Clang-tidy should not guess compile commands

[t-a-james]

If clang-tidy fails to find an entry in the compile commands database it guesses at some flags. For my project (and, I suspect, for any non-trivial project) these flags are always catastrophically wrong. clang-tidy then gives back garbage results.

I would like clang-tidy to simply error out if it's given a compile commands database and a file, but fails to find an entry for the file.

That would be a change in default behaviour. I think the new behaviour would be simpler and less unexpected than the current behaviour, but if that's considered too big a change we could introduce a flag to re-instate the old behaviour, or keep the old default and hide the new behaviour behind a flag.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Tom (t-a-james)

If clang-tidy fails to find an entry in the compile commands database it guesses at some flags. For my project (and, I suspect, for any non-trivial project) these flags are always catastrophically wrong. clang-tidy then gives back garbage results.

I would like clang-tidy to simply error out if it's given a compile commands database and a file, but fails to find an entry for the file.

That would be a change in default behaviour. I think the new behaviour would be simpler and less unexpected than the current behaviour, but if that's considered too big a change we could introduce a flag to re-instate the old behaviour, or keep the old default and hide the new behaviour behind a flag.

[vbvictor]

For clang-tidy-diff there is a flag -only-check-in-db.
Maybe we need to add a policy on what to do if the file is not found in database:

• silently skip file
• show error and skip file
• guess flags and process file (default "legacy")

Particular variants of behavior can be discussed further, but I'm in favor of multiple values in a flag instead of binary flag.

[t-a-james]

The fragmentation between the different ways to run clang-tidy is something I'd like to address, but not in this ticket.

I think you're right about the flag