[dev.typeparams] cmd/compile/internal/types2: accept embedded interface elements

Accept embedded interface elements of the form ~T or A|B and
treat them like type lists: for now the elements of a union
cannot be interfaces. Also, translate existing style "type"-
lists in interfaces into interface elements: "type a, b, c"
becomes a union element "~a|~b|~c" which in turn is handled
internally like a type list.

For now, "~" is still ignored and type lists are mapped to
Sum types as before, thus ensuring that all existing tests
work as before (with some minor adjustments).

Introduced a new Union type to represent union elements.
For now they don't make it past interface completion where
they are represented as a Sum type. Thus, except for printing
(and the respective tests) and substitution for interfaces,
the various type switches ignore Union types. In a next step,
we'll replace Sum types with union types and then consider
the ~ functionality as well.

Because union elements are no different from embedded interfaces
we don't need a separate Interface.types field anymore. Removed.

For #45346.

Change-Id: I98ac3286aea9d706e98aee80241d4712ed99af08
Reviewed-on: https://go-review.googlesource.com/c/go/+/321689
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/types2/testdata/examples/constraints.go2 b/src/cmd/compile/internal/types2/testdata/examples/constraints.go2
new file mode 100644
index 0000000000..e8b3912884
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/examples/constraints.go2
@@ -0,0 +1,25 @@
+// 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.
+
+// This file shows some examples of generic constraint interfaces.
+
+package p
+
+type (
+	// Arbitrary types may be embedded like interfaces.
+	_ interface{int}
+	_ interface{~int}
+
+	// Types may be combined into a union.
+	_ interface{int|~string}
+
+	// Union terms must be unique independent of whether they are ~ or not.
+	_ interface{int|int /* ERROR duplicate term int */ }
+	_ interface{int|~ /* ERROR duplicate term int */ int }
+	_ interface{~int|~ /* ERROR duplicate term int */ int }
+
+	// For now we do not permit interfaces with ~ or in unions.
+	_ interface{~ /* ERROR cannot use interface */ interface{}}
+	_ interface{int|interface /* ERROR cannot use interface */ {}}
+)