cmd/compile: fix import/export of ODYNAMICDOTTYPE

The RType field isn't needed when performing type assertions from
non-empty interface types, because we use the ITab field instead. But
the inline body exporter didn't know to expect this.

It's possible we could use a single bool to distinguish whether
we're serializing the RType or ITab field, but using two is simpler
and seems safer.

Fixes #54302.

Change-Id: I9ddac72784fb2241fee0a0dee30493d868a2c259
Reviewed-on: https://go-review.googlesource.com/c/go/+/421755
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/typecheck/iexport.go

@@ -1928,7 +1928,9 @@ func (w *exportWriter) expr(n ir.Node) {
 		w.op(n.Op())
 		w.pos(n.Pos())
 		w.expr(n.X)
+		if w.bool(n.RType != nil) {
 			w.expr(n.RType)
+		}
 		if w.bool(n.ITab != nil) {
 			w.expr(n.ITab)
 		}

src/cmd/compile/internal/typecheck/iimport.go

@@ -1460,7 +1460,10 @@ func (r *importReader) node() ir.Node {
 		return n
 
 	case ir.ODYNAMICDOTTYPE, ir.ODYNAMICDOTTYPE2:
-		n := ir.NewDynamicTypeAssertExpr(r.pos(), op, r.expr(), r.expr())
+		n := ir.NewDynamicTypeAssertExpr(r.pos(), op, r.expr(), nil)
+		if r.bool() {
+			n.RType = r.expr()
+		}
 		if r.bool() {
 			n.ITab = r.expr()
 		}

test/typeparam/issue54302.dir/a.go

@@ -0,0 +1,20 @@
+// Copyright 2022 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 a
+
+func A() {
+	B[int](new(G[int]))
+}
+
+func B[T any](iface interface{ M(T) }) {
+	x, ok := iface.(*G[T])
+	if !ok || iface != x {
+		panic("FAIL")
+	}
+}
+
+type G[T any] struct{}
+
+func (*G[T]) M(T) {}

test/typeparam/issue54302.dir/main.go

@@ -0,0 +1,11 @@
+// Copyright 2022 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 "./a"
+
+func main() {
+	a.A()
+}

test/typeparam/issue54302.go

@@ -0,0 +1,7 @@
+// rundir
+
+// Copyright 2022 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 p