[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, 10 insertions, 11 deletions

diff --git a/src/cmd/compile/internal/types2/typestring.go b/src/cmd/compile/internal/types2/typestring.go
index c534b04130..55858b7b42 100644
--- a/src/cmd/compile/internal/types2/typestring.go
+++ b/src/cmd/compile/internal/types2/typestring.go
@@ -158,11 +158,17 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		writeSignature(buf, t, qf, visited)
 
 	case *Sum:
-		for i, t := range t.types {
+		writeTypeList(buf, t.types, qf, visited)
+
+	case *Union:
+		for i, e := range t.terms {
 			if i > 0 {
-				buf.WriteString(", ")
+				buf.WriteString("|")
 			}
-			writeType(buf, t, qf, visited)
+			if t.tilde[i] {
+				buf.WriteByte('~')
+			}
+			writeType(buf, e, qf, visited)
 		}
 
 	case *Interface:
@@ -207,14 +213,6 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				writeSignature(buf, m.typ.(*Signature), qf, visited)
 				empty = false
 			}
-			if !empty && t.types != nil {
-				buf.WriteString("; ")
-			}
-			if t.types != nil {
-				buf.WriteString("type ")
-				writeType(buf, t.types, qf, visited)
-				empty = false
-			}
 			if !empty && len(t.embeddeds) > 0 {
 				buf.WriteString("; ")
 			}
@@ -307,6 +305,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 
 	default:
 		// For externally defined implementations of Type.
+		// Note: In this case cycles won't be caught.
 		buf.WriteString(t.String())
 	}
 }