[llvm-bugs] [Bug 27793] New: Long typenames produce extremely hard to read diagnostics and slow down compilation time

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27793

            Bug ID: 27793
           Summary: Long typenames produce extremely hard to read
                    diagnostics and slow down compilation time
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: vittorio.romeo at outlook.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Long typenames, usually generated by heavy template metaprogramming code,
result in errors that are extremely hard to read and parse. Furthermore, they
slow down compilation time significantly.

Here's a benchmark and example from the boost::di project:
http://melpon.org/wandbox/permlink/7Fh0u2oaQbDmkNV0

The benchmark shows:
* How unnecessarily long and hard-to-understand errors are.
* How typename erasure techniques can improve compilation times (define
TYPENAME_ERASURE to see compilation time improvements).

I've encountered this same issue in one of my projects (ECST) - errors were
impossible to understand before GCC 6 was released. GCC 6's produced errors
pinpoint the issue more accurately, but still produce an enormous amount of
unnecessary output.

I think this is primarily a defect in error reporting. A flag to control long
typename output would be desired and possibly necessary for projects that
require the generation of long typenames.

I also think that having compilation times speed up when erasing typenames
signals some sort of potential compilation optimization for long typenames.

P.S.: gcc has similar issues.

Links:
boost::di -> https://github.com/boost-experimental/di
ECST -> https://github.com/SuperV1234/ecst

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