cmd/compile: fix conversions from TypeParam to interface

If the TypeParam has all the methods of an interface, allow conversions from TypeParam to interface

Fixes #47708

Change-Id: I40a82a31f6ea9354130dbe3bcfc83537094bf12c
Reviewed-on: https://go-review.googlesource.com/c/go/+/342509
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Trust: Keith Randall <khr@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

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

@@ -740,9 +740,16 @@ func implements(t, iface *types.Type, m, samename **types.Field, ptr *int) bool
 
 	if t.IsInterface() || t.IsTypeParam() {
 		if t.IsTypeParam() {
-			// A typeparam satisfies an interface if its type bound
-			// has all the methods of that interface.
-			t = t.Bound()
+			// If t is a simple type parameter T, its type and underlying is the same.
+			// If t is a type definition:'type P[T any] T', its type is P[T] and its
+			// underlying is T. Therefore we use 't.Underlying() != t' to distinguish them.
+			if t.Underlying() != t {
+				CalcMethods(t)
+			} else {
+				// A typeparam satisfies an interface if its type bound
+				// has all the methods of that interface.
+				t = t.Bound()
+			}
 		}
 		i := 0
 		tms := t.AllMethods().Slice()

test/typeparam/issue47708.go

@@ -0,0 +1,40 @@
+// 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 (
+	"fmt"
+)
+
+type FooType[T any] interface {
+	Foo(BarType[T])string
+}
+type BarType[T any] interface {
+	Bar(FooType[T])string
+}
+
+type Baz[T any] T
+func (l Baz[T]) Foo(v BarType[T]) string {
+	return v.Bar(l)
+}
+type Bob[T any] T
+func (l Bob[T]) Bar(v FooType[T]) string {
+	if v,ok := v.(Baz[T]);ok{
+		return fmt.Sprintf("%v%v",v,l)
+	}
+	return ""
+}
+
+
+func main() {
+	var baz Baz[int] = 123
+	var bob Bob[int] = 456
+
+	if got, want := baz.Foo(bob), "123456"; got != want {
+		panic(fmt.Sprintf("got %s want %s", got, want))
+	}
+}