feat validate Do & DoReturn args (#558)

Previous behavior was a panic would happen if the number of args
did not match. Now the test should fail more gracefully and allow
for better debugging.

Fixes: #71

gomock/call.go

@@ -113,8 +113,14 @@ func (c *Call) DoAndReturn(f interface{}) *Call {
 	v := reflect.ValueOf(f)
 
 	c.addAction(func(args []interface{}) []interface{} {
+		c.t.Helper()
 		vArgs := make([]reflect.Value, len(args))
 		ft := v.Type()
+		if c.methodType.NumIn() != ft.NumIn() {
+			c.t.Fatalf("wrong number of arguments in DoAndReturn func for %T.%v: got %d, want %d [%s]",
+				c.receiver, c.method, ft.NumIn(), c.methodType.NumIn(), c.origin)
+			return nil
+		}
 		for i := 0; i < len(args); i++ {
 			if args[i] != nil {
 				vArgs[i] = reflect.ValueOf(args[i])
@@ -142,6 +148,12 @@ func (c *Call) Do(f interface{}) *Call {
 	v := reflect.ValueOf(f)
 
 	c.addAction(func(args []interface{}) []interface{} {
+		c.t.Helper()
+		if c.methodType.NumIn() != v.Type().NumIn() {
+			c.t.Fatalf("wrong number of arguments in Do func for %T.%v: got %d, want %d [%s]",
+				c.receiver, c.method, v.Type().NumIn(), c.methodType.NumIn(), c.origin)
+			return nil
+		}
 		vArgs := make([]reflect.Value, len(args))
 		ft := v.Type()
 		for i := 0; i < len(args); i++ {

gomock/call_test.go

@@ -142,7 +142,7 @@ var testCases []testCase = []testCase{
 		doFunc:      func(x int) {},
 		callFunc:    func(x int, y int) {},
 		args:        []interface{}{0, 1},
-		expectPanic: true,
+		expectPanic: false,
 	}, {
 		description: "number of args for Do func don't match Call func",
 		doFunc: func(x int) bool {
@@ -152,7 +152,7 @@ var testCases []testCase = []testCase{
 			return true
 		},
 		args:        []interface{}{0, 1},
-		expectPanic: true,
+		expectPanic: false,
 	}, {
 		description: "arg type for Do func incompatible with Call func",
 		doFunc:      func(x int) {},
@@ -481,6 +481,104 @@ func TestCall_Do(t *testing.T) {
 	}
 }
 
+func TestCall_Do_NumArgValidation(t *testing.T) {
+	tests := []struct {
+		name       string
+		methodType reflect.Type
+		doFn       interface{}
+		args       []interface{}
+		wantErr    bool
+	}{
+		{
+			name:       "too few",
+			methodType: reflect.TypeOf(func(one, two string) {}),
+			doFn:       func(one string) {},
+			args:       []interface{}{"too", "few"},
+			wantErr:    true,
+		},
+		{
+			name:       "too many",
+			methodType: reflect.TypeOf(func(one, two string) {}),
+			doFn:       func(one, two, three string) {},
+			args:       []interface{}{"too", "few"},
+			wantErr:    true,
+		},
+		{
+			name:       "just right",
+			methodType: reflect.TypeOf(func(one, two string) {}),
+			doFn:       func(one string, two string) {},
+			args:       []interface{}{"just", "right"},
+			wantErr:    false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tr := &mockTestReporter{}
+			call := &Call{
+				t:          tr,
+				methodType: tt.methodType,
+			}
+			call.Do(tt.doFn)
+			call.actions[0](tt.args)
+			if tt.wantErr && tr.fatalCalls != 1 {
+				t.Fatalf("expected call to fail")
+			}
+			if !tt.wantErr && tr.fatalCalls != 0 {
+				t.Fatalf("expected call to pass")
+			}
+		})
+	}
+}
+
+func TestCall_DoAndReturn_NumArgValidation(t *testing.T) {
+	tests := []struct {
+		name       string
+		methodType reflect.Type
+		doFn       interface{}
+		args       []interface{}
+		wantErr    bool
+	}{
+		{
+			name:       "too few",
+			methodType: reflect.TypeOf(func(one, two string) string { return "" }),
+			doFn:       func(one string) {},
+			args:       []interface{}{"too", "few"},
+			wantErr:    true,
+		},
+		{
+			name:       "too many",
+			methodType: reflect.TypeOf(func(one, two string) string { return "" }),
+			doFn:       func(one, two, three string) string { return "" },
+			args:       []interface{}{"too", "few"},
+			wantErr:    true,
+		},
+		{
+			name:       "just right",
+			methodType: reflect.TypeOf(func(one, two string) string { return "" }),
+			doFn:       func(one string, two string) string { return "" },
+			args:       []interface{}{"just", "right"},
+			wantErr:    false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			tr := &mockTestReporter{}
+			call := &Call{
+				t:          tr,
+				methodType: tt.methodType,
+			}
+			call.DoAndReturn(tt.doFn)
+			call.actions[0](tt.args)
+			if tt.wantErr && tr.fatalCalls != 1 {
+				t.Fatalf("expected call to fail")
+			}
+			if !tt.wantErr && tr.fatalCalls != 0 {
+				t.Fatalf("expected call to pass")
+			}
+		})
+	}
+}
+
 func TestCall_DoAndReturn(t *testing.T) {
 	for _, tc := range testCases {
 		t.Run(tc.description, func(t *testing.T) {