go/analysis/passes/test: remove some false positives

Alternative build systems like blaze don't always provide a way to determine
the relationship between a package and the package it's testing. This means
that sometimes the check for misspelled Example function names over-reports
because it doesn't find the object being exemplified. Don't report errors
unless a object can't be found in any of the imports. This means that there
won't be any false positives though of course this comes at the cost of
false positives.

Change-Id: I7435eeb2333b6dd72e06bb6383fff2ac17bee845
Reviewed-on: https://go-review.googlesource.com/c/tools/+/168404
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

go/analysis/passes/tests/testdata/src/b/b.go

@@ -0,0 +1,8 @@
+package b
+
+type Foo struct {
+}
+
+func (f *Foo) F() {
+
+}

go/analysis/passes/tests/testdata/src/b_x_test/b_test.go

@@ -0,0 +1,16 @@
+package b_x_test
+
+import "b"
+
+func ExampleFoo_F() {
+	var x b.Foo
+	x.F()
+}
+
+func ExampleFoo_G() { // want "ExampleFoo_G refers to unknown field or method: Foo.G"
+
+}
+
+func ExampleBar_F() { // want "ExampleBar_F refers to unknown identifier: Bar"
+
+}

go/analysis/passes/tests/tests.go

@@ -84,23 +84,25 @@ func isTestParam(typ ast.Expr, wantType string) bool {
 	return false
 }
 
-func lookup(pkg *types.Package, name string) types.Object {
+func lookup(pkg *types.Package, name string) []types.Object {
 	if o := pkg.Scope().Lookup(name); o != nil {
-		return o
+		return []types.Object{o}
 	}
 
-	// If this package is ".../foo_test" and it imports a package
-	// ".../foo", try looking in the latter package.
-	// This heuristic should work even on build systems that do not
-	// record any special link between the packages.
-	if basePath := strings.TrimSuffix(pkg.Path(), "_test"); basePath != pkg.Path() {
+	var ret []types.Object
+	// Search through the imports to see if any of them define name.
+	// It's hard to tell in general which package is being tested, so
+	// for the purposes of the analysis, allow the object to appear
+	// in any of the imports. This guarantees there are no false positives
+	// because the example needs to use the object so it must be defined
+	// in the package or one if its imports. On the other hand, false
+	// negatives are possible, but should be rare.
 	for _, imp := range pkg.Imports() {
-			if imp.Path() == basePath {
-				return imp.Scope().Lookup(name)
+		if obj := imp.Scope().Lookup(name); obj != nil {
+			ret = append(ret, obj)
 		}
 	}
-	}
-	return nil
+	return ret
 }
 
 func checkExample(pass *analysis.Pass, fn *ast.FuncDecl) {
@@ -121,9 +123,9 @@ func checkExample(pass *analysis.Pass, fn *ast.FuncDecl) {
 		exName = strings.TrimPrefix(fnName, "Example")
 		elems  = strings.SplitN(exName, "_", 3)
 		ident  = elems[0]
-		obj    = lookup(pass.Pkg, ident)
+		objs   = lookup(pass.Pkg, ident)
 	)
-	if ident != "" && obj == nil {
+	if ident != "" && len(objs) == 0 {
 		// Check ExampleFoo and ExampleBadFoo.
 		pass.Reportf(fn.Pos(), "%s refers to unknown identifier: %s", fnName, ident)
 		// Abort since obj is absent and no subsequent checks can be performed.
@@ -145,10 +147,17 @@ func checkExample(pass *analysis.Pass, fn *ast.FuncDecl) {
 	mmbr := elems[1]
 	if !isExampleSuffix(mmbr) {
 		// Check ExampleFoo_Method and ExampleFoo_BadMethod.
-		if obj, _, _ := types.LookupFieldOrMethod(obj.Type(), true, obj.Pkg(), mmbr); obj == nil {
+		found := false
+		// Check if Foo.Method exists in this package or its imports.
+		for _, obj := range objs {
+			if obj, _, _ := types.LookupFieldOrMethod(obj.Type(), true, obj.Pkg(), mmbr); obj != nil {
+				found = true
+			}
+			if !found {
 				pass.Reportf(fn.Pos(), "%s refers to unknown field or method: %s.%s", fnName, ident, mmbr)
 			}
 		}
+	}
 	if len(elems) == 3 && !isExampleSuffix(elems[2]) {
 		// Check ExampleFoo_Method_suffix and ExampleFoo_Method_Badsuffix.
 		pass.Reportf(fn.Pos(), "%s has malformed example suffix: %s", fnName, elems[2])

go/analysis/passes/tests/tests_test.go

@@ -16,6 +16,7 @@ func Test(t *testing.T) {
 
 	analysistest.Run(t, testdata, tests.Analyzer,
 		"a",        // loads "a", "a [a.test]", and "a.test"
+		"b_x_test", // loads "b" and "b_x_test"
 		"divergent",
 	)
 }