fix cross-file

Author: bkreitch

go/analysis/passes/shadow/shadow.go

@@ -245,31 +245,34 @@ func checkShadowing(pass *analysis.Pass, spans map[types.Object]span, ident *ast
 	if shadowed.Parent() == types.Universe {
 		return
 	}
+
+	shadowedPos := pass.Fset.Position(shadowed.Pos())
+	identPos := pass.Fset.Position(ident.Pos())
+
 	if strict {
 		// The shadowed identifier must appear before this one to be an instance of shadowing.
 		if shadowed.Pos() > ident.Pos() {
 			return
 		}
 	} else {
 		// Don't complain if the span of validity of the shadowed identifier doesn't include
-		// the shadowing identifier.
+		// the shadowing identifier, except for cross-file shadowing where file processing
+		// order affects span checks.
 		span, ok := spans[shadowed]
 		if !ok {
 			pass.ReportRangef(ident, "internal error: no range for %q", ident.Name)
 			return
 		}
-		if !span.contains(ident.Pos()) {
+
+		if shadowedPos.Filename == identPos.Filename && !span.contains(ident.Pos()) {
 			return
 		}
 	}
 	// Don't complain if the types differ: that implies the programmer really wants two different things.
 	if types.Identical(obj.Type(), shadowed.Type()) {
-		shadowedPos := pass.Fset.Position(shadowed.Pos())
-
 		// Build the message, adding filename only if in a different file
 		message := fmt.Sprintf("declaration of %q shadows declaration at line %d", obj.Name(), shadowedPos.Line)
-		currentFile := pass.Fset.Position(ident.Pos()).Filename
-		if shadowedPos.Filename != currentFile {
+		if shadowedPos.Filename != identPos.Filename {
 			message += fmt.Sprintf(" in %s", filepath.Base(shadowedPos.Filename))
 		}