bug#37633: Column part interpreted wrong in compilation mode

[Bernd Paysan via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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Am Sonntag, 6. Oktober 2019, 21:16:47 CEST schrieb Eli Zaretskii:
> > From: Bernd Paysan <bernd@net2o.de>
> > Cc: anton@mips.complang.tuwien.ac.at, 37633@debbugs.gnu.org
> > Date: Sun, 06 Oct 2019 21:02:14 +0200
> > 
> > if the editor mistook a UTF-8 file for an iso8859-1, it will see an
> > UTF-8 string "äöü" (6 bytes UTF-8) as "äöü" (6 bytes iso8859-1).
> > But it's still 6 bytes.
> 
> Not inside the Emacs buffer, it isn't.

I created a unicode file:

void main() {
        char *b="ha", *c="ho";
        printf("test %i", b);
        printf("testäöü %i", c);
}

I loaded this into emacs, and reverted the buffer using iso8859-1 coding 
(simulating a wrongly detected encoding).

It then looks like this:

void main() {
	char *b="ha", *c="ho";
	printf("test %i", b);
	printf("testäöü %i", c);
}

I compiled it with gcc -Wall test-utf8.c into a compile-mode buffer.

-*- mode: compilation; default-directory: "~/tmp/" -*-
Compilation started at Sun Oct  6 21:18:24

gcc -Wall test-utf.c 
test-utf.c:1:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
    1 | void main() {
      |      ^~~~
test-utf.c: In function ‘main’:
test-utf.c:3:2: warning: implicit declaration of function ‘printf’ [-
Wimplicit-function-declaration]
    3 |  printf("test %i", b);
      |  ^~~~~~
test-utf.c:3:2: warning: incompatible implicit declaration of built-in 
function ‘printf’
test-utf.c:1:1: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’
  +++ |+#include <stdio.h>
    1 | void main() {
test-utf.c:3:16: warning: format ‘%i’ expects argument of type ‘int’, but 
argument 2 has type ‘char *’ [-Wformat=]
    3 |  printf("test %i", b);
      |               ~^   ~
      |                |   |
      |                int char *
      |               %s
test-utf.c:4:22: warning: format ‘%i’ expects argument of type ‘int’, but 
argument 2 has type ‘char *’ [-Wformat=]
    4 |  printf("testäöü %i", c);
      |                     ~^   ~
      |                      |   |
      |                      int char *
      |                     %s

Compilation finished at Sun Oct  6 21:18:24

If I click on the test-utf.c:4:22 label, I get exactly where I want to: On the 
i of %i.

If I revert this buffer with the correct encoding utf-8-unix, then it still 
navigates to the i of %i, so it's all agnostic to whether the encoding 
detected was correct or wrong.

-- 
Bernd Paysan
"If you want it done right, you have to do it yourself"
net2o id: kQusJzA;7*?t=uy@X}1GWr!+0qqp_Cn176t4(dQ*
https://net2o.de/

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