Invalid SMARTS generated by MolToSmarts

[nielskm]

Describe the bug
Chem.MolToSmarts returns a SMARTS string which is invalid when converted back to a molecule using Chem.MolFromSmarts.

Original SMARTS: [NX3H2+0,NX4H3+;!$([N][!C]);!$([N]*~[#7,#8,#15,#16])]
RDKit SMARTS generated by Chem.MolToSmarts: [N&X3&H2&+0,N&X4&H3&+;!$(N!C);!$(N*~[#7,#8,#15,#16])]

The error is in the $(N!C) part. If I change back to $([N][!C]) the SMARTS become valid.

I have seen this bug with multiple different SMARTS strings, and all seem to involve a removal of square brackets around a !C (or other atoms).

To Reproduce

smarts = [NX3H2+0,NX4H3+;!$([N][!C]);!$([N]*~[#7,#8,#15,#16])]
mol = Chem.MolFromSmarts(smarts)
rdsmarts = Chem.MolToSmarts(mol)
mol2 = Chem.MolFromSmarts(rdsmarts)  # mol2 is None

Expected behavior
The SMARTS returned by Chem.MolToSmarts should be valid.

Configuration (please complete the following information):

• RDKit version: 2021.09.4
• OS: Ubuntu 20.04 on WSL2 (Windows 10)
• Python version: 3.9.10
• Are you using conda? Yes
• If you are using conda, which channel did you install the rdkit from? conda-forge

[greglandrum]

confirmed. Thanks for the bug report.

[greglandrum]

And here's an even simpler simple reproducible:

In [6]: Chem.MolToSmarts(Chem.MolFromSmarts('[!C]'))
Out[6]: '!C'