cmd/compile: fix CONVIFACE case converting interface to empty interface

We need an extra case in convertToDictionary. In the case of an operand
which is an interface and converting to an empty interface, we don't
want to get the run-time type from the dictionary (which would be the
run-time type of the interface). We want to do a type-assert to the
empty interface.

Change-Id: I414247210168153151272fab198bfe82ad7b1567
Reviewed-on: https://go-review.googlesource.com/c/go/+/342009
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>

src/cmd/compile/internal/noder/stencil.go

@@ -1224,6 +1224,11 @@ func convertUsingDictionary(info *instInfo, dictParam *ir.Name, pos src.XPos, v
 		}
 		assert(ix >= 0)
 		rt = getDictionaryEntry(pos, dictParam, ix, info.dictLen)
+	} else if v.Type().IsInterface() {
+		ta := ir.NewTypeAssertExpr(pos, v, nil)
+		ta.SetType(dst)
+		ta.SetTypecheck(1)
+		return ta
 	} else {
 		ix := findDictType(info, src)
 		assert(ix >= 0)

test/typeparam/issue47272.go

@@ -0,0 +1,55 @@
+// run -gcflags=-G=3
+
+// Copyright 2021 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 (
+	"errors"
+	"fmt"
+)
+
+type Option[T any] struct {
+	ok  bool
+	val T
+}
+
+func (o Option[T]) String() string {
+	if o.ok {
+		return fmt.Sprintf("Some(%v)", o.val)
+	}
+	return "None"
+}
+
+func Some[T any](val T) Option[T] { return Option[T]{ok: true, val: val} }
+func None[T any]() Option[T]      { return Option[T]{ok: false} }
+
+type Result[T, E any] struct {
+	ok  bool
+	val T
+	err E
+}
+
+func (r Result[T, E]) String() string {
+	if r.ok {
+		return fmt.Sprintf("Ok(%v)", r.val)
+	}
+	return fmt.Sprintf("Err(%v)", r.err)
+}
+
+func Ok[T, E any](val T) Result[T, E]  { return Result[T, E]{ok: true, val: val} }
+func Err[T, E any](err E) Result[T, E] { return Result[T, E]{ok: false, err: err} }
+
+func main() {
+	a := Some[int](1)
+	b := None[int]()
+	fmt.Println(a, b)
+
+	x := Ok[int, error](1)
+	y := Err[int, error](errors.New("test"))
+	fmt.Println(x, y)
+	// fmt.Println(x)
+	_, _, _, _ = a, b, x, y
+}

test/typeparam/issue47272.out

@@ -0,0 +1,2 @@
+Some(1) None
+Ok(1) Err(test)