cmd/compile: in append(f()), type convert appended items

The second and subsequent return values from f() need to be
converted to the element type of the first return value from f()
(which must be a slice).

Fixes #22327

Change-Id: I5c0a424812c82c1b95b6d124c5626cfc4408bdb6
Reviewed-on: https://go-review.googlesource.com/c/142718Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/walk.go

@@ -2884,6 +2884,8 @@ func walkappend(n *Node, init *Nodes, dst *Node) *Node {
 	}
 	walkexprlistsafe(n.List.Slice()[1:], init)
 
+	nsrc := n.List.First()
+
 	// walkexprlistsafe will leave OINDEX (s[n]) alone if both s
 	// and n are name or literal, but those may index the slice we're
 	// modifying here. Fix explicitly.
@@ -2892,11 +2894,14 @@ func walkappend(n *Node, init *Nodes, dst *Node) *Node {
 	// before we begin to modify the slice in a visible way.
 	ls := n.List.Slice()[1:]
 	for i, n := range ls {
-		ls[i] = cheapexpr(n, init)
+		n = cheapexpr(n, init)
+		if !eqtype(n.Type, nsrc.Type.Elem()) {
+			n = assignconv(n, nsrc.Type.Elem(), "append")
+			n = walkexpr(n, init)
+		}
+		ls[i] = n
 	}
 
-	nsrc := n.List.First()
-
 	argc := n.List.Len() - 1
 	if argc < 1 {
 		return nsrc

test/fixedbugs/issue22327.go

+// compile
+
+// Copyright 2018 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.
+
+// Using a multi-result function as an argument to
+// append should compile successfully. Previously there
+// was a missing *int -> interface{} conversion that caused
+// the compiler to ICE.
+
+package p
+
+func f() ([]interface{}, *int) {
+	return nil, nil
+}
+
+var _ = append(f())