transaction balancing with a balance assignment not using costs #2039

Reported by Rik Snel on the mail list:

RS:

While I was sorting this all out, I think I may have
stumbled on a bug regarding a three commodity trade where the prices are
specified with @@.

Here is a minimal example, note that the last posting only has a
balance assertion:

2023/01/02 * Trade
        Assets                  AAA-1.1 @@ CCC 2
        Assets                  BBB-1.2 @@ CCC 3
        Expenses:Fees           CCC 0.1
        Assets                  = CCC 4.9

hledger says:

hledger: Error: /home/user/journal:1-5:
1 | 2023-01-02 * Trade
  |     Assets                AAA-1.1
  |     Assets                BBB-1.2
  |     Expenses:Fees         CCC 0.1
  |     Assets                CCC 4.9 = CCC 4.9

Consider adjusting this entry's amounts, adding missing postings,
or recording conversion price(s) with @, @@ or equity postings.

SM:

That's an interesting example. The implicit last amount is constrained in two ways:

1. by the balance assignment (it must be the amount that brings Assets's CCC balance to 4.9)

2. and by transaction balancing (it must be the amount that brings AAA -1.1 @@ CCC 2 + BBB -1.2 @@ CCC 3 + CCC 0.1 to 0).

Ledger interprets the transaction this way:

$ ledger -f a.j print
2023/01/02 * Trade
    Assets                                   AAA-1.1 @@ CCC 2.0
    Assets                                   BBB-1.2 @@ CCC 3.0
    Expenses:Fees                            CCC 0.1
    Assets                                   CCC 4.9 = CCC 4.9

and as you say, hledger accepts this interpretation - if it's given with all amounts explicit.

Is transaction balancing supposed to balance costs, ie convert all amounts to cost before balancing ? I never quite thought of it that way, and it isn't spelled out at https://hledger.org/dev/hledger.html#costs or in Ledger docs, but yes, I think that is the customary behaviour.

So the interpretation above is correct, and yes, this does seem like a hledger bug. Also, the actual error message includes this info which seems like it should mention the costs, instead:

The real postings' sum should be 0 but is: AAA-1.1, BBB-1.2, CCC 5.0