C# v7.x: Local functions

[RexJaeschke]

No description provided.

[BillWagner]

This looks great @RexJaeschke Should we mark this ready for review?

I've got one note for how local functions interacts with generalized async return types.

I've also got one second question that won't need to be addressed until C# 8.

[BillWagner]

Add notes from committee meeting.

[BillWagner]

This is now ready for final review.

@jskeet Should I tag others for review?

[gafter]

A lot of the work on this feature was done by @agocke. It would be good to get his feedback.

[agocke]

I'll try to take a look. It looks like the definite assignment section needs work. Unlike lambdas, local functions use a more precise definite assignment when not converted to delegates. I'll try to write up some appropriate spec language for this. Overview of the differences is at https://commentout.com/localfunc.html

[RexJaeschke]

@BillWagner I think the header and modifiers rules needs tweaking:

We have

local_function_header
    : local_function_modifiers? return_type identifier type_parameter_list?
        ( formal_parameter_list? ) type_parameter_constraints_clause*
    ;
local_function_modifiers
    : 'async'
    | 'unsafe'
    ;

That allows a local function to have no modifiers, async, or unsafe, but it does not allow async unsafe, which I think is a valid combination. [In V8, we'll add static, and in V9, extern.]

Following the corresponding pattern for modifiers on methods (14.16.1), the rule should be renamed local_function_modifier (singular), with 0 or more occurrences allowed, with both rules changed, as follows:

local_function_header
    : local_function_modifier* return_type identifier type_parameter_list?
        ( formal_parameter_list? ) type_parameter_constraints_clause*
    ;
local_function_modifier
    : 'async'
    | 'unsafe'
    ;

[BillWagner]

@jskeet Check the final commit, and when you're ready, let's

[jskeet]

Just a few nits left; happy for it to be merged after that.