[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, 6 insertions, 6 deletions

diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index e497e17463..6c5f756491 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -145,12 +145,12 @@ func (subst *subster) typ(typ Type) Type {
 		}
 
 	case *Union:
-		terms, copied := subst.termList(t.terms)
+		terms, copied := subst.termlist(t.terms)
 		if copied {
-			// TODO(gri) Remove duplicates that may have crept in after substitution
-			//           (unlikely but possible). This matters for the Identical
-			//           predicate on unions.
-			return &Union{terms}
+			// term list substitution may introduce duplicate terms (unlikely but possible).
+			// This is ok; lazy type set computation will determine the actual type set
+			// in normal form.
+			return &Union{terms, nil}
 		}
 
 	case *Interface:
@@ -387,7 +387,7 @@ func (subst *subster) typeList(in []Type) (out []Type, copied bool) {
 	return
 }
 
-func (subst *subster) termList(in []*term) (out []*term, copied bool) {
+func (subst *subster) termlist(in []*term) (out []*term, copied bool) {
 	out = in
 	for i, t := range in {
 		if u := subst.typ(t.typ); u != t.typ {