workaround MSVC compiler bug

[srnwk]

On recent MSVC versions, the thrift compiler fails to build under C++17 or C++20 -- as far as I can tell this is a bug in MSVC.
Splitting that clever "read the whole file into a string"-line into several seems to avoid it, with no functional change.

• Did you create an Apache Jira ticket? (not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[Jens-G]

I use it with VS2022 and had not experienced any issues.

Could you enlighten us what the problem is exactly? "fails fails to build" is a bit too vague (and grammatically questionable :-))

[srnwk]

I use it with VS2022 and had not experienced any issues.

Could you enlighten us what the problem is exactly? "fails fails to build" is a bit too vague (and grammatically questionable :-))

Here's a minimal repro https://godbolt.org/z/b76n9oaKv

<source>(9,102): error C2064: term does not evaluate to a function taking 0 arguments
        std::string const old_file_contents(static_cast<std::ostringstream const&>(std::ostringstream() << old_file.rdbuf()).str());
                                                                                                     ^

When compiling with /std:c++20 (or C++17 with /permissive-), this blows up with the above error.
(There's no reason why it should, as far as I can tell; clang and GCC agree).

[emmenlau]

Its an unlikely problem, but at the same time the fix seems relatively harmless (and makes the code even a bit more readable). I'm not opposed to merging. @Jens-G what do you think?