compiler.run produces a runresult with undefined stdout and stderr rather than an error on use

[apache-hb]

Describe the bug
The result of compiler.run has a stdout field that contains UNDEFINED

To Reproduce

project(
  'configure-file-int-repro',
  'c',
  meson_version : '>= 1.3.0',
  version : '0.1',
  default_options : ['warning_level=3'],
)

cc = meson.get_compiler('c')

result = cc.run(
'''
int main() {
  printf("5\n");
  return 0;
}
'''
)

cdata = configuration_data()
cdata.set('VALUE', result.stdout().strip())

config = configure_file(
  output : 'thing.h',
  configuration : cdata,
)

sources = [
  'configure_file_int_repro.c',
]

exe = executable(
  'configure-file-int-repro',
  sources,
  install : true,
)

#include <stdio.h>

#include "thing.h"

#define PROJECT_NAME "configure-file-int-repro"

#if VALUE != 5
#error "VALUE is not defined"
#endif

int main(int argc, char **argv) {
    if (argc != 1) {
        printf("%s takes no arguments.\n", argv[0]);
        return 1;
    }
    printf("This is project %s.\n", PROJECT_NAME);
    return 0;
}

This fails with ../configure_file_int_repro.c:8:2: error: "VALUE is not defined" and thing.h contains

/*
 * Autogenerated by the Meson build system.
 * Do not edit, your changes will be lost.
 */

#pragma once

#define VALUE UNDEFINED

Expected behavior
Using the runresult of a failed run should produce an error.

system parameters

elliothb@ELLIOT-SERVER:~/github/configure-file-int-repro$ meson --version
1.10.0
elliothb@ELLIOT-SERVER:~/github/configure-file-int-repro$ ninja --version
1.10.1
elliothb@ELLIOT-SERVER:~/github/configure-file-int-repro$ uname -a
Linux ELLIOT-SERVER 6.16.5-WSL2-STABLE+ #1 SMP PREEMPT_DYNAMIC Fri Sep  5 02:14:51 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
elliothb@ELLIOT-SERVER:~/github/configure-file-int-repro$ python3 --version
Python 3.10.12

[eli-schwartz]

This is documented and intended behavior:

runresult.compiled()

If true, the compilation succeeded, if false it did not and the other methods return unspecified data. This is only available for compiler.run() results.

It is undefined behavior to access the results without fits checking. Accordingly meson literally prints "undefined" (behavior).

Arguably it's a weird decision to make it Undefined Behavior instead of defining it as an error. I don't like it but I wasn't around when this was decided. Changing it would be a breaking behavior change.

@thesamesam proof that if you live long enough you become the thing you hate. We are the C/C++ standard, or something.

... We should deprecate this behavior and make it raise an unconditional warning, targeted for removal in meson 2.0. I've known it for a long time, just never got around to it.