Bug: cannot implicitly cast error union payloads

[fengb]

Basic test cases:

const std = @import("std");
const testing = std.testing;

test "int assignment" {
    var a: anyerror!i32 = @as(i16, 1); // error: expected type 'anyerror!i32', found 'i16'
    _ = a catch {};
}

test "error union assignment" {
    var a: anyerror!i16 = 1;
    var b: anyerror!i32 = a; // error: expected type 'anyerror!i32', found 'anyerror!i16'
    // note: error union payload 'i16' cannot cast into error union payload 'i32'
    _ = b;
}

fn makeErrorUnion(comptime T: type) !T {
    return error.OutOfMemory;
}

fn doesNotWork() !u32 {
    return makeErrorUnion(u16); // error: expected type '@typeOf(doesNotWork).ReturnType.ErrorSet!u32', found '@typeOf(makeError).ReturnType.ErrorSet!u16'
    // note: error union payload 'u16' cannot cast into error union payload 'u32'
}

test "func call" {
    _ = doesNotWork() catch null;
}

fn allocExtended(allocator: std.mem.Allocator, memsize: usize) !u8 {
    return allocator.alignedAlloc(u8, 4096, memsize); // error: expected type '@typeOf(allocExtended).ReturnType.ErrorSet!u8', found '@typeOf(std.mem.Allocator.alignedAlloc).ReturnType.ErrorSet![]align(4096) u8'
    // note: error union payload '[]align(4096) u8' cannot cast into error union payload 'u8'

    // adding try fixes this
    // return try allocator.alignedAlloc(u8, 4096, memsize);
}

test "func call alignment" {
    var buf: [100]u8 = undefined;
    var fba = std.heap.FixedBufferAllocator.init(&buf);
    const allocator = fba.allocator();

    _ = allocExtended(allocator, 123) catch []u8{};
}

I had a hard time reproducing the alignment case with simple assignments.