From: Robert Griesemer <gri@golang.org>
Date: Fri, 24 Feb 2023 02:55:11 +0000 (-0800)
Subject: go/types, types2: simplify unification with constraints
X-Git-Tag: go1.21rc1~1402
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=e2f2123e256094c64377008c17ca20538c4742d8;p=gostls13.git

go/types, types2: simplify unification with constraints

Change-Id: I399f0ac12e65713f3018a89da55ecd3cdb855c50
Reviewed-on: https://go-review.googlesource.com/c/go/+/471017
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
---

diff --git a/src/cmd/compile/internal/types2/infer.go b/src/cmd/compile/internal/types2/infer.go
index 1aa8e31f5d..4d842fa388 100644
--- a/src/cmd/compile/internal/types2/infer.go
+++ b/src/cmd/compile/internal/types2/infer.go
@@ -198,16 +198,16 @@ func (check *Checker) infer(pos syntax.Pos, tparams []*TypeParam, targs []Type,
 				tx := u.at(tpar)
 				switch {
 				case tx != nil:
-					// The corresponding type argument tx is known.
-					// In this case, if the core type has a tilde, the type argument's underlying
-					// type must match the core type, otherwise the type argument and the core type
-					// must match.
-					// If tx is an (external) type parameter, don't consider its underlying type
-					// (which is an interface). The unifier will use the type parameter's core
-					// type automatically.
-					if core.tilde && !isTypeParam(tx) {
-						tx = under(tx)
-					}
+					// The corresponding type argument tx is known. There are 2 cases:
+					// 1) If the core type has a tilde, per spec requirement for tilde
+					//    elements, the core type is an underlying (literal) type.
+					//    And because of the tilde, the underlying type of tx must match
+					//    against the core type.
+					//    But because unify automatically matches a defined type against
+					//    an underlying literal type, we can simply unify tx with the
+					//    core type.
+					// 2) If the core type doesn't have a tilde, we also must unify tx
+					//    with the core type.
 					if !u.unify(tx, core.typ) {
 						check.errorf(pos, CannotInferTypeArgs, "%s does not match %s", tpar, core.typ)
 						return nil
diff --git a/src/go/types/infer.go b/src/go/types/infer.go
index af1001cd3c..59f982b584 100644
--- a/src/go/types/infer.go
+++ b/src/go/types/infer.go
@@ -200,16 +200,16 @@ func (check *Checker) infer(posn positioner, tparams []*TypeParam, targs []Type,
 				tx := u.at(tpar)
 				switch {
 				case tx != nil:
-					// The corresponding type argument tx is known.
-					// In this case, if the core type has a tilde, the type argument's underlying
-					// type must match the core type, otherwise the type argument and the core type
-					// must match.
-					// If tx is an (external) type parameter, don't consider its underlying type
-					// (which is an interface). The unifier will use the type parameter's core
-					// type automatically.
-					if core.tilde && !isTypeParam(tx) {
-						tx = under(tx)
-					}
+					// The corresponding type argument tx is known. There are 2 cases:
+					// 1) If the core type has a tilde, per spec requirement for tilde
+					//    elements, the core type is an underlying (literal) type.
+					//    And because of the tilde, the underlying type of tx must match
+					//    against the core type.
+					//    But because unify automatically matches a defined type against
+					//    an underlying literal type, we can simply unify tx with the
+					//    core type.
+					// 2) If the core type doesn't have a tilde, we also must unify tx
+					//    with the core type.
 					if !u.unify(tx, core.typ) {
 						check.errorf(posn, CannotInferTypeArgs, "%s does not match %s", tpar, core.typ)
 						return nil