[dev.typeparams] cmd/compile/internal/types2: replace Sum type with Union type

- We still mostly ignore the tilde information.

- More consistent naming: A Union term is the pair (type, tilde).
  Rename Union.terms to Union.types; the Union.types and Union.tilde
  slices make up the Union terms.

- Replace Sum.is with Union.underIs: underIs iterates through all
  union terms and calls its argument function with the underlying
  type of the term (and thus can ignore the tilde information).
  This also eliminates the need to call under in the argument
  function.

- Added Union.is for situations where we need to consider the tilde
  information for each Union term.

Change-Id: I70fcf1813e072651dc0f61d52d5555642ee762fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/323274
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

1 files changed, 10 insertions, 6 deletions

diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index 94fb506d80..1779e32c5c 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -178,9 +178,9 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 				mode = value
 			}
 
-		case *Sum:
-			if t.is(func(t Type) bool {
-				switch t := under(t).(type) {
+		case *Union:
+			if t.underIs(func(t Type) bool {
+				switch t := t.(type) {
 				case *Basic:
 					if isString(t) && id == _Len {
 						return true
@@ -460,8 +460,8 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 				m = 2
 			case *Map, *Chan:
 				m = 1
-			case *Sum:
-				return t.is(valid)
+			case *Union:
+				return t.underIs(valid)
 			default:
 				return false
 			}
@@ -749,10 +749,14 @@ func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
 	if tp := asTypeParam(x); tp != nil {
 		// Test if t satisfies the requirements for the argument
 		// type and collect possible result types at the same time.
+		// TODO(gri) This needs to consider the ~ information if we
+		//           have a union type.
 		var rtypes []Type
+		var tilde []bool
 		if !tp.Bound().is(func(x Type) bool {
 			if r := f(x); r != nil {
 				rtypes = append(rtypes, r)
+				tilde = append(tilde, true) // for now - see TODO above
 				return true
 			}
 			return false
@@ -768,7 +772,7 @@ func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
 		// construct a suitable new type parameter
 		tpar := NewTypeName(nopos, nil /* = Universe pkg */, "<type parameter>", nil)
 		ptyp := check.NewTypeParam(tpar, 0, &emptyInterface) // assigns type to tpar as a side-effect
-		tsum := NewSum(rtypes)
+		tsum := newUnion(rtypes, tilde)
 		ptyp.bound = &Interface{allMethods: markComplete, allTypes: tsum}
 
 		return ptyp