[Bug c++/89550] New: Spurious array-bounds warning when using __PRETTY_FUNCTION__ as a string_view.

[aaron at bestateless dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89550

            Bug ID: 89550
           Summary: Spurious array-bounds warning when using
                    __PRETTY_FUNCTION__ as a string_view.
           Product: gcc
           Version: 8.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aaron at bestateless dot com
  Target Milestone: ---

Created attachment 45870
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45870&action=edit
Preprocessed source.

Compiler Information:
  $ g++ -v
  Using built-in specs.
  COLLECT_GCC=g++
  COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/8.2.1/lto-wrapper
  Target: x86_64-pc-linux-gnu
  Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --enable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
--enable-cet=auto
  Thread model: posix
  gcc version 8.2.1 20181127 (GCC)

Example:
  #include <string>

  void foo() {
      std::string_view s(__PRETTY_FUNCTION__);
      s.remove_prefix(s.find_last_of(' '));
      std::string().find(s.data());
  }

Command Line:
  g++ -std=c++17 -Warray-bounds -Werror -O2 -c test.cpp -o test.o

  Does not fail for optimization levels -O0, -O1, or -O3. Only fails for -O2.

Expected:
  Compiles successfully.

  From experiments on https://godbolt.org/ this code compiles on 7.1, 7.2, 7.3,
  7.4, 8.1, 8.2, and trunk. It fails to compile on 8.2.1 and 8.3.0.

Actual:
  test.cpp: In function ‘void foo()’:
  test.cpp:3:6: warning: offset ‘-1’ outside bounds of constant string
[-Warray-bounds]
   void foo() {
        ^~~

Details:
  This does not fail if the line with "remove_prefix" is commented out. In this
  case __PRETTY_FUNCTION__ does contain a space, so it should not get npos from
  the "find_last_of" call. Also does not fail if the call to "find" is
commented
  out. It is the combination of both lines that causes the warning.

  I can get this to fail in the same way on -O3 if I wrap the function in a
  namespace name that is six characters in length or more. If it is five
  characters or less then it does compile on -O3.

  This example succeeds with "g++ -std=c++17 -Warray-bounds -Werror -O3":

    #include <string>

    namespace nnnnn
    {

    void foo() {
        std::string_view s(__PRETTY_FUNCTION__);
        s.remove_prefix(s.find_last_of(' '));
        std::string().find(s.data());
    }

    }

  This example fails with "g++ -std=c++17 -Warray-bounds -Werror -O3" (note the
  namespace name is longer by one character):

    #include <string>

    namespace nnnnnn
    {

    void foo() {
        std::string_view s(__PRETTY_FUNCTION__);
        s.remove_prefix(s.find_last_of(' '));
        std::string().find(s.data());
    }

    }