sane inferred error set name

[DerryAlex]

#21345

const std = @import("std");

fn div(a: i32, b: i32) !i32 {
    return if (b == 0) error.DivisionByZero else @divFloor(a, b);
}

fn wrap(op: anytype, a: i32, b: i32) !i32 {
    return op(i32, a, b);
}

pub fn main() !void {
    const x = std.math.divFloor(i32, 1, 0);
    std.log.debug("{}", .{@TypeOf(x)});
    std.log.debug("{}", .{@TypeOf(std.math.divFloor)});
    const y = div(1, 0);
    std.log.debug("{}", .{@TypeOf(y)});
    std.log.debug("{}", .{@TypeOf(div)});
    const z = wrap(std.math.divFloor, 1, 0);
    std.log.debug("{}", .{@TypeOf(z)});
    std.log.debug("{}", .{@TypeOf(wrap)});
}

Before

debug: @typeInfo(@typeInfo(@TypeOf(math.divFloor__anon_1601)).@"fn".return_type.?).error_union.error_set!i32
debug: fn (comptime type, anytype, anytype) @typeInfo(@typeInfo(@TypeOf(math.divFloor)).@"fn".return_type.?).error_union.error_set!anytype
debug: @typeInfo(@typeInfo(@TypeOf(mytest.div)).@"fn".return_type.?).error_union.error_set!i32
debug: fn (i32, i32) @typeInfo(@typeInfo(@TypeOf(mytest.div)).@"fn".return_type.?).error_union.error_set!i32
debug: @typeInfo(@typeInfo(@TypeOf(mytest.wrap__anon_1658)).@"fn".return_type.?).error_union.error_set!i32
debug: fn (anytype, i32, i32) @typeInfo(@typeInfo(@TypeOf(mytest.wrap)).@"fn".return_type.?).error_union.error_set!i32

After

debug: error{DivisionByZero,Overflow}!i32
debug: fn (comptime type, anytype, anytype) @typeInfo(@typeInfo(@TypeOf(math.divFloor)).@"fn".return_type.?).error_union.error_set!anytype
debug: error{DivisionByZero}!i32
debug: fn (i32, i32) error{DivisionByZero}!i32
debug: error{DivisionByZero,Overflow}!i32
debug: fn (anytype, i32, i32) @typeInfo(@typeInfo(@TypeOf(mytest.wrap)).@"fn".return_type.?).error_union.error_set!i32

[mlugg]

This approach is not acceptable. Whether an IES is resolved when a compile error is reported depends on analysis order, which would make errors change based on seemingly arbitrary factors, and would make errors non-deterministic once we introduce threading to semantic analysis.