cmd/compile: retain source positions of arguments to inlined calls

Arguments to inlined calls are hidden from setPos as follows:

    args := as.Rlist
    as.Rlist.Set(nil)
    // setPos...
    as.Rlist.Set(args.Slice())

Previously, this code had no effect since the value of as was
overwritten by the assignment in the retvars loop.

Fixes #19799.

Change-Id: Iaf97259f82fdba8b236136337cc42b2774c7fef5
Reviewed-on: https://go-review.googlesource.com/44351
Run-TryBot: David Lazar <lazard@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -726,9 +726,9 @@ func mkinlcall1(n *Node, fn *Node, isddd bool) *Node {
 
 	// Zero the return parameters.
 	for _, n := range retvars {
-		as = nod(OAS, n, nil)
-		as = typecheck(as, Etop)
-		ninit.Append(as)
+		ras := nod(OAS, n, nil)
+		ras = typecheck(ras, Etop)
+		ninit.Append(ras)
 	}
 
 	retlabel := autolabel(".i")

test/fixedbugs/issue19799.go

@@ -0,0 +1,71 @@
+// run
+
+// Copyright 2017 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
+
+import (
+	"os"
+	"runtime"
+)
+
+func foo(x int) int {
+	return x + 1
+}
+
+func test() {
+	defer func() {
+		if r := recover(); r != nil {
+			pcs := make([]uintptr, 10)
+			n := runtime.Callers(0, pcs)
+			pcs = pcs[:n]
+			frames := runtime.CallersFrames(pcs)
+			for {
+				f, more := frames.Next()
+				if f.Function == "main.foo" {
+					println("did not expect to see call to foo in stack trace")
+					os.Exit(1)
+				}
+				if !more {
+					break
+				}
+			}
+		}
+	}()
+	var v []int
+	foo(v[0])
+}
+
+func bar(x ...int) int {
+	return x[0] + 1
+}
+
+func testVariadic() {
+	defer func() {
+		if r := recover(); r != nil {
+			pcs := make([]uintptr, 10)
+			n := runtime.Callers(0, pcs)
+			pcs = pcs[:n]
+			frames := runtime.CallersFrames(pcs)
+			for {
+				f, more := frames.Next()
+				if f.Function == "main.bar" {
+					println("did not expect to see call to bar in stack trace")
+					os.Exit(1)
+				}
+				if !more {
+					break
+				}
+			}
+		}
+	}()
+	var v []int
+	bar(v[0])
+}
+
+func main() {
+	test()
+	testVariadic()
+}