internal/gcimporter: don't panic when exporting a builtin 'Error' method

The 'Error' method on the builtin error type is special, because it does
not have a package. Fall back to the old behavior in this case, by
changing the objectpath encoding from (pkg, objectpath) to (objectpath,
pkg), and using "" for objects without a package.

Change-Id: I3d7e7ffe4eb3523d3ba758b09372abb7fc80be89
Reviewed-on: https://go-review.googlesource.com/c/tools/+/510416
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>

gopls/internal/regtest/marker/testdata/references/issue60676.txt

@@ -25,12 +25,15 @@ type E struct {
 type AI interface {
 	M() //@loc(MDef, "M")
 	EI
+	error
 }
 
 type EI interface {
 	N() //@loc(NDef, "N")
 }
 
+type Error error
+
 -- b/b.go --
 package b
 

internal/gcimporter/iexport.go

@@ -907,9 +907,10 @@ func (w *exportWriter) doTyp(t types.Type, pkg *types.Package) {
 // type of B, they are encoded as objectPaths and later looked up in the
 // importer. The same problem applies to interface methods.
 func (w *exportWriter) objectPath(obj types.Object) {
-	w.pkg(obj.Pkg())
-	if obj.Pkg() == w.p.localpkg {
-		// If obj is declared in the local package, no need to encode.
+	if obj.Pkg() == nil || obj.Pkg() == w.p.localpkg {
+		// obj.Pkg() may be nil for the builtin error.Error.
+		// In this case, or if obj is declared in the local package, no need to
+		// encode.
 		w.string("")
 		return
 	}
@@ -934,6 +935,7 @@ func (w *exportWriter) objectPath(obj types.Object) {
 		}
 	}
 	w.string(string(objectPath))
+	w.pkg(obj.Pkg())
 }
 
 func (w *exportWriter) signature(sig *types.Signature) {

internal/gcimporter/iimport.go

@@ -978,11 +978,11 @@ func (r *importReader) kind() itag {
 // imported package. See the doc for exportWriter.objectPath for a full
 // explanation.
 func (r *importReader) objectPathObject() types.Object {
-	pkg := r.pkg()
 	objPath := objectpath.Path(r.string())
 	if objPath == "" {
 		return nil
 	}
+	pkg := r.pkg()
 	obj, err := objectpath.Object(pkg, objPath)
 	if err != nil {
 		if r.p.reportf != nil {