Summary:

When attempting to add a non-ranged descriptor with a start/end range of [0,0] via AddWalletDescriptor, an error may appear:

UpdateWalletDescriptor: new range must include current range = [0,0].

This is unexpected for a non-ranged descriptor, as [0,0] should effectively mean “no range.” In some cases, changing the end to 1 (i.e. [0,1]) circumvents the issue, but that implies a single-index range rather than a truly non-ranged descriptor.

Further debugging revealed this seems tied to:

1. Multiple calls to AddWalletDescriptor for the same descriptor.
2. Internal handling of descriptor ranges (the wallet expects a valid range for ranged descriptors, but still enforces checks on start/end even if the descriptor is meant to be non-ranged).
3. A possible bug in UpdateWalletDescriptor when it encounters repeated or zero-width ranges.

What was expected:

• Specifying [0,0] for a non-ranged descriptor (or not specifying a range at all) should not fail.
• A truly non-ranged descriptor should not require these range checks.
• AddWalletDescriptor should be idempotent

What actually happens:

• Repeatedly adding a descriptor (or calling it in a way that triggers UpdateWalletDescriptor) with [0,0] can cause the wallet to complain that the “new range must include current range = [0,0]”.

Why this matters:

• Non-ranged descriptor workflows should be consistent and should not require a dummy range.
• The wallet code should either cleanly ignore range parameters for non-ranged descriptors or handle a [0,0] range gracefully.

Proposed fix or investigation:

• Review AddWalletDescriptor and UpdateWalletDescriptor logic to ensure zero-width ([0,0]) or absent ranges are handled properly for non-ranged descriptors.
• Confirm if the internal code should treat [0,0] as a valid non-ranged descriptor or if that scenario should simply be disallowed/ignored in favor of no range parameters at all.
• Add tests to cover repeatedly adding the same descriptor and verifying correct behavior for non-ranged descriptors.
• Change the wallet::WalletDescriptor to have std::optional<std::pair<start,end>> to differentiate unranged v.s. empty ranged descriptors.

This appears to be a minor bug, but it can lead to confusion and error messages when using or experimenting with non-ranged descriptors in custom wallet workflows.