[llvm-bugs] [Bug 52466] New: clang-format not idempotent with long comment containing tabs

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52466

            Bug ID: 52466
           Summary: clang-format not idempotent with long comment
                    containing tabs
           Product: clang
           Version: 13.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Formatter
          Assignee: unassignedclangbugs at nondot.org
          Reporter: felix.widmaier at tue.mpg.de
                CC: djasper at google.com, klimek at google.com,
                    llvm-bugs at lists.llvm.org

Created attachment 25437
  --> https://bugs.llvm.org/attachment.cgi?id=25437&action=edit
Example file for which the described issue occurs.

The output of `clang-format` changes multiple times on this example containing
a long comment with tabs (this is extracted from a real-world example, reduced
to the offending line):

> // 	//								for(auto const& value: sze) cout << value<<" ";

(the file is also added as attachment).

I am testing with version "13.0.0 (https://github.com/llvm/llvm-project
d7b669b3a30345cfcdb2fde2af6f48aa4b94845d)".


Expected behaviour
==================

Calling `clang-format` a second time on any file should not result in any
additional changes, i.e. the output of

    $ clang-format file.cpp | clang-format --dry-run

should always be empty.


Actual behaviour
================

On the given example, 

    $ clang-format file.cpp | clang-format --dry-run

results in the following output:

> <stdin>:1:37: warning: code should be clang-formatted [-Wclang-format-violations]
> //      //                                                              for(auto const& value: sze)
>                                                                                               ^
> <stdin>:1:42: warning: code should be clang-formatted [-Wclang-format-violations]
> //      //                                                              for(auto const& value: sze)
>                                                                                                    ^
> <stdin>:2:1: warning: code should be clang-formatted [-Wclang-format-violations]
> // cout << value<<" ";
> ^

Actually, I have to call it four times until there are no changes anymore.


One run
-------

    $ clang-format file.cpp
    //  //                                                             
for(auto const& value: sze)
    // cout << value<<" ";

Two runs
--------

    $ clang-format file.cpp | clang-format
    //  //                                                             
for(auto const& value:
    // sze) cout << value<<" ";

Three runs
----------

    $ clang-format file.cpp | clang-format | clang-format
    //  //                                                             
for(auto const&
    // value: sze) cout << value<<" ";

Four runs
---------

    $ clang-format file.cpp | clang-format | clang-format | clang-format
    //  //                                                             
for(auto
    // const& value: sze) cout << value<<" ";

>From here there are no more changes.

I noticed that when I replace the tabs with spaces, I don't have this issue.

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