Re: [analyzer PATCH] Restore bootstrap with g++ 4.8.

[Jonathan Wakely <jwakely@redhat.com>]

On 14/06/24 10:36 +0100, Jonathan Wakely wrote:
>On 07/06/24 19:40 +0100, Roger Sayle wrote:
>>
>>This patch restores bootstrap when using g++ 4.8 as a host compiler.
>>Returning a std::unique_ptr requires a std::move on C++ compilers
>>(pre-C++17) that don't guarantee copy elision/return value optimization.
>
>It doesn't though.  The C++17 guaranteed copy elision rules are not
>relevant here.  This is about lookup for the constructor used in the
>return statement, and whether that lookup considers the variable to be
>an lvalue or an rvalue.  C++11 already says this is valid:
>
>i#include <memory>
>
>std::unique_ptr<int> f()
>{
>    std::unique_ptr<int> m;
>    return m;
>}
>
>See C++11 12.8 [class.copy] p31:
>
>  This elision of copy/move operations, called copy elision, is
>  permitted in the following circumstances (which may be combined to
>  eliminate multiple copies):
>
>  - in a return statement in a function with a class return type, when
>  the expression is the name of a non-volatile automatic object (other
>  than a function or catch-clause parameter) with the same cv-
>  unqualified type as the function return type, the copy/move
>  operation can be omitted by constructing the automatic object
>  directly into the function’s return value
>
>and then p32:
>
>  When the criteria for elision of a copy operation are met or would
>  be met save for the fact that the source object is a function
>  parameter, and the object to be copied is designated by an lvalue,
>  overload resolution to select the constructor for the copy is first
>  performed as if the object were designated by an rvalue.
>
>The constructor isn't required to be elided in C++11, but the compiler
>is required to use a move constructor instead of a copy constructor,
>if a move constructor is available. So you don't need to use std::move
>on the return value.
>
>And the code above compiles fine with gcc 4.8.5 on CentOS 7 (and even
>with 4.7.1).
>
>So the std::move calls you've added are redundant, and will cause
>-Wredundant-move warnings.
>
>What's the error you were seeing?

I can reproduce it:

/home/test/src/gcc/gcc/analyzer/constraint-manager.cc: In member function ‘std::unique_ptr<text_art::widget> ana::equiv_class::make_dump_widget(const text_art::dump_widget_info&, unsigned int) const’:
/home/test/src/gcc/gcc/analyzer/constraint-manager.cc:1179:10: error: cannot bind ‘std::unique_ptr<text_art::tree_widget>’ lvalue to ‘std::unique_ptr<text_art::tree_widget>&&’
    return ec_widget;
           ^

Odd ... I'm looking into it ...