[llvm-bugs] [Bug 42427] New: clanc-cl with -Xclang -isystem forgets meaning of `or`, `and`, etc.

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42427

            Bug ID: 42427
           Summary: clanc-cl with -Xclang -isystem forgets meaning of
                    `or`, `and`, etc.
           Product: clang
           Version: 8.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: artur.ryt at r2software.pl
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Suppose you have two files, `main.cpp`

#include <lib.h>
int main(){}

and external library `external/include/foo.h`:

#pragma once
bool foo(int x) {
    int y;  // generate some warning to check whenever -isystem works
    return x or x;
}

You can compile it with `clang-cl /Iexternal\include /W4 test.cpp` and it will
issue an warning about `y`.
external\include\lib.h(3,9): warning: unused variable 'y' [-Wunused-variable]

You can try to suppress it by adding `-Xclang -isystemexternal\include`, but it
will lead to strange error:

> clang-cl /Iexternal\include /W4 test.cpp -Xclang -isystemexternal\include
In file included from test.cpp:1:
external\include\lib.h(4,13): error: expected ';' after return statement
    return x or x;
            ^
1 error generated.

It turns out that alternative tokens are breaking compilation (only) for
headers which are under `-isystem` flagged directories.
`or` in main.cpp always compiles.
I've also tested `and` and `bitand` out of
https://en.cppreference.com/w/cpp/language/operator_alternative, same result.

You can work it around with preprocessor "-Dor=||":
> clang-cl /Iexternal\include /W4 test.cpp -Xclang -isystemexternal\include "-Dor=||"

PS. I hope I've tagged this issue correctly to frontend category.

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