RFR: 8258010: Debug build failure with clang-10 due to -Wdeprecated-copy and -Wimplicit-int-float-conversion

[Kim Barrett]

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/AArch64 machines.

I think the two issues described here are distinct and should be dealt
with in separate bugs and PRs.  Their only relation is that both arise
with using clang-10.  But they are very different problems, in very
different parts of the code, and probably ought to be reviewed by
folks from different teams.

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