lcov considers Coverage's function data inconsistent with its line data #2394

Describe the bug

Calling lcov --add-tracefile on a file generated by coverage lcov for executing a Python file with a function that is not called generates errors about "inconsistent" data in lcov complaining that the "line data" does not match the "function data".

To Reproduce

Locally I used lcov 2.6.0, but this reproduces on lcov 2.4-3 from the Ubuntu Resolute repos (26.04, as in GitHub's ubuntu-26.04) too.

File f.py:

def f():
    return True

coverage run -- f.py
coverage lcov
lcov -a coverage.lcov

This complains about:

Combining tracefiles.
... found 1 files to aggregate.
Reading tracefile coverage.lcov
lcov: ERROR: (corrupt) unable to read trace file 'coverage.lcov': lcov: ERROR: (inconsistent) "f.py":1: function 'f' is not hit but line 1 is.
   [... truncated ...]

The generated coverage.lcov file is

SF:f.py
DA:1,1
DA:2,0
LF:2,
FH:1
FN:1,2,f
FNDA:0,f
FNF:1
FNH:0
end_of_record

Answer the questions below:

1. Python 3.13
2. coverage==7.17.3
3. no other Python packages
4. repro above

Expected behavior

Combination should be successful.

Additional context

I'm not familiar with the coverage or lcov internals, but I suspect that changing the "function" information to register that the function starts on the line that contains the first non-signature line, as opposed to the line that includes the def statement would be suitable.

Explicitly: in my generated coverage.lcov file above, if I change the FN:1,2,f line to FN:2,2,f, lcov is happy to combine it. I think this is semantically valid: line coverage still shows that the def f(): line is hit without "contradicting" the function-hit data.