more interface checks:

	- pointer to interface cannot have methods
	- record type names for better runtime error

R=r,ken
DELTA=85  (80 added, 0 deleted, 5 changed)
OCL=16658
CL=16722

src/cmd/6g/obj.c

@@ -466,6 +466,7 @@ dumpsignatures(void)
 	Addr at, ao, ac, ad;
 	Prog *p;
 	char *sp;
+	char buf[NSYMB];
 
 	// copy externdcl list to signatlist
 	for(d=externdcl; d!=D; d=d->forw) {
@@ -608,7 +609,9 @@ dumpsignatures(void)
 		p->to.offset = stringo;
 		ot += widthptr;
 
-		datastring("", 1);
+		// save type name for runtime error message
+		snprint(buf, sizeof buf, "%T", t);
+		datastring(buf, strlen(buf)+1);
 
 		if(et == TINTER) {
 			// first field of an interface signature
@@ -733,6 +736,7 @@ dumpsignatures(void)
 				ot += widthptr;
 			}
 			datastring(b->name, strlen(b->name)+1);
+
 		}
 
 		// nil field name at end

src/cmd/gc/subr.c

@@ -1406,6 +1406,10 @@ ismethod(Type *t)
 	if(t == T)
 		return T;
 
+	// no interfaces
+	if(t->etype == TINTER || (t->etype == tptr && t->type->etype == TINTER))
+		return T;
+
 	a = algtype(t);
 
 	// direct receiver

src/runtime/iface.c

@@ -40,7 +40,7 @@ static	Map*	hash[1009];
 static void
 printsigi(Sigi *si)
 {
-	int32 i, n;
+	int32 i;
 	byte *name;
 
 	sys·printpointer(si);
@@ -125,7 +125,7 @@ hashmap(Sigi *si, Sigt *st)
 	m->sigi = si;
 	m->sigt = st;
 
-	nt = 0;
+	nt = 1;
 	for(ni=1; (iname=si[ni].name) != nil; ni++) {	// ni=1: skip first word
 		// pick up next name from
 		// interface signature
@@ -136,9 +136,14 @@ hashmap(Sigi *si, Sigt *st)
 			// from structure signature
 			sname = st[nt].name;
 			if(sname == nil) {
+				prints("cannot convert type ");
+				prints((int8*)st[0].name);
+				prints(" to interface ");
+				prints((int8*)si[0].name);
+				prints(": missing method ");
 				prints((int8*)iname);
-				prints(": ");
-				throw("hashmap: failed to find method");
+				prints("\n");
+				throw("interface conversion");
 				m->bad = 1;
 				m->link = hash[h];
 				hash[h] = m;

test/golden.out

@@ -2,6 +2,20 @@
 =========== ./helloworld.go
 hello, world
 
+=========== ./interface1.go
+./interface1.go:5: syntax error near package
+./interface1.go:31: illegal types for operand: AS
+	interface { Next () (*Inst) }
+	*Inst
+
+=========== ./interface2.go
+cannot convert type S to interface I: missing method Foo
+throw: interface conversion
+SIGSEGV: segmentation violation
+Faulting address: 0x0
+pc: xxx
+
+
 =========== ./peano.go
 0! = 1
 1! = 1
@@ -64,6 +78,7 @@ BUG: compilation should succeed
 
 =========== bugs/bug074.go
 bugs/bug074.go:6: syntax error near string
+bugs/bug074.go:6: syntax error near string
 bugs/bug074.go:7: x: undefined
 BUG: compiler crashes - Bus error
 

test/interface1.go

+// errchk $G $D/$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 main
+
+package main
+
+type Inst interface {
+	Next()	*Inst;
+}
+
+type Regexp struct {
+	code *[]Inst;
+	start	Inst;
+}
+
+type Start struct {
+	foo	*Inst;
+}
+
+func (start *Start) Next() *Inst { return nil }
+
+
+func AddInst(Inst) *Inst {
+	print("ok in addinst\n");
+	return nil
+}
+
+func main() {
+	re := new(Regexp);
+	print("call addinst\n");
+	var x Inst = AddInst(new(Start));
+	print("return from  addinst\n");
+}

test/interface2.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
+
+type S struct
+
+type I interface {
+	Foo()
+}
+
+func main() {
+	var s *S;
+	var i I;
+	i = s;
+}
+
+// hide S down here to avoid static warning
+type S struct {
+}