[release-branch.go1.14] go/doc: fix detection of whole file examples

After CL 211357 (commit 499dc1c),
hasTests and numDecl were not updated properly for function
declarations with parameters, which affected the whole file
example detection logic. This caused examples like

	package foo_test

	func Foo(x int) {
	}

	func Example() {
		fmt.Println("Hello, world!")
		// Output: Hello, world!
	}

to not be detected as whole file ones.

Fixes #38418.
For #38409.

Change-Id: I9ebd47e52d7ee9d91eb6f8e0257511de69b2a402
GitHub-Last-Rev: cc71c31
GitHub-Pull-Request: #37730
Reviewed-on: https://go-review.googlesource.com/c/go/+/222477
Reviewed-by: Agniva De Sarker <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Run-TryBot: Agniva De Sarker <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
(cherry picked from commit c4961dc)
Reviewed-on: https://go-review.googlesource.com/c/go/+/232868
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Gregory Petrosyan <[email protected]>

Author: flyingmutant

src/go/doc/example.go

@@ -62,9 +62,6 @@ func Examples(testFiles ...*ast.File) []*Example {
 			if !ok || f.Recv != nil {
 				continue
 			}
-			if params := f.Type.Params; len(params.List) != 0 {
-				continue // function has params; not a valid example
-			}
 			numDecl++
 			name := f.Name.Name
 			if isTest(name, "Test") || isTest(name, "Benchmark") {
@@ -74,6 +71,9 @@ func Examples(testFiles ...*ast.File) []*Example {
 			if !isTest(name, "Example") {
 				continue
 			}
+			if params := f.Type.Params; len(params.List) != 0 {
+				continue // function has params; not a valid example
+			}
 			if f.Body == nil { // ast.File.Body nil dereference (see issue 28044)
 				continue
 			}

src/go/doc/example_test.go

@@ -331,25 +331,65 @@ func main() {
 }
 `
 
+const exampleWholeFileFunction = `package foo_test
+
+func Foo(x int) {
+}
+
+func Example() {
+	fmt.Println("Hello, world!")
+	// Output: Hello, world!
+}
+`
+
+const exampleWholeFileFunctionOutput = `package main
+
+func Foo(x int) {
+}
+
+func main() {
+	fmt.Println("Hello, world!")
+}
+`
+
+var exampleWholeFileTestCases = []struct {
+	Title, Source, Play, Output string
+}{
+	{
+		"Methods",
+		exampleWholeFile,
+		exampleWholeFileOutput,
+		"Hello, world!\n",
+	},
+	{
+		"Function",
+		exampleWholeFileFunction,
+		exampleWholeFileFunctionOutput,
+		"Hello, world!\n",
+	},
+}
+
 func TestExamplesWholeFile(t *testing.T) {
-	fset := token.NewFileSet()
-	file, err := parser.ParseFile(fset, "test.go", strings.NewReader(exampleWholeFile), parser.ParseComments)
-	if err != nil {
-		t.Fatal(err)
-	}
-	es := doc.Examples(file)
-	if len(es) != 1 {
-		t.Fatalf("wrong number of examples; got %d want 1", len(es))
-	}
-	e := es[0]
-	if e.Name != "" {
-		t.Errorf("got Name == %q, want %q", e.Name, "")
-	}
-	if g, w := formatFile(t, fset, e.Play), exampleWholeFileOutput; g != w {
-		t.Errorf("got Play == %q, want %q", g, w)
-	}
-	if g, w := e.Output, "Hello, world!\n"; g != w {
-		t.Errorf("got Output == %q, want %q", g, w)
+	for _, c := range exampleWholeFileTestCases {
+		fset := token.NewFileSet()
+		file, err := parser.ParseFile(fset, "test.go", strings.NewReader(c.Source), parser.ParseComments)
+		if err != nil {
+			t.Fatal(err)
+		}
+		es := doc.Examples(file)
+		if len(es) != 1 {
+			t.Fatalf("%s: wrong number of examples; got %d want 1", c.Title, len(es))
+		}
+		e := es[0]
+		if e.Name != "" {
+			t.Errorf("%s: got Name == %q, want %q", c.Title, e.Name, "")
+		}
+		if g, w := formatFile(t, fset, e.Play), c.Play; g != w {
+			t.Errorf("%s: got Play == %q, want %q", c.Title, g, w)
+		}
+		if g, w := e.Output, c.Output; g != w {
+			t.Errorf("%s: got Output == %q, want %q", c.Title, g, w)
+		}
 	}
 }