken
OCL=34465
CL=34470

src/cmd/5g/ggen.c

@@ -48,7 +48,7 @@ compile(Node *fn)
 
 	hasdefer = 0;
 	walk(curfn);
-	if(nerrors != 0)
+	if(nerrors != 0 || isblank(curfn->nname))
 		goto ret;
 
 	allocparams();

src/cmd/6g/ggen.c

@@ -49,7 +49,7 @@ compile(Node *fn)
 
 	hasdefer = 0;
 	walk(curfn);
-	if(nerrors != 0)
+	if(nerrors != 0 || isblank(curfn->nname))
 		goto ret;
 
 	allocparams();

src/cmd/8g/ggen.c

@@ -47,7 +47,7 @@ compile(Node *fn)
 
 	hasdefer = 0;
 	walk(curfn);
-	if(nerrors != 0)
+	if(nerrors != 0 || isblank(curfn->nname))
 		goto ret;
 
 	allocparams();

src/cmd/gc/align.c

@@ -139,7 +139,7 @@ dowidth(Type *t)
 	w = 0;
 	switch(et) {
 	default:
-		fatal("dowidth: unknown type: %E", t->etype);
+		fatal("dowidth: unknown type: %T", t);
 		break;
 
 	/* compiler-specific stuff */

src/cmd/gc/dcl.c

@@ -164,6 +164,9 @@ declare(Node *n, int ctxt)
 	int gen;
 	static int typegen, vargen;
 
+	if(isblank(n))
+		return;
+
 	s = n->sym;
 	gen = 0;
 	if(ctxt == PEXTERN) {
@@ -301,7 +304,6 @@ variter(NodeList *vl, Node *t, NodeList *el)
 	int doexpr;
 	Node *v, *e;
 	NodeList *init;
-	Sym *s;
 
 	init = nil;
 	doexpr = el != nil;
@@ -317,8 +319,6 @@ variter(NodeList *vl, Node *t, NodeList *el)
 			e = N;
 
 		v = vl->n;
-		s = v->sym;
-
 		v->op = ONAME;
 		declare(v, dclcontext);
 		v->ntype = t;
@@ -550,6 +550,8 @@ colasdefn(NodeList *left, Node *defn)
 	nnew = 0;
 	for(l=left; l; l=l->next) {
 		n = l->n;
+		if(isblank(n))
+			continue;
 		if(!colasname(n)) {
 			yyerror("non-name %#N on left side of :=", n);
 			continue;
@@ -838,7 +840,7 @@ stotype(NodeList *l, int et, Type **t)
 			f->sym = f->nname->sym;
 			if(pkgimportname != S && !exportname(f->sym->name))
 				f->sym = pkglookup(f->sym->name, structpkg);
-			if(f->sym) {
+			if(f->sym && !isblank(f->nname)) {
 				for(t1=*t0; t1!=T; t1=t1->down) {
 					if(t1->sym == f->sym) {
 						yyerror("duplicate field %s", t1->sym->name);
@@ -963,6 +965,8 @@ checkarglist(NodeList *all)
 			t = n;
 			n = N;
 		}
+		if(isblank(n))
+			n = N;
 		if(n != N && n->sym == S) {
 			t = n;
 			n = N;

src/cmd/gc/gen.c

@@ -430,6 +430,60 @@ cgen_dcl(Node *n)
 	cgen_as(n->heapaddr, n->alloc);
 }
 
+/*
+ * generate discard of value
+ */
+void
+cgen_discard(Node *nr)
+{
+	Node tmp;
+
+	if(nr == N)
+		return;
+
+	switch(nr->op) {
+	case ONAME:
+		break;
+
+	// unary
+	case OADD:
+	case OAND:
+	case ODIV:
+	case OEQ:
+	case OGE:
+	case OGT:
+	case OLE:
+	case OLSH:
+	case OLT:
+	case OMOD:
+	case OMUL:
+	case ONE:
+	case OOR:
+	case ORSH:
+	case OSUB:
+	case OXOR:
+		cgen_discard(nr->left);
+		cgen_discard(nr->right);
+		break;
+
+	// binary
+	case OCAP:
+	case OCOM:
+	case OLEN:
+	case OMINUS:
+	case ONOT:
+	case OPLUS:
+		cgen_discard(nr->left);
+		break;
+
+	// special enough to just evaluate
+	default:
+		tempname(&tmp, nr->type);
+		cgen_as(&tmp, nr);
+		gused(&tmp);
+	}
+}
+
 /*
  * generate assignment:
  *	nl = nr
@@ -450,6 +504,11 @@ cgen_as(Node *nl, Node *nr)
 		dump("cgen_as = ", nr);
 	}
 
+	if(isblank(nl)) {
+		cgen_discard(nr);
+		return;
+	}
+
 	iszer = 0;
 	if(nr == N || isnil(nr)) {
 		// externals and heaps should already be clear

src/cmd/gc/go.h

@@ -6,6 +6,10 @@
 #include	<libc.h>
 #include	<bio.h>
 
+// avoid <ctype.h>
+#undef isblank
+#define isblank goisblank
+
 #ifndef	EXTERN
 #define	EXTERN	extern
 #endif
@@ -421,6 +425,7 @@ enum
 	// pseudo-types for literals
 	TIDEAL,
 	TNIL,
+	TBLANK,
 
 	NTYPE,
 };
@@ -467,6 +472,7 @@ enum
 	Ecall = 1<<4,	// call-only expressions are ok
 	Efnstruct = 1<<5,	// multivalue function returns are ok
 	Eiota = 1<<6,		// iota is ok
+	Easgn = 1<<7,		// assigning to expression
 };
 
 #define	BITS	5
@@ -803,6 +809,7 @@ int	isinter(Type*);
 int	isnilinter(Type*);
 int	isddd(Type*);
 int	isideal(Type*);
+int	isblank(Node*);
 Type*	maptype(Type*, Type*);
 Type*	methtype(Type*);
 Node*	typename(Type*);

src/cmd/gc/lex.c

@@ -1352,6 +1352,13 @@ lexinit(void)
 	// (the type of x in const x = "hello").
 	// TODO(rsc): this may need some more thought.
 	idealstring = typ(TSTRING);
+
+	s = lookup("_");
+	s->block = -100;
+	s->def = nod(ONAME, N, N);
+	s->def->sym = s;
+	types[TBLANK] = typ(TBLANK);
+	s->def->type = types[TBLANK];
 }
 
 struct
@@ -1431,6 +1438,9 @@ mkpackage(char* pkg)
 	int32 h;
 	char *p;
 
+	if(strcmp(pkg, "_") == 0)
+		yyerror("invalid package name _");
+
 	if(package == nopackage) {
 		// redefine all names to be this package.
 		for(h=0; h<NHASH; h++)

src/cmd/gc/range.c

@@ -19,7 +19,7 @@ typecheckrange(Node *n)
 	// delicate little dance.  see typecheckas2
 	for(ll=n->list; ll; ll=ll->next)
 		if(ll->n->defn != n)
-			typecheck(&ll->n, Erv);
+			typecheck(&ll->n, Erv | Easgn);
 
 	typecheck(&n->right, Erv);
 	if((t = n->right->type) == T)
@@ -121,7 +121,7 @@ walkrange(Node *n)
 
 	case TARRAY:
 		hv1 = nod(OXXX, N, n);
-		tempname(hv1, v1->type);
+		tempname(hv1, types[TINT]);
 
 		init = list(init, nod(OAS, hv1, N));
 		n->ntest = nod(OLT, hv1, nod(OLEN, ha, N));
@@ -169,7 +169,7 @@ walkrange(Node *n)
 
 	case TCHAN:
 		hv1 = nod(OXXX, N, n);
-		tempname(hv1, v1->type);
+		tempname(hv1, t->type);
 
 		n->ntest = nod(ONOT, nod(OCLOSED, ha, N), N);
 		n->ntest->ninit = list1(nod(OAS, hv1, nod(ORECV, ha, N)));

src/cmd/gc/subr.c

@@ -953,6 +953,7 @@ basicnames[] =
 	[TSTRING]		= "string",
 	[TNIL]		= "nil",
 	[TIDEAL]		= "ideal",
+	[TBLANK]		= "blank",
 };
 
 int
@@ -1421,6 +1422,19 @@ isslice(Type *t)
 	return t != T && t->etype == TARRAY && t->bound < 0;
 }
 
+int
+isblank(Node *n)
+{
+	char *p;
+
+	if(n == N || n->sym == S)
+		return 0;
+	p = n->sym->name;
+	if(p == nil)
+		return 0;
+	return p[0] == '_' && p[1] == '\0';
+}
+
 int
 isselect(Node *n)
 {

src/cmd/gc/typecheck.c

@@ -61,7 +61,10 @@ typecheck(Node **np, int top)
 	n = *np;
 	if(n == N)
 		return N;
-	if(n->typecheck == 1 && n->op != ONAME)	// XXX for test/func4.go
+	
+	// Skip typecheck if already done.
+	// But re-typecheck ONAME node in case context has changed.
+	if(n->typecheck == 1 && n->op != ONAME)
 		return n;
 	if(n->typecheck == 2)
 		fatal("typecheck loop");
@@ -85,8 +88,10 @@ reswitch:
 	 */
 	case OLITERAL:
 		ok |= Erv;
-		if(n->iota && !(top & Eiota))
+		if(n->iota && !(top & Eiota)) {
 			yyerror("use of iota not in constant initializer");
+			goto error;
+		}
 		if(n->val.ctype == CTSTR)
 			n->type = idealstring;
 		goto ret;
@@ -100,6 +105,10 @@ reswitch:
 			ok |= Ecall;
 			goto ret;
 		}
+		if(isblank(n) && !(top & Easgn)) {
+			yyerror("cannot use _ as value");
+			goto error;
+		}
 		ok |= Erv;
 		goto ret;
 
@@ -581,7 +590,7 @@ reswitch:
 		}
 		yyerror("cannot slice %#N (type %T)", l, t);
 		goto error;
-
+	
 	/*
 	 * call and call like
 	 */
@@ -970,6 +979,7 @@ ret:
 		case TFORW:
 		case TIDEAL:
 		case TNIL:
+		case TBLANK:
 			break;
 		default:
 			checkwidth(t);
@@ -1165,6 +1175,11 @@ checkconv(Type *nt, Type *t, int explicit, int *op, int *et)
 	 */
 	if(nt == T)
 		return 0;
+	
+	if(t->etype == TBLANK) {
+		*op = OCONVNOP;
+		return 0;
+	}
 
 	if(eqtype(t, nt)) {
 		exportassignok(t);
@@ -1804,7 +1819,7 @@ typecheckas(Node *n)
 	// will not look at defn, so it is okay (and desirable,
 	// so that the conversion below happens).
 	if(n->left->defn != n || n->left->ntype)
-		typecheck(&n->left, Erv);
+		typecheck(&n->left, Erv | Easgn);
 
 	checkassign(n->left);
 	typecheck(&n->right, Erv);
@@ -1820,7 +1835,7 @@ typecheckas(Node *n)
 	// just to get it over with.  see dance above.
 	n->typecheck = 1;
 	if(n->left->typecheck == 0)
-		typecheck(&n->left, Erv);
+		typecheck(&n->left, Erv | Easgn);
 }
 
 static void
@@ -1835,7 +1850,7 @@ typecheckas2(Node *n)
 	for(ll=n->list; ll; ll=ll->next) {
 		// delicate little dance.
 		if(ll->n->defn != n || ll->n->ntype)
-			typecheck(&ll->n, Erv);
+			typecheck(&ll->n, Erv | Easgn);
 	}
 	cl = count(n->list);
 	cr = count(n->rlist);
@@ -1946,7 +1961,7 @@ typecheckfunc(Node *n)
 {
 	Type *t, *rcvr;
 
-	typecheck(&n->nname, Erv);
+	typecheck(&n->nname, Erv | Easgn);
 	if((t = n->nname->type) == T)
 		return;
 	n->type = t;

src/cmd/gc/walk.c

@@ -1103,7 +1103,7 @@ ascompatee(int op, NodeList *nl, NodeList *nr, NodeList **init)
 }
 
 /*
- * n is an lv and t is the type of an rv
+ * l is an lv and rt is the type of an rv
  * return 1 if this implies a function call
  * evaluating the lv or a function call
  * in the conversion of the types
@@ -1141,6 +1141,10 @@ ascompatet(int op, NodeList *nl, Type **nr, int fp, NodeList **init)
 		if(r == T)
 			break;
 		l = ll->n;
+		if(isblank(l)) {
+			r = structnext(&saver);
+			continue;
+		}
 
 		// any lv that causes a fn call must be
 		// deferred until all the return arguments

test/blank.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+var call string
+
+type T struct {
+	_, _, _ int;
+}
+
+const (
+	c0 = iota;
+	_;
+	_;
+	_;
+	c4;
+)
+
+var ints = []string {
+	"1",
+	"2",
+	"3"
+}
+
+func f() (int, int) {
+	call += "f";
+	return 1,2
+}
+
+func g() (float, float) {
+	call += "g";
+	return 3,4
+}
+
+func h(_ int, _ float) {
+}
+
+func i() int {
+	call += "i";
+	return 23;
+}
+
+func main()
+{
+	_, _ = f();
+	a, _ := f();
+	if a != 1 {panic(a)}
+	b, _ := g();
+	if b != 3 {panic(b)}
+	_, a = f();
+	if a != 2 {panic(a)}
+	_, b = g();
+	if b != 4 {panic(b)}
+	_ = i();
+	if call != "ffgfgi" {panic(call)}
+	if c4 != 4 {panic(c4)}
+
+	out := "";
+	for _, s := range ints {
+		out += s;
+	}
+	if out != "123" {panic(out)}
+
+	sum := 0;
+	for s, _ := range ints {
+		sum += s;
+	}
+	if sum != 3 {panic(sum)}
+
+	h(a,b);
+}
+
+// useless but legal
+var _ int = 1;
+var _ = 2;
+var _, _ = 3, 4;
+const _ = 3;
+const _, _ = 4, 5;
+type _ int;
+func _() {
+	panic("oops")
+}
+
+func ff() {
+	var _ int = 1;
+}

test/blank1.go

+// errchk $G -e $D/$F.go
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package _	// ERROR "invalid package name _"
+
+func main() {
+	_();	// ERROR "cannot use _ as value"
+	x := _+1;	// ERROR "cannot use _ as value"
+}

test/fixedbugs/bug154.go

@@ -16,7 +16,7 @@ func f0() string {
 
 func f1() string {
 	const f = 3.141592;
-	_ := float64(float32(f));  // appears to change the precision of f
+	x := float64(float32(f));  // appears to change the precision of f
 	return fmt.Sprintf("%v", float64(f));
 }