[dev.typeparams] cmd/compile/internal/types2: remove Type.Under method in favor of function

This removes the need for the aType embedded type and brings the types2.Type
API in sync with the go/types.Type API.

For reasons not fully understood yet, introducing the new under function
causes a very long initialization cycle error, which doesn't exist in
go/types. For now, circumvent the problem through a helper function variable.

This CL also eliminates superflous (former) Under() method calls
inside optype calls (optype takes care of this).

Plus some minor misc. cleanups and comment adjustments.

Change-Id: I86e13ccf6f0b34d7496240ace61a1c84856b6033
Reviewed-on: https://go-review.googlesource.com/c/go/+/293470
Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>

1 files changed, 7 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index e9fc08df37..677172d40f 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -445,7 +445,7 @@ func (check *Checker) constDecl(obj *Const, typ, init syntax.Expr, inherited boo
 		if !isConstType(t) {
 			// don't report an error if the type is an invalid C (defined) type
 			// (issue #22090)
-			if t.Under() != Typ[Invalid] {
+			if under(t) != Typ[Invalid] {
 				check.errorf(typ, "invalid constant type %s", t)
 			}
 			obj.typ = Typ[Invalid]
@@ -545,13 +545,13 @@ func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.Expr) {
 	check.initVars(lhs, []syntax.Expr{init}, nopos)
 }
 
-// Under returns the expanded underlying type of n0; possibly by following
+// under returns the expanded underlying type of n0; possibly by following
 // forward chains of named types. If an underlying type is found, resolve
 // the chain by setting the underlying type for each defined type in the
 // chain before returning it. If no underlying type is found or a cycle
 // is detected, the result is Typ[Invalid]. If a cycle is detected and
 // n0.check != nil, the cycle is reported.
-func (n0 *Named) Under() Type {
+func (n0 *Named) under() Type {
 	u := n0.underlying
 	if u == nil {
 		return Typ[Invalid]
@@ -584,6 +584,8 @@ func (n0 *Named) Under() Type {
 
 		if i, ok := seen[n]; ok {
 			// cycle
+			// TODO(gri) revert this to a method on Checker. Having a possibly
+			// nil Checker on Named and TypeParam is too subtle.
 			if n0.check != nil {
 				n0.check.cycleError(path[i:])
 			}
@@ -667,7 +669,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 		// any forward chain.
 		// TODO(gri) Investigate if we can just use named.origin here
 		//           and rely on lazy computation of the underlying type.
-		named.underlying = named.Under()
+		named.underlying = under(named)
 	}
 
 }
@@ -716,7 +718,7 @@ func (check *Checker) collectTypeParams(list []*syntax.Field) (tparams []*TypeNa
 		//           we may not have a complete interface yet:
 		//           type C(type T C) interface {}
 		//           (issue #39724).
-		if _, ok := bound.Under().(*Interface); ok {
+		if _, ok := under(bound).(*Interface); ok {
 			// set the type bounds
 			for i < j {
 				tparams[i].typ.(*TypeParam).bound = bound