[llvm-bugs] [Bug 42311] New: BitStream reader ignores errors

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42311

            Bug ID: 42311
           Summary: BitStream reader ignores errors
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: llvm-bugzilla at jfbastien.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

The bitstream reader handles errors poorly. The has two effects:

 * Bugs in file handling (especially modules) manifest as an "unexpected end of
file" crash
 * Users of clang as a library end up aborting because the code unconditionally
calls `report_fatal_error`

The bitstream reader should be more resilient and return Expected / Error as
soon as an error is encountered, not way late like it does now.

This will take a few changes to get to: propagating the errors is a huge patch!
As we propagate them there's places where we can offer better diagnostics (for
example, the file's name, or where we were in the file / what we were parsing).

Further, many places ignore filesystem errors. Those should be nodiscard.

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