cmd/compile: nilcheck interface value in go/defer interface call for SSA

This matches the behavior of the legacy backend.

Fixes #15975 (if this is the intended behavior)

Change-Id: Id277959069b8b8bf9958fa8f2cbc762c752a1a19
Reviewed-on: https://go-review.googlesource.com/23820Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -2573,6 +2573,9 @@ func (s *state) call(n *Node, k callKind) *ssa.Value {
 		}
 		i := s.expr(fn.Left)
 		itab := s.newValue1(ssa.OpITab, Types[TUINTPTR], i)
+		if k != callNormal {
+			s.nilCheck(itab)
+		}
 		itabidx := fn.Xoffset + 3*int64(Widthptr) + 8 // offset of fun field in runtime.itab
 		itab = s.newValue1I(ssa.OpOffPtr, Types[TUINTPTR], itabidx, itab)
 		if k == callNormal {

test/fixedbugs/issue15975.go

+// run
+
+// Copyright 2016 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
+
+var fail bool
+
+type Closer interface {
+	Close()
+}
+
+func nilInterfaceDeferCall() {
+	var x Closer
+	defer x.Close()
+	// if it panics when evaluating x.Close, it should not reach here
+	fail = true
+}
+
+func shouldPanic(f func()) {
+	defer func() {
+		if recover() == nil {
+			panic("did not panic")
+		}
+	}()
+	f()
+}
+
+func main() {
+	shouldPanic(nilInterfaceDeferCall)
+	if fail {
+		panic("fail")
+	}
+}