Fix #71 Do + DoAndReturn signature change error msg

Update the error handling for Call.Do in the case where
the argument passed to Call.Do does not match expectations.

* panic if the argument is not a function
* panic if the number of input arguments do not match those expected by Call
* panic if the types of the input arguments do not match those expected
by Call
* panic if the number of return arguments do not match those expected by
Call
* panic if the types of return arguments do not match those expected by
Call

Author: cvgw

go.mod

@@ -1,6 +1,7 @@
 module github.com/golang/mock
 
 require (
+	github.com/pkg/errors v0.9.1
 	golang.org/x/tools v0.0.0-20190425150028-36563e24a262
 	rsc.io/quote/v3 v3.1.0
 )

go.sum

@@ -1,3 +1,5 @@
+github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
+github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=

gomock/call.go

@@ -19,6 +19,8 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
+
+	"github.com/golang/mock/gomock/internal/calldo"
 )
 
 // Call represents an expected call to a mock.
@@ -106,9 +108,20 @@ func (c *Call) MaxTimes(n int) *Call {
 // The return values from this function are returned by the mocked function.
 // It takes an interface{} argument to support n-arity functions.
 func (c *Call) DoAndReturn(f interface{}) *Call {
-	// TODO: Check arity and types here, rather than dying badly elsewhere.
 	v := reflect.ValueOf(f)
 
+	switch v.Kind() {
+	case reflect.Func:
+		mt := c.methodType
+
+		ft := v.Type()
+		if err := calldo.ValidateInputAndOutputSig(ft, mt); err != nil {
+			panic(fmt.Sprintf("DoAndReturn: %s", err))
+		}
+	default:
+		panic("DoAndReturn: argument must be a function")
+	}
+
 	c.addAction(func(args []interface{}) []interface{} {
 		vargs := make([]reflect.Value, len(args))
 		ft := v.Type()
@@ -135,9 +148,20 @@ func (c *Call) DoAndReturn(f interface{}) *Call {
 // return values call DoAndReturn.
 // It takes an interface{} argument to support n-arity functions.
 func (c *Call) Do(f interface{}) *Call {
-	// TODO: Check arity and types here, rather than dying badly elsewhere.
 	v := reflect.ValueOf(f)
 
+	switch v.Kind() {
+	case reflect.Func:
+		mt := c.methodType
+
+		ft := v.Type()
+		if err := calldo.ValidateInputAndOutputSig(ft, mt); err != nil {
+			panic(fmt.Sprintf("Do: %s", err))
+		}
+	default:
+		panic("Do: argument must be a function")
+	}
+
 	c.addAction(func(args []interface{}) []interface{} {
 		vargs := make([]reflect.Value, len(args))
 		ft := v.Type()