go/types, types2: implement field access for struct structural constraints

This change implements field the access p.f where the type of p
is a type parameter with a structural constraint that is a struct
with a field f. This is only the fix for the type checker. The
compiler will need a separate CL.

This makes the behavior consistent with the fact that we can
write struct composite literals for type parameters with a
struct structural type.

For #50417.
For #50233.

Change-Id: I87d07e016f97cbf19c45cde19165eae3ec0bad2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/375795
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/test/typeparam/issue50417.go b/test/typeparam/issue50417.go
new file mode 100644
index 0000000000..bf6ac424c5
--- /dev/null
+++ b/test/typeparam/issue50417.go
@@ -0,0 +1,70 @@
+// run
+
+// 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
+
+func main() {}
+
+type Sf struct {
+	f int
+}
+
+func f0[P Sf](p P) {
+	_ = p.f
+	p.f = 0
+}
+
+func f0t[P ~struct{ f int }](p P) {
+	_ = p.f
+	p.f = 0
+}
+
+// TODO(danscales) enable once the compiler is fixed
+// var _ = f0[Sf]
+// var _ = f0t[Sf]
+
+func f1[P interface {
+	Sf
+	m()
+}](p P) {
+	_ = p.f
+	p.f = 0
+	p.m()
+}
+
+type Sm struct{}
+
+func (Sm) m() {}
+
+type Sfm struct {
+	f int
+}
+
+func (Sfm) m() {}
+
+func f2[P interface {
+	Sfm
+	m()
+}](p P) {
+	_ = p.f
+	p.f = 0
+	p.m()
+}
+
+// TODO(danscales) enable once the compiler is fixed
+// var _ = f2[Sfm]
+
+// special case: structural type is a named pointer type
+
+type PSfm *Sfm
+
+func f3[P interface{ PSfm }](p P) {
+	_ = p.f
+	p.f = 0
+}
+
+// TODO(danscales) enable once the compiler is fixed
+// var _ = f3[PSfm]