cmd/gc: allow append and complex builtins to accept 2-result call expression as first argument.

Fixes #5793.

LGTM=rsc
R=rsc, adonovan, dave
CC=golang-codereviews
https://golang.org/cl/13367051

src/cmd/gc/inl.c

@@ -392,6 +392,8 @@ inlnode(Node **np)
 	case OCALLFUNC:
 	case OCALLMETH:
 	case OCALLINTER:
+	case OAPPEND:
+	case OCOMPLEX:
 		// if we just replaced arg in f(arg()) or return arg with an inlined call
 		// and arg returns multiple values, glue as list
 		if(count(n->list) == 1 && n->list->n->op == OINLCALL && count(n->list->n->rlist) > 1) {

src/cmd/gc/order.c

@@ -344,6 +344,8 @@ orderexpr(Node **np, NodeList **out)
 	case OCALLFUNC:
 	case OCALLMETH:
 	case OCALLINTER:
+	case OAPPEND:
+	case OCOMPLEX:
 		ordercall(n, out);
 		n = copyexpr(n, n->type, out);
 		break;

src/cmd/gc/typecheck.c

@@ -1212,17 +1212,29 @@ reswitch:
 
 	case OCOMPLEX:
 		ok |= Erv;
-		if(twoarg(n) < 0)
-			goto error;
-		l = typecheck(&n->left, Erv | (top & Eiota));
-		r = typecheck(&n->right, Erv | (top & Eiota));
-		if(l->type == T || r->type == T)
-			goto error;
-		defaultlit2(&l, &r, 0);
-		if(l->type == T || r->type == T)
-			goto error;
-		n->left = l;
-		n->right = r;
+		if(count(n->list) == 1) {
+			typechecklist(n->list, Efnstruct);
+			t = n->list->n->left->type;
+			if(t->outtuple != 2) {
+				yyerror("invalid operation: complex expects two arguments, %N returns %d results", n->list->n, t->outtuple);
+				goto error;
+			}
+			t = n->list->n->type->type;
+			l = t->nname;
+			r = t->down->nname;
+		} else {
+			if(twoarg(n) < 0)
+				goto error;
+			l = typecheck(&n->left, Erv | (top & Eiota));
+			r = typecheck(&n->right, Erv | (top & Eiota));
+			if(l->type == T || r->type == T)
+				goto error;
+			defaultlit2(&l, &r, 0);
+			if(l->type == T || r->type == T)
+				goto error;
+			n->left = l;
+			n->right = r;
+		}
 		if(!eqtype(l->type, r->type)) {
 			yyerror("invalid operation: %N (mismatched types %T and %T)", n, l->type, r->type);
 			goto error;
@@ -1301,9 +1313,22 @@ reswitch:
 			yyerror("missing arguments to append");
 			goto error;
 		}
-		typechecklist(args, Erv);
+
+		if(count(args) == 1 && !n->isddd)
+			typecheck(&args->n, Erv | Efnstruct);
+		else
+			typechecklist(args, Erv);
+
 		if((t = args->n->type) == T)
 			goto error;
+
+		// Unpack multiple-return result before type-checking.
+		if(istype(t, TSTRUCT)) {
+			t = t->type;
+			if(istype(t, TFIELD))
+				t = t->type;
+		}
+
 		n->type = t;
 		if(!isslice(t)) {
 			if(isconst(args->n, CTNIL)) {

src/cmd/gc/walk.c

@@ -493,6 +493,11 @@ walkexpr(Node **np, NodeList **init)
 	case OADD:
 	case OCOMPLEX:
 	case OLROT:
+		// Use results from call expression as arguments for complex.
+		if(n->op == OCOMPLEX && n->left == N && n->right == N) {
+			n->left = n->list->n;
+			n->right = n->list->next->n;
+		}
 		walkexpr(&n->left, init);
 		walkexpr(&n->right, init);
 		goto ret;
@@ -2772,6 +2777,10 @@ append(Node *n, NodeList **init)
 		l->n = cheapexpr(l->n, init);
 
 	nsrc = n->list->n;
+
+	// Resolve slice type of multi-valued return.
+	if(istype(nsrc->type, TSTRUCT))
+		nsrc->type = nsrc->type->type->type;
 	argc = count(n->list) - 1;
 	if (argc < 1) {
 		return nsrc;

test/fixedbugs/issue5793.go

@@ -0,0 +1,36 @@
+// run
+
+// Copyright 2013 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.
+
+// Issue 5793: calling 2-arg builtin with multiple-result f() call expression gives
+// spurious error.
+
+package main
+
+func complexArgs() (float64, float64) {
+	return 5, 7
+}
+
+func appendArgs() ([]string, string) {
+	return []string{"foo"}, "bar"
+}
+
+func appendMultiArgs() ([]byte, byte, byte) {
+	return []byte{'a', 'b'}, '1', '2'
+}
+
+func main() {
+	if c := complex(complexArgs()); c != 5+7i {
+		panic(c)
+	}
+
+	if s := append(appendArgs()); len(s) != 2 || s[0] != "foo" || s[1] != "bar" {
+		panic(s)
+	}
+
+	if b := append(appendMultiArgs()); len(b) != 4 || b[0] != 'a' || b[1] != 'b' || b[2] != '1' || b[3] != '2' {
+		panic(b)
+	}
+}