test.zig

const std = @import("std");
const stdout = std.io.getStdOut().writer();
const print = stdout.print;
const Allocator = std.mem.Allocator;

pub var A: *Allocator = undefined; // allocator

fn hello() !void {
    try print("hello", .{});
}

fn buildFuncMap() type {
    return std.StringArrayHashMap(*const @TypeOf(hello)).init(A);
}

pub fn main() !void {
    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
    defer arena.deinit();
    A = &arena.allocator;

    var funcmap = buildFuncMap();
}

$ zig run scratch.zig
./scratch.zig:13:63: error: unable to evaluate constant expression
    return std.StringArrayHashMap(*const @TypeOf(hello)).init(A);
                                                              ^
./scratch.zig:21:31: note: called from here
    var funcmap = buildFuncMap();
                              ^
./scratch.zig:16:21: note: called from here
pub fn main() !void {
                    ^
./scratch.zig:13:62: note: referenced here
    return std.StringArrayHashMap(*const @TypeOf(hello)).init(A);
                                                             ^
./scratch.zig:21:31: note: referenced here
    var funcmap = buildFuncMap();