[release-branch.go1.21] go/types, types2: don't lose position info of interface embeddings

Accurate position information for embedded types in interfaces is
crucial to identify the corresponding source file, and with that
the Go language version associated with that file. (The position
information is also important for proper error messages.)

Before this CL, the position information for embedded types was
discarded after type set computation, in the assumption that it
was not needed anymore. However, substitutions that update the
interface may lead to repeated type set computations which then
won't have the correct position information.

This CL does preserve the position information for embedded
types until the end of type checking (cleanup phase), and also
copy the position information during a substitution of the
interface.

The respective bug (#64759) doesn't seem to appear in 1.22 (most
likely because it's hidden by some of the changes made with respect
to the file version logic), but the existing code is still wrong.
The backport of this code to 1.21 and 1.20 fixes the issue in those
releases.

For #64759.
Fixes #65053.

Change-Id: I80f4004c9d79cb02eac6739c324c477706615102
Reviewed-on: https://go-review.googlesource.com/c/go/+/555296
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>
Reviewed-on: https://go-review.googlesource.com/c/go/+/555415
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/types2/issues_test.go

@@ -964,3 +964,20 @@ func f[I *T, T any]() {
 		t.Fatalf("types of v and T are not pointer-identical: %p != %p", v.Type().(*TypeParam), T)
 	}
 }
+
+func TestIssue64759(t *testing.T) {
+	const src = `
+//go:build go1.18
+package p
+
+func f[S ~[]E, E any](S) {}
+
+func _() {
+	f([]string{})
+}
+`
+	// Per the go:build directive, the source must typecheck
+	// even though the (module) Go version is set to go1.17.
+	conf := Config{GoVersion: "go1.17"}
+	mustTypecheck(src, &conf, nil)
+}

src/cmd/compile/internal/types2/subst.go

@@ -169,6 +169,7 @@ func (subst *subster) typ(typ Type) Type {
 		if mcopied || ecopied {
 			iface := subst.check.newInterface()
 			iface.embeddeds = embeddeds
+			iface.embedPos = t.embedPos
 			iface.implicit = t.implicit
 			iface.complete = t.complete
 			// If we've changed the interface type, we may need to replace its

src/cmd/compile/internal/types2/typeset.go

@@ -304,7 +304,6 @@ func computeInterfaceTypeSet(check *Checker, pos syntax.Pos, ityp *Interface) *_
 		// separately. Here we only need to intersect the term lists and comparable bits.
 		allTerms, allComparable = intersectTermLists(allTerms, allComparable, terms, comparable)
 	}
-	ityp.embedPos = nil // not needed anymore (errors have been reported)
 
 	ityp.tset.comparable = allComparable
 	if len(allMethods) != 0 {

src/go/types/issues_test.go

@@ -974,3 +974,20 @@ func f[I *T, T any]() {
 		t.Fatalf("types of v and T are not pointer-identical: %p != %p", v.Type().(*TypeParam), T)
 	}
 }
+
+func TestIssue64759(t *testing.T) {
+	const src = `
+//go:build go1.18
+package p
+
+func f[S ~[]E, E any](S) {}
+
+func _() {
+	f([]string{})
+}
+`
+	// Per the go:build directive, the source must typecheck
+	// even though the (module) Go version is set to go1.17.
+	conf := Config{GoVersion: "go1.17"}
+	mustTypecheck(src, &conf, nil)
+}

src/go/types/subst.go

@@ -171,6 +171,7 @@ func (subst *subster) typ(typ Type) Type {
 		if mcopied || ecopied {
 			iface := subst.check.newInterface()
 			iface.embeddeds = embeddeds
+			iface.embedPos = t.embedPos
 			iface.implicit = t.implicit
 			iface.complete = t.complete
 			// If we've changed the interface type, we may need to replace its

src/go/types/typeset.go

@@ -302,7 +302,6 @@ func computeInterfaceTypeSet(check *Checker, pos token.Pos, ityp *Interface) *_T
 		// separately. Here we only need to intersect the term lists and comparable bits.
 		allTerms, allComparable = intersectTermLists(allTerms, allComparable, terms, comparable)
 	}
-	ityp.embedPos = nil // not needed anymore (errors have been reported)
 
 	ityp.tset.comparable = allComparable
 	if len(allMethods) != 0 {