go/doc: resolve imports before predeclared identifiers in examples

Fixes #70611
Fixes #70630

Change-Id: I868e68dbdf50ce34259eeef7b3d7985ede1f4c0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/639175
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/go/doc/example.go

@@ -192,13 +192,6 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 	// Find unresolved identifiers and uses of top-level declarations.
 	depDecls, unresolved := findDeclsAndUnresolved(body, topDecls, typMethods)
 
-	// Remove predeclared identifiers from unresolved list.
-	for n := range unresolved {
-		if predeclaredTypes[n] || predeclaredConstants[n] || predeclaredFuncs[n] {
-			delete(unresolved, n)
-		}
-	}
-
 	// Use unresolved identifiers to determine the imports used by this
 	// example. The heuristic assumes package names match base import
 	// paths for imports w/o renames (should be good enough most of the time).
@@ -251,6 +244,13 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 		}
 	}
 
+	// Remove predeclared identifiers from unresolved list.
+	for n := range unresolved {
+		if predeclaredTypes[n] || predeclaredConstants[n] || predeclaredFuncs[n] {
+			delete(unresolved, n)
+		}
+	}
+
 	// If there are other unresolved identifiers, give up because this
 	// synthesized file is not going to build.
 	if len(unresolved) > 0 {

src/go/doc/testdata/examples/shadow_predeclared.go

@@ -0,0 +1,19 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package foo_test
+
+import (
+	"fmt"
+
+	"example.com/error"
+)
+
+func Print(s string) {
+	fmt.Println(s)
+}
+
+func Example() {
+	Print(error.Hello)
+}

src/go/doc/testdata/examples/shadow_predeclared.golden

@@ -0,0 +1,16 @@
+-- .Play --
+package main
+
+import (
+	"fmt"
+
+	"example.com/error"
+)
+
+func Print(s string) {
+	fmt.Println(s)
+}
+
+func main() {
+	Print(error.Hello)
+}