gopls/internal/golang: fix bug in freeRefs algorithm

The special case for dot imports was spuriously matching
struct field names from other packages.

+ regression test

Change-Id: Ib125e22f092b793007f6ee60d8e58890762c37a4
Reviewed-on: https://go-review.googlesource.com/c/tools/+/586780
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

gopls/internal/golang/freesymbols.go

@@ -358,7 +358,7 @@ func freeRefs(pkg *types.Package, info *types.Info, file *ast.File, start, end t
 
 		// Compute dotted path.
 		objects := append(suffix, obj)
-		if obj.Pkg() != nil && obj.Pkg() != pkg { // dot import
+		if obj.Pkg() != nil && obj.Pkg() != pkg && isPackageLevel(obj) { // dot import
 			// Synthesize the implicit PkgName.
 			pkgName := types.NewPkgName(token.NoPos, pkg, obj.Pkg().Name(), obj.Pkg())
 			parent = fileScope

gopls/internal/golang/freesymbols_test.go

@@ -54,6 +54,16 @@ func TestFreeRefs(t *testing.T) {
 			`package p; import . "errors"; var _ = « New»`,
 			[]string{"file pkgname errors.New"},
 		},
+		{
+			// struct field (regression test for overzealous dot import logic)
+			`package p; import "net/url"; var _ = «url.URL{Host: ""}»`,
+			[]string{"file pkgname url.URL"},
+		},
+		{
+			// dot imports (another regression test of same)
+			`package p; import . "net/url"; var _ = «URL{Host: ""}»`,
+			[]string{"file pkgname url.URL"},
+		},
 		{
 			// dot import of unsafe (a corner case)
 			`package p; import . "unsafe"; var _ « Pointer»`,