go/analysis/passes/httpresponse: fix ReceiverNamed usage

Fixes a bug where ReceiverNamed returns a nil *types.Named if the type is
not a Named type.

Updates related documentation in httpmux.

Fixes golang/go#66259

Change-Id: I512feeb11473f2278edf4d9ef3312319d9dd1edf
Reviewed-on: https://go-review.googlesource.com/c/tools/+/571057
Run-TryBot: Tim King <taking@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

go/analysis/passes/httpmux/httpmux.go

@@ -84,26 +84,29 @@ func isServeMuxRegisterCall(pass *analysis.Pass, call *ast.CallExpr) bool {
 	if !isMethodNamed(fn, "net/http", "Handle", "HandleFunc") {
 		return false
 	}
-	isPtr, named := typesinternal.ReceiverNamed(fn.Type().(*types.Signature).Recv())
+	recv := fn.Type().(*types.Signature).Recv() // isMethodNamed() -> non-nil
+	isPtr, named := typesinternal.ReceiverNamed(recv)
 	return isPtr && analysisutil.IsNamedType(named, "net/http", "ServeMux")
 }
 
+// isMethodNamed reports when a function f is a method,
+// in a package with the path pkgPath and the name of f is in names.
 func isMethodNamed(f *types.Func, pkgPath string, names ...string) bool {
 	if f == nil {
 		return false
 	}
 	if f.Pkg() == nil || f.Pkg().Path() != pkgPath {
-		return false
+		return false // not at pkgPath
 	}
 	if f.Type().(*types.Signature).Recv() == nil {
-		return false
+		return false // not a method
 	}
 	for _, n := range names {
 		if f.Name() == n {
 			return true
 		}
 	}
-	return false
+	return false // not in names
 }
 
 // stringConstantExpr returns expression's string constant value.

go/analysis/passes/httpresponse/httpresponse.go

@@ -119,7 +119,7 @@ func isHTTPFuncOrMethodOnClient(info *types.Info, expr *ast.CallExpr) bool {
 		return false // the function called does not return two values.
 	}
 	isPtr, named := typesinternal.ReceiverNamed(res.At(0))
-	if !isPtr || !analysisutil.IsNamedType(named, "net/http", "Response") {
+	if !isPtr || named == nil || !analysisutil.IsNamedType(named, "net/http", "Response") {
 		return false // the first return type is not *http.Response.
 	}
 

go/analysis/passes/httpresponse/testdata/src/a/a.go

@@ -110,3 +110,12 @@ func badUnwrapResp() {
 		log.Fatal(err)
 	}
 }
+
+type i66259 struct{}
+
+func (_ *i66259) Foo() (*int, int) { return nil, 1 }
+
+func issue66259() {
+	var f *i66259
+	f.Foo()
+}