Integrated: 8258010: Debug build failure with clang-10 due to -Wdeprecated-copy

[Hao Sun]

On Wed, 23 Dec 2020 01:45:58 GMT, Hao Sun <github.com+16932759+shqking at openjdk.org> wrote:

> 1. '-Wdeprecated-copy'
> As specified in C++11 [1], "the generation of the implicitly-defined
> copy constructor is deprecated if T has a user-defined destructor or
> user-defined copy assignment operator". The rationale behind is the
> well-known Rule of Three [2].
> 
> Introduced since gcc-9 [3] and clang-10 [4], flag '-Wdeprecated-copy'
> warns about the C++11 deprecation of implicitly declared copy
> constructor and assignment operator if one of them is user-provided.
> Defining an explicit copy constructor would suppress this warning.
> 
> The main reason why debug build with gcc-9 or higher succeeds lies in
> the inconsistent warning behaviors between gcc and clang. See the
> reduced code example [5]. We suspect it might be return value
> optimization/copy elision [6] that drives gcc not to declare implicit
> copy constructor for this case.
> 
> Note that flag '-Wdeprecated' in clang-8 and clang-9 would also raise
> warnings for deprecated defintions of copy constructors. However,
> '-Wdeprecated' is not enabled by '-Wall' or '-Wextra'. Hence, clang-8
> and clang-9 are not affected.
> 
> ~~2. '-Wimplicit-int-float-conversion'~~
> ~~Making the conversion explicit would fix it.~~
> 
> ~~Flag '-Wimplicit-int-float-conversion' is first introduced in clang-10.~~
> ~~Therefore clang-8 and clang-9 are not affected. The flag with similar~~
> ~~functionality in gcc is '-Wfloat-conversion', but it is not enabled by~~
> ~~'-Wall' or '-Wextra'. That's why this warning does not apprear when~~
> ~~building with gcc.~~
> 
> [1] https://en.cppreference.com/w/cpp/language/copy_constructor
> [2] https://en.cppreference.com/w/cpp/language/rule_of_three
> [3] https://www.gnu.org/software/gcc/gcc-9/changes.html
> [4] https://releases.llvm.org/10.0.0/tools/clang/docs/ReleaseNotes.html
> [5] https://godbolt.org/z/err4jM
> [6] https://en.wikipedia.org/wiki/Copy_elision#Return_value_optimization
> 
> 
> Note that we have tested with this patch, debug build succeeded with clang-10 on Linux X86-64/AArch64 machines.
> Note that '--with-extra-cxxflags=-Wno-implicit-int-float-conversion' should be added when configuration. It's another issue (See JDK-8259288)

This pull request has now been integrated.

Changeset: 5513f989
Author:    Hao Sun <hao.sun at arm.com>
Committer: Ningsheng Jian <njian at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/5513f989
Stats:     11 lines in 1 file changed: 8 ins; 1 del; 2 mod

8258010: Debug build failure with clang-10 due to -Wdeprecated-copy

Reviewed-by: xliu, kvn, kbarrett

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PR: https://git.openjdk.java.net/jdk/pull/1874