compilers: remove opinionated c++ warning flag

-Wnon-virtual-dtor is not what people think of as a standard warning
flag. It was previously removed from -Wall in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=16190 on the grounds that
people didn't like it and were refusing to use -Wall at all because it
forced this warning. Instead, it is enabled by -Weffc++ which is
typically not enabled and even comes with GCC documentation warnings
stating that the standard library doesn't obey it, and you might need to
`grep -v` and filter out warnings. (!!!)

It doesn't fit into the typical semantics of Meson's warning_level
option, which usually aligns with compiler standard warning levels
rather than a niche ideological warning level.

It was originally added in commit 22af56e05a,
but without any specific rationale included, and has gone unquestioned
since then -- except by the Meson users who see it, assume there is a
finely crafted design behind it, and quietly opt out by rolling their own
warning options with `add_project_arguments('-Wall', ...)`.

Furthermore a GCC component maintainer for the C++ standard library
opened a Meson bug report specially to tell us that this warning flag is
a "dumb option" and "broken by design" and "doesn't warn about the right
thing anyway", thus it should not be used. This is a reasonably
authoritative source that maybe, just maybe, this flag... is too
opinionated to force upon Meson users without recourse. It's gone beyond
opinionated and into the realm of compiler vendors seem to think that
the state of the language would be better if the flag did not exist at
all, whether default or not.

Fixes #11096

docs/markdown/snippets/cxx_warn_args_dtor.md

@@ -0,0 +1,3 @@
+## Flags removed from cpp/objcpp warning level 1
+
+`-Wnon-virtual-dtor` is no longer implied by `meson setup -Dwarning_level=1`.

mesonbuild/compilers/cpp.py

@@ -190,7 +190,7 @@ class ClangCPPCompiler(ClangCompiler, CPPCompiler):
         CPPCompiler.__init__(self, ccache, exelist, version, for_machine, is_cross,
                              info, exe_wrapper, linker=linker, full_version=full_version)
         ClangCompiler.__init__(self, defines)
-        default_warn_args = ['-Wall', '-Winvalid-pch', '-Wnon-virtual-dtor']
+        default_warn_args = ['-Wall', '-Winvalid-pch']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
                           '2': default_warn_args + ['-Wextra'],
@@ -310,7 +310,7 @@ class ArmclangCPPCompiler(ArmclangCompiler, CPPCompiler):
         CPPCompiler.__init__(self, ccache, exelist, version, for_machine, is_cross,
                              info, exe_wrapper, linker=linker, full_version=full_version)
         ArmclangCompiler.__init__(self)
-        default_warn_args = ['-Wall', '-Winvalid-pch', '-Wnon-virtual-dtor']
+        default_warn_args = ['-Wall', '-Winvalid-pch']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
                           '2': default_warn_args + ['-Wextra'],
@@ -356,7 +356,7 @@ class GnuCPPCompiler(GnuCompiler, CPPCompiler):
         CPPCompiler.__init__(self, ccache, exelist, version, for_machine, is_cross,
                              info, exe_wrapper, linker=linker, full_version=full_version)
         GnuCompiler.__init__(self, defines)
-        default_warn_args = ['-Wall', '-Winvalid-pch', '-Wnon-virtual-dtor']
+        default_warn_args = ['-Wall', '-Winvalid-pch']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
                           '2': default_warn_args + ['-Wextra'],
@@ -536,7 +536,7 @@ class IntelCPPCompiler(IntelGnuLikeCompiler, CPPCompiler):
         IntelGnuLikeCompiler.__init__(self)
         self.lang_header = 'c++-header'
         default_warn_args = ['-Wall', '-w3', '-diag-disable:remark',
-                             '-Wpch-messages', '-Wnon-virtual-dtor']
+                             '-Wpch-messages']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
                           '2': default_warn_args + ['-Wextra'],

mesonbuild/compilers/objcpp.py

@@ -63,7 +63,7 @@ class GnuObjCPPCompiler(GnuCompiler, ObjCPPCompiler):
         ObjCPPCompiler.__init__(self, ccache, exelist, version, for_machine, is_cross,
                                 info, exe_wrapper, linker=linker, full_version=full_version)
         GnuCompiler.__init__(self, defines)
-        default_warn_args = ['-Wall', '-Winvalid-pch', '-Wnon-virtual-dtor']
+        default_warn_args = ['-Wall', '-Winvalid-pch']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
                           '2': default_warn_args + ['-Wextra'],
@@ -81,7 +81,7 @@ class ClangObjCPPCompiler(ClangCompiler, ObjCPPCompiler):
         ObjCPPCompiler.__init__(self, ccache, exelist, version, for_machine, is_cross,
                                 info, exe_wrapper, linker=linker, full_version=full_version)
         ClangCompiler.__init__(self, defines)
-        default_warn_args = ['-Wall', '-Winvalid-pch', '-Wnon-virtual-dtor']
+        default_warn_args = ['-Wall', '-Winvalid-pch']
         self.warn_args = {'0': [],
                           '1': default_warn_args,
                           '2': default_warn_args + ['-Wextra'],