[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, 2 insertions, 2 deletions

diff --git a/test/typeparam/mincheck.dir/main.go b/test/typeparam/mincheck.dir/main.go
index 72d8effcc5..9cf2c6bafd 100644
--- a/test/typeparam/mincheck.dir/main.go
+++ b/test/typeparam/mincheck.dir/main.go
@@ -28,11 +28,11 @@ func main() {
 	}
 
 	const want2 = "ay"
-	if got := a.Min[string]("bb", "ay"); got != want2 { // ERROR "string does not satisfy interface{int|int64|float64}"
+	if got := a.Min[string]("bb", "ay"); got != want2 { // ERROR "string does not satisfy"
 		panic(fmt.Sprintf("got %d, want %d", got, want2))
 	}
 
-	if got := a.Min("bb", "ay"); got != want2 { // ERROR "string does not satisfy interface{int|int64|float64}"
+	if got := a.Min("bb", "ay"); got != want2 { // ERROR "string does not satisfy"
 		panic(fmt.Sprintf("got %d, want %d", got, want2))
 	}
 }