gc: complex(0)

Fixes #1232.

R=ken2
CC=golang-dev
https://golang.org/cl/3621041

src/cmd/gc/const.c

@@ -202,8 +202,6 @@ convlit1(Node **np, Type *t, int explicit)
 				goto bad;
 			case CTFLT:
 			case CTINT:
-				if(explicit)
-					goto bad;
 				n->val = tocplx(n->val);
 				break;
 			case CTCPLX:
@@ -300,7 +298,7 @@ toflt(Val v)
 		f = mal(sizeof(*f));
 		mpmovefltflt(f, &v.u.cval->real);
 		if(mpcmpfltc(&v.u.cval->imag, 0) != 0)
-			yyerror("constant %#F truncated to real", v.u.fval);
+			yyerror("constant %#F%+#Fi truncated to real", &v.u.cval->real, &v.u.cval->imag);
 		v.ctype = CTFLT;
 		v.u.fval = f;
 		break;
@@ -324,9 +322,9 @@ toint(Val v)
 	case CTCPLX:
 		i = mal(sizeof(*i));
 		if(mpmovefltfix(i, &v.u.cval->real) < 0)
-			yyerror("constant %#F truncated to integer", v.u.fval);
+			yyerror("constant %#F%+#Fi truncated to integer", &v.u.cval->real, &v.u.cval->imag);
 		if(mpcmpfltc(&v.u.cval->imag, 0) != 0)
-			yyerror("constant %#F truncated to real", v.u.fval);
+			yyerror("constant %#F%+#Fi truncated to real", &v.u.cval->real, &v.u.cval->imag);
 		v.ctype = CTINT;
 		v.u.xval = i;
 		break;

src/cmd/gc/mparith1.c

@@ -475,6 +475,8 @@ Fconv(Fmt *fp)
 		// for well in range, convert to double and use print's %g
 		if(-900 < fvp->exp && fvp->exp < 900) {
 			d = mpgetflt(fvp);
+			if(d >= 0 && (fp->flags & FmtSign))
+				fmtprint(fp, "+");
 			return fmtprint(fp, "%g", d);
 		}
 		// TODO(rsc): for well out of range, print

test/cmplx.go

@@ -22,6 +22,7 @@ func main() {
 	c64 = cmplx(f32, f32)
 	c128 = cmplx(f64, f64)
 
+	_ = complex(0) // ok
 	_ = cmplx(f, f32)	// ERROR "cmplx"
 	_ = cmplx(f, f64)	// ERROR "cmplx"
 	_ = cmplx(f32, f)	// ERROR "cmplx"