[dev.typeparams] cmd/compile/internal/types2: implement type sets with term lists

This CL resolves several known issues and TODOs.

- Represent type sets with term lists and using term list abstractions.

- Represent Unions internally as a list of (syntactical) terms.
  Use term operations to print terms and detect overlapping union
  entries.

- Compute type sets corresponding to unions lazily, on demand.

- Adjust code throughout.

- Adjusted error check in test/typeparam/mincheck.dir/main.go
  to make test pass.

Change-Id: Ib36fb7e1d343c2b6aec51d304f0f7d1ad415f999
Reviewed-on: https://go-review.googlesource.com/c/go/+/338310
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 17 insertions, 8 deletions

diff --git a/src/cmd/compile/internal/types2/typestring.go b/src/cmd/compile/internal/types2/typestring.go
index b7e32c9860..558da50528 100644
--- a/src/cmd/compile/internal/types2/typestring.go
+++ b/src/cmd/compile/internal/types2/typestring.go
@@ -158,9 +158,10 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		writeSignature(buf, t, qf, visited)
 
 	case *Union:
-		if t.IsEmpty() {
-			buf.WriteString("⊥")
-			break
+		// Unions only appear as (syntactic) embedded elements
+		// in interfaces and syntactically cannot be empty.
+		if t.NumTerms() == 0 {
+			panic("internal error: empty union")
 		}
 		for i, t := range t.terms {
 			if i > 0 {
@@ -198,13 +199,21 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				writeSignature(buf, m.typ.(*Signature), qf, visited)
 				empty = false
 			}
-			if !empty && tset.types != nil {
+			if !empty && tset.hasTerms() {
 				buf.WriteString("; ")
 			}
-			if tset.types != nil {
-				buf.WriteString("type ")
-				writeType(buf, tset.types, qf, visited)
-			}
+			first := true
+			tset.is(func(t *term) bool {
+				if !first {
+					buf.WriteByte('|')
+				}
+				first = false
+				if t.tilde {
+					buf.WriteByte('~')
+				}
+				writeType(buf, t.typ, qf, visited)
+				return true
+			})
 		} else {
 			// print explicit interface methods and embedded types
 			for i, m := range t.methods {