List(string) cannot be coerced with String anymore since 1.0 in Concat

[ion-elgreco]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

Before v1.0 (on 0.20.10)

df_a = pl.DataFrame({
    "foo":['a','b','c']
})
df_b = pl.DataFrame({
    "foo":[['a'],['b','d'],['c']]
})

pl.concat(
    [df_a, df_b],
    how="diagonal_relaxed",
)

shape: (6, 1)
┌────────────┐
│ foo        │
│ ---        │
│ list[str]  │
╞════════════╡
│ ["a"]      │
│ ["b"]      │
│ ["c"]      │
│ ["a"]      │
│ ["b", "d"] │
│ ["c"]      │
└────────────┘

After v1.0 we get this:

InvalidOperationError: cannot cast List type (inner: 'String', to: 'String')

Log output

No response

Issue description

Before you could coerce string and list[string] to list[string], but since v1.0 this is not possible anymore. @stinodego

Expected behavior

Be able to coerce list[string] and string.

Installed versions

--------Version info---------
Polars:               1.0.0
Index type:           UInt32
Platform:             Linux-5.15.153.1-microsoft-standard-WSL2-x86_64-with-glibc2.39
Python:               3.10.12 (main, Jun 28 2024, 11:13:01) [GCC 13.2.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          3.0.0
connectorx:           <not installed>
deltalake:            0.18.0
fastexcel:            0.10.4
fsspec:               2024.2.0
gevent:               <not installed>
great_tables:         <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         1.6.0
numpy:                1.26.4
openpyxl:             3.1.5
pandas:               1.5.3
pyarrow:              16.1.0
pydantic:             2.7.4
pyiceberg:            <not installed>
sqlalchemy:           2.0.31
torch:                2.2.2+cpu
xlsx2csv:             0.8.2
xlsxwriter:           3.2.0

[stinodego]

Thanks for the report. This is intentional, see #16918

I will add an example to make this clear.

[ion-elgreco]

@stinodego ah that's a shame, I really liked how it behaved before :)