https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111474
Bug ID: 111474
Summary: Consider stripping from the error message the
identifiers of namespaces brought with
using-directives
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mateusz.pusz at gmail dot com
Target Milestone: ---
It would be awesome if the compiler was able to strip the identifiers for
namespaces brought with using-directive in the source code that produces an
error. It does it for some, but not all names already.
If the full names are considered useful in some scenarios (to better point to
the source type in an unknown code base), maybe a command line option for the
verbosity of messages could be introduced?
Let's consider the following example:
```cpp
using namespace mp_units;
using namespace mp_units::si::unit_symbols;
auto q = (5 * m) * (1. * s);
quantity<si::metre / si::second> v1 = q;
quantity<isq::speed[m / s]> v2 = q;
```
`quantity` in the above code is declared in the `mp_units` namespace but in the
following error messages, it is printed without the namespace identifier, which
is awesome. However, the rest of the identifiers from this namespace (even
though it is brought to the current scope with the using-directive are using
this namespace identifier).
```
<source>: In function 'int main()':
<source>:9:41: error: conversion from
'quantity<mp_units::derived_unit<mp_units::si::metre,
mp_units::si::second>(),[...]>' to non-scalar type
'quantity<mp_units::derived_unit<mp_units::si::metre,
mp_units::per<mp_units::si::second> >(),[...]>' requested
9 | quantity<si::metre / si::second> v1 = q;
| ^
<source>:10:36: error: conversion from
'quantity<mp_units::derived_unit<mp_units::si::metre,
mp_units::si::second>(),[...]>' to non-scalar type
'quantity<mp_units::reference<mp_units::isq::speed(),
mp_units::derived_unit<mp_units::si::metre, mp_units::per<mp_units::si::second>
>()>(),[...]>' requested
10 | quantity<isq::speed[m / s]> v2 = q;
| ^
Compiler returned: 1
```
If the compiler was able to strip such namespaces, the error message would look
like this:
```
<source>: In function 'int main()':
<source>:9:41: error: conversion from 'quantity<derived_unit<si::metre,
si::second>(),[...]>' to non-scalar type 'quantity<derived_unit<si::metre,
per<si::second> >(),[...]>' requested
9 | quantity<si::metre / si::second> v1 = q;
| ^
<source>:10:36: error: conversion from 'quantity<derived_unit<si::metre,
si::second>(),[...]>' to non-scalar type 'quantity<reference<isq::speed(),
derived_unit<si::metre, per<si::second> >()>(),[...]>' requested
10 | quantity<isq::speed[m / s]> v2 = q;
| ^
Compiler returned: 1
```
which is much terser and easier to read.
As the user brings the `mp-units` namespace to the current scope, he/she should
not be surprised by the meaning of the shorter type identifiers and would
certainly appreciate easier-to-understand error messages.
Please note that GCC already removed some important information from the error
messages with the `[...]` anyway now, so there is a precedence for preferring
terse error messages already.
Consider also how it would improve error messages for `std::filesystem` of
`std::chrono` in the standard library. As they are quite long, users already
often bring them in the current scope with the using-declaration or use
namespace aliases, which is probably too much for the error message generator
to follow?
