go.tools/go/ssa: perform nil check when taking value of interface method.

+ test.

Fixes golang/go#7269

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/84650046

cmd/ssadump/main.go

@@ -130,8 +130,7 @@ func doMain() error {
 		case 'R':
 			interpMode |= interp.DisableRecover
 		default:
-			fmt.Fprintf(os.Stderr, "ssadump: unknown -interp option: '%c'.", c)
-			os.Exit(1)
+			return fmt.Errorf("unknown -interp option: '%c'", c)
 		}
 	}
 

go/ssa/builder.go

@@ -631,9 +631,16 @@ func (b *builder) expr0(fn *Function, e ast.Expr) Value {
 			// e.f where e is an expression and f is a method.
 			// The result is a bound method closure.
 			obj := sel.Obj().(*types.Func)
-			wantAddr := isPointer(recvType(obj))
+			rt := recvType(obj)
+			wantAddr := isPointer(rt)
 			escaping := true
 			v := b.receiver(fn, e.X, wantAddr, escaping, sel)
+			if _, ok := rt.Underlying().(*types.Interface); ok {
+				// If v has interface type I,
+				// we must emit a check that v is non-nil.
+				// We use: typeassert v.(I).
+				emitTypeAssert(fn, v, rt, token.NoPos)
+			}
 			c := &MakeClosure{
 				Fn:       boundMethodWrapper(fn.Prog, obj),
 				Bindings: []Value{v},

go/ssa/interp/testdata/boundmeth.go

@@ -2,6 +2,8 @@
 
 package main
 
+import "fmt"
+
 func assert(b bool) {
 	if !b {
 		panic("oops")
@@ -100,6 +102,32 @@ func regress1(x error) func() string {
 	return x.Error
 }
 
+// Regression test for b/7269:
+// taking the value of an interface method performs a nil check.
+func nilInterfaceMethodValue() {
+	err := fmt.Errorf("ok")
+	f := err.Error
+	if got := f(); got != "ok" {
+		panic(got)
+	}
+
+	err = nil
+	if got := f(); got != "ok" {
+		panic(got)
+	}
+
+	defer func() {
+		r := fmt.Sprint(recover())
+		// runtime panic string varies across toolchains
+		if r != "runtime error: interface conversion: interface is nil, not error" &&
+			r != "runtime error: invalid memory address or nil pointer dereference" {
+			panic("want runtime panic from nil interface method value, got " + r)
+		}
+	}()
+	f = err.Error // runtime panic: err is nil
+	panic("unreachable")
+}
+
 func main() {
 	valueReceiver()
 	pointerReceiver()
@@ -111,4 +139,6 @@ func main() {
 	if e := regress1(errString("hi"))(); e != "hi" {
 		panic(e)
 	}
+
+	nilInterfaceMethodValue()
 }