reflect: fix interface to interface conversion in Call

Call is meant to mirror the language semantics, which allow:

	var r io.ReadWriter
	f := func(io.Reader){}
	f(r)

even though the conversion from io.ReadWriter to io.Reader is
being applied to a nil interface. This is different from an explicit
conversion:

	_ = r.(io.Reader)
	f(r.(io.Reader))

Both of those lines panic, but the implicit conversion does not.

By using E2I, which is the implementation of the explicit conversion,
the reflect.Call equivalent of f(r) was inadvertently panicking.
Avoid the panic.

Fixes #22143.

Change-Id: I6b2f5b808e0cd3b89ae8bc75881e307bf1c25558
Reviewed-on: https://go-review.googlesource.com/80736
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/reflect/all_test.go

@@ -1631,6 +1631,15 @@ func TestFunc(t *testing.T) {
 	}
 }
 
+func TestCallConvert(t *testing.T) {
+	v := ValueOf(new(io.ReadWriter)).Elem()
+	f := ValueOf(func(r io.Reader) io.Reader { return r })
+	out := f.Call([]Value{v})
+	if len(out) != 1 || out[0].Type() != TypeOf(new(io.Reader)).Elem() || !out[0].IsNil() {
+		t.Errorf("expected [nil], got %v", out)
+	}
+}
+
 type emptyStruct struct{}
 
 type nonEmptyStruct struct {

src/reflect/value.go

@@ -2197,6 +2197,12 @@ func (v Value) assignTo(context string, dst *rtype, target unsafe.Pointer) Value
 		if target == nil {
 			target = unsafe_New(dst)
 		}
+		if v.Kind() == Interface && v.IsNil() {
+			// A nil ReadWriter passed to nil Reader is OK,
+			// but using ifaceE2I below will panic.
+			// Avoid the panic by returning a nil dst (e.g., Reader) explicitly.
+			return Value{dst, nil, flag(Interface)}
+		}
 		x := valueInterface(v, false)
 		if dst.NumMethod() == 0 {
 			*(*interface{})(target) = x