apidiff: fix comparison of a type in an other package with a basic type

A named type in a package other than the one being compared can
never correspond to a basic type.

Fixes golang/go#61385.

Change-Id: I1c440dfc52e3c9e2b58a7f013d9809974e94efeb
Reviewed-on: https://go-review.googlesource.com/c/exp/+/513675
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

apidiff/correspondence.go

@@ -119,13 +119,7 @@ func (d *differ) corr(old, new types.Type, p *ifacePair) bool {
 		}
 
 	case *types.Named:
-		if new, ok := new.(*types.Named); ok {
-			return d.establishCorrespondence(old, new)
-		}
-		if new, ok := new.(*types.Basic); ok {
-			// Basic types are defined types, too, so we have to support them.
-			return d.establishCorrespondence(old, new)
-		}
+		return d.establishCorrespondence(old, new)
 
 	case *types.TypeParam:
 		if new, ok := new.(*types.TypeParam); ok {
@@ -191,7 +185,9 @@ func (d *differ) establishCorrespondence(old *types.Named, new types.Type) bool
 	// of the same path, doesn't correspond to something other than the new type.
 	// That is a bit hard, because there is no easy way to find a new package
 	// matching an old one.
-	if newn, ok := new.(*types.Named); ok {
+	switch new := new.(type) {
+	case *types.Named:
+		newn := new
 		oobj := old.Obj()
 		nobj := newn.Obj()
 		if oobj.Pkg() != d.old || nobj.Pkg() != d.new {
@@ -233,6 +229,14 @@ func (d *differ) establishCorrespondence(old *types.Named, new types.Type) bool
 				return false
 			}
 		}
+	case *types.Basic:
+		if old.Obj().Pkg() != d.old {
+			// A named type from a package other than old never corresponds to a basic type.
+			return false
+		}
+	default:
+		// Only named and basic types can correspond.
+		return false
 	}
 	// If there is no correspondence, create one.
 	d.correspondMap.Set(old, new)

apidiff/testdata/aliases.go

@@ -13,3 +13,17 @@ type A = t1
 // new
 // i t1: changed from int to bool
 type A = t2
+
+// old
+type B = int
+
+// new
+// i B: changed from int to B
+type B int
+
+// old
+type C int
+
+// new
+// OK: merging types
+type C = int

apidiff/testdata/other_packages.go

@@ -1,9 +1,9 @@
-package p
-
-// This test demonstrates correct handling of symbols
+// These tests demonstrate the correct handling of symbols
 // in packages other than two being compared.
-// See the lines in establishCorrespondence after
-//   	if newn, ok := new.(*types.Named); ok
+// See the lines in establishCorrespondence beginning
+//
+//	if newn, ok := new.(*types.Named); ok
+package p
 
 // both
 
@@ -14,6 +14,10 @@ import (
 	"text/tabwriter"
 )
 
+// Here we have two named types in different packages.
+// They have the same package-relative name, but we compare
+// the package-qualified names.
+
 // old
 var V io.Writer
 var _ tabwriter.Writer
@@ -22,3 +26,19 @@ var _ tabwriter.Writer
 // i V: changed from io.Writer to text/tabwriter.Writer
 var V tabwriter.Writer
 var _ io.Writer
+
+// Here one type is a basic type.
+// Example from https://go.dev/issue/61385.
+// apidiff would previously report
+//	 F2: changed from func(io.ReadCloser) to func(io.ReadCloser)
+//   io.ReadCloser: changed from interface{io.Reader; io.Closer} to int
+
+// old
+func F1(io.ReadCloser) {}
+
+// new
+// i F1: changed from func(io.ReadCloser) to func(int)
+func F1(int) {}
+
+// both
+func F2(io.ReadCloser) {}