[Development] On the use of the inline keyword

[Marc Mutz]

Hi,

Every now and then we get a sporadic MinGW error because someone writes 
the moral equivalent of the following:

    class Q_FOO_EXPORT QMeep {
        ~~~~
        QBar bar() const;
        ~~~~
    };
    inline QBar QMeep::bar() const;

Resulting in something like this:

     qmeep.h: error: 'QBar QMeep::bar() const' redeclared without 
dllimport attribute after being referenced with dll linkage [-Werror]

The last time I remember this came up was in 5.8 
(https://bugreports.qt.io/browse/QTBUG-56459). It's so sporadic that 
Ivan didn't manage to repro when I pointed this out as a potential problem in 
API review, until it suddenly hit today in code that was unchanged since Qt 
5.0: https://testresults.qt.io/coin/integration/qt/qtbase/tasks/1698267962

I don't claim to know what's the cause (it's probably use of the 
function in inline implementation), but I do know the fix, and that's to 
put `inline` on the _declaration_, but _not_ the definition. While it 
doesn't hurt to put it on the definition, it's exactly this practice 
that lets other platforms compile this code that MinGW then out of a 
sudden starts to complain about. By _not_ putting `inline` on the 
definition, only on the declaration, we cause all platforms to complain 
if we get it wrong ("multiple definition errors at link time" or "inline 
function not defined").

So, if we want this as a minimal-complexity rule:

- the `inline` keyword goes _only_ on declarations, never on definitions
- the `inline` keyword should be omitted on the following declarations:
   - of constexpr functions (they're implicitly inline)
   - of consteval functions (ditto)
   - of in-class-body member function definitions (they, too, are 
implicitly inline)

Please add this to your things to look out for in reviews.

Thanks,
Marc

-- 
Marc Mutz <marc.mutz at qt.io>
Principal Software Engineer

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