[llvm-bugs] [Bug 34062] New: -Wdouble-promotion false positive on C++11 brace initialization

via llvm-bugs llvm-bugs at lists.llvm.org
Fri Aug 4 01:55:28 PDT 2017


            Bug ID: 34062
           Summary: -Wdouble-promotion false positive on C++11 brace
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: abigail.buccaneer at gmail.com
                CC: llvm-bugs at lists.llvm.org

Clang's -Wdouble-promotion flags all cases where a float is implicitly
converted to a double. The conversion is lossless but often unintentional and
can be a performance issue. When the conversion is intentional, the solution
is to explicitly cast with static_cast or a C-style cast. Unfortunately, a
C++11-style brace-init of double is flagged as an implicit conversion. This
should be considered an explicit conversion for the sake of -Wdouble-promotion
as some modern C++ guidelines recommend using brace-init for casting when the
cast cannot change the value.

    $ cat test.cpp
    double f() { return double{1.f}; }
    $ clang++-6.0 -c -std=c++2a -Wdouble-promotion -Werror test.cpp
    test.cpp:1:28: error: implicit conversion increases floating-point
precision: 'float' to 'double'
    double f() { return double{1.f}; }
    1 error generated.
    $ clang++-6.0 --version | head -n 1
    clang version 6.0.0-svn309896-1~exp1 (trunk)

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