Document the arguments number limit of 15 in documentation

[PiotrNycz]

Does the feature exist in the most recent commit?

No

Why do we need this feature?

This is just for documentation. I recently spent some time of investigation why MOCK_METHOD stops compiling after adding new argument to function type. It occured that limit for arguments number is 15 which is nowhere documented (maybe it is but I cannot find it.

Describe the proposal.

Add information that MOCK_METHOD macro has limit when talking about number of arguments.
And that this limit current is 15.
This can be in form of FAQ and just short info in MOCK_METHOD doc.

For FAQ I'd propose to add this QA:

Q: I encounter this error .... while adding new argument to function mocked by MOCK_METHOD macro. What does it mean and how to work around this issue?

A:
You crossed the limit of 15 arguments in function mocked by MOCK_METHOD macro.
You can work around this limit by the following methods:

1. The best is to refactor your code - 15 is quite the enough number as for function arguments...
2. Other solution - if you have arguments that are irrelevant - always substituted by any matcher ::testing::_ - then your mocked function can just have subset of arguments:
   void func(int a1, ..., int a16) override { mocked_func(a1, ... , a15); }
   MOCK_METHOD(mocked_func, void, (int a1, .., int a15), ());
3. OR - you can have MOCK_METHOD that takes std::tuple of its arguments:
   Ret func(T1 a1, ...., T19 a19) override { return mocked_func(std::tuple<T1, ..., T19>(a1, ..., a19)); }
   MOCK_METHOD(mocked_func, Ret, ((std::tuple<T1, ..., T19>)), ());
   And use FieldsAre to match this packed arguments - but here again - we have limit of 19 when using FieldsAre (as I can see in the code)

Is the feature specific to an operating system, compiler, or build system version?

No

[miacodehub]

Hi can i work on this issue?

[PiotrNycz]

@miacodehub sure, it'd be great, if you are asking me

[miacodehub]

Awesome! I'll take it.