anytype does not propogate through functions

[iansimonson]

Given a function which takes an anytype and a function with signature dependent on that type, it's currently an unexpected compiler error to bind a function which takes anytype

Minimal Example:

const AType = struct {};

fn should_match(ctx: anytype, l: AType, r: AType) bool {
    return true;
}

fn foo(
    comptime T: type,
    context: anytype,
    comptime f: fn (context: @TypeOf(context), l: T, r: T) bool,
) void {}
pub fn main() void {
    foo(AType, .{}, should_match);
}

results in:

./anytype.zig:13:21: error: expected type 'fn(struct:13:17, AType, AType) bool', found 'fn(anytype,anytype,anytype) anytype'
    foo(AType, .{}, should_match);
                    ^
./anytype.zig:13:21: note: only one of the functions is generic
    foo(AType, .{}, should_match);
                    ^
./anytype.zig:13:8: note: referenced here
    foo(AType, .{}, should_match);
       ^

A more practical example is the use of std.sort.sort which takes a context. If no context is required it should be possible to pass it an empty struct literal (.{}). However the comparator used would then need to be able to handle the various literals passed through.

e.g. this should work:

const AType = struct { index: usize, };

fn compare_by_index(ignore_context: anytype, lhs: AType, rhs: AType) bool {
    return lhs.index < rhs.index;
}

// impl
var list: []AType = //make a list
std.sort.sort(AType, list, .{}, compare_by_index);

[Vexu]

Related #4828.