[release-branch.go1.21] go/types, types2: use exact unification when comparing interface methods

Irrespective of whether unification is exact or inexact, method
signatures of interfaces must always match exactly: a type never
satisfies/implements an interface if relevant method signatures
are different (i.e., not identical, possibly after substitution).

Fixes #61959.

Change-Id: I20c0aa28ac86e2edec615b40f2269938e4a96938
Reviewed-on: https://go-review.googlesource.com/c/go/+/519435
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/519416
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>

3 files changed, 63 insertions, 6 deletions

diff --git a/src/cmd/compile/internal/types2/unify.go b/src/cmd/compile/internal/types2/unify.go
index 94830e94f7..eddd808cb7 100644
--- a/src/cmd/compile/internal/types2/unify.go
+++ b/src/cmd/compile/internal/types2/unify.go
@@ -505,7 +505,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 			}
 			// All xmethods must exist in ymethods and corresponding signatures must unify.
 			for _, xm := range xmethods {
-				if ym := ymap[xm.Id()]; ym == nil || !u.nify(xm.typ, ym.typ, emode, p) {
+				if ym := ymap[xm.Id()]; ym == nil || !u.nify(xm.typ, ym.typ, exact, p) {
 					return false
 				}
 			}
@@ -526,7 +526,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 			xmethods := xi.typeSet().methods
 			for _, xm := range xmethods {
 				obj, _, _ := LookupFieldOrMethod(y, false, xm.pkg, xm.name)
-				if ym, _ := obj.(*Func); ym == nil || !u.nify(xm.typ, ym.typ, emode, p) {
+				if ym, _ := obj.(*Func); ym == nil || !u.nify(xm.typ, ym.typ, exact, p) {
 					return false
 				}
 			}
@@ -685,7 +685,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 				}
 				for i, f := range a {
 					g := b[i]
-					if f.Id() != g.Id() || !u.nify(f.typ, g.typ, emode, q) {
+					if f.Id() != g.Id() || !u.nify(f.typ, g.typ, exact, q) {
 						return false
 					}
 				}
diff --git a/src/go/types/unify.go b/src/go/types/unify.go
index bef851f423..56b258ae35 100644
--- a/src/go/types/unify.go
+++ b/src/go/types/unify.go
@@ -507,7 +507,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 			}
 			// All xmethods must exist in ymethods and corresponding signatures must unify.
 			for _, xm := range xmethods {
-				if ym := ymap[xm.Id()]; ym == nil || !u.nify(xm.typ, ym.typ, emode, p) {
+				if ym := ymap[xm.Id()]; ym == nil || !u.nify(xm.typ, ym.typ, exact, p) {
 					return false
 				}
 			}
@@ -528,7 +528,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 			xmethods := xi.typeSet().methods
 			for _, xm := range xmethods {
 				obj, _, _ := LookupFieldOrMethod(y, false, xm.pkg, xm.name)
-				if ym, _ := obj.(*Func); ym == nil || !u.nify(xm.typ, ym.typ, emode, p) {
+				if ym, _ := obj.(*Func); ym == nil || !u.nify(xm.typ, ym.typ, exact, p) {
 					return false
 				}
 			}
@@ -687,7 +687,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 				}
 				for i, f := range a {
 					g := b[i]
-					if f.Id() != g.Id() || !u.nify(f.typ, g.typ, emode, q) {
+					if f.Id() != g.Id() || !u.nify(f.typ, g.typ, exact, q) {
 						return false
 					}
 				}
diff --git a/src/internal/types/testdata/fixedbugs/issue61879.go b/src/internal/types/testdata/fixedbugs/issue61879.go
new file mode 100644
index 0000000000..542bc2d68a
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue61879.go
@@ -0,0 +1,57 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "fmt"
+
+type Interface[T any] interface {
+	m(Interface[T])
+}
+
+func f[S []Interface[T], T any](S) {}
+
+func _() {
+	var s []Interface[int]
+	f(s) // panic here
+}
+
+// Larger example from issue
+
+type InterfaceA[T comparable] interface {
+	setData(string) InterfaceA[T]
+}
+
+type ImplA[T comparable] struct {
+	data string
+	args []any
+}
+
+func NewInterfaceA[T comparable](args ...any) InterfaceA[T] {
+	return &ImplA[T]{
+		data: fmt.Sprintf("%v", args...),
+		args: args,
+	}
+}
+
+func (k *ImplA[T]) setData(data string) InterfaceA[T] {
+	k.data = data
+	return k
+}
+
+func Foo[M ~map[InterfaceA[T]]V, T comparable, V any](m M) {
+	// DO SOMETHING HERE
+	return
+}
+
+func Bar() {
+	keys := make([]InterfaceA[int], 0, 10)
+	m := make(map[InterfaceA[int]]int)
+	for i := 0; i < 10; i++ {
+		keys = append(keys, NewInterfaceA[int](i))
+		m[keys[i]] = i
+	}
+
+	Foo(m) // panic here
+}