[CMake] Set correct flags for clang-cl (#278)

clang-cl produce binaries with MSVC ABI and wants to be as flag-compatible with pure MSVC as possible, so this leads to all sorts of weird cases.

clang-cl alias /Wall as clang's -Weverything which is way too verbose, so it needs /W3 like pure MSVC.
clang-cl only understand GCC style warning flags (-W[no]blah) and just silent drop MSVC style warning flags (/wd[num]).
clang-cl needs MSVC define flags since it is consuming the same header files as pure MSVC.
CMake set CMAKE_CXX_COMPILER_ID as Clang when clang-cl is detected, so need extra if (MSVC) to differentiate it.

We are not doing clang-cl specialization in Bazel as currently there is no reliable way to detect clang-cl in Bazel..

This PR should be NFC for LLVM/GCC users on Unix platforms.

Other changes:

Add ABSL_ prefix to variable names to avoid name collision in CMake.

6 files changed, 346 insertions, 169 deletions

diff --git a/.gitignore b/.gitignore
index 31f33741408d..d54fa5a91fe6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,5 +11,5 @@ CMakeLists.txt.user
 # Ignore VS Code files
 .vscode/*
 # Ignore generated python artifacts
-copts/copts.pyc
+*.pyc
 copts/__pycache__/
diff --git a/absl/copts/AbseilConfigureCopts.cmake b/absl/copts/AbseilConfigureCopts.cmake
index f68895d917fd..5084958c3581 100644
--- a/absl/copts/AbseilConfigureCopts.cmake
+++ b/absl/copts/AbseilConfigureCopts.cmake
@@ -5,26 +5,33 @@ set(ABSL_LSAN_LINKOPTS "")
 set(ABSL_HAVE_LSAN OFF)
 
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
-  set(ABSL_DEFAULT_COPTS "${GCC_FLAGS}")
-  set(ABSL_TEST_COPTS "${GCC_FLAGS};${GCC_TEST_FLAGS}")
-  set(ABSL_EXCEPTIONS_FLAG "${GCC_EXCEPTIONS_FLAGS}")
+  set(ABSL_DEFAULT_COPTS "${ABSL_GCC_FLAGS}")
+  set(ABSL_TEST_COPTS "${ABSL_GCC_FLAGS};${ABSL_GCC_TEST_FLAGS}")
+  set(ABSL_EXCEPTIONS_FLAG "${ABSL_GCC_EXCEPTIONS_FLAGS}")
 elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
   # MATCHES so we get both Clang and AppleClang
-  set(ABSL_DEFAULT_COPTS "${LLVM_FLAGS}")
-  set(ABSL_TEST_COPTS "${LLVM_FLAGS};${LLVM_TEST_FLAGS}")
-  set(ABSL_EXCEPTIONS_FLAG "${LLVM_EXCEPTIONS_FLAGS}")
-  if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
-    # AppleClang doesn't have lsan
-    # https://developer.apple.com/documentation/code_diagnostics
-    if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 3.5)
-      set(ABSL_LSAN_LINKOPTS "-fsanitize=leak")
-      set(ABSL_HAVE_LSAN ON)
+  if (MSVC)
+    # clang-cl is half MSVC, half LLVM
+    set(ABSL_DEFAULT_COPTS "${ABSL_CLANG_CL_FLAGS}")
+    set(ABSL_TEST_COPTS "${ABSL_CLANG_CL_FLAGS};${ABSL_CLANG_CL_TEST_FLAGS}")
+    set(ABSL_EXCEPTIONS_FLAG "${ABSL_CLANG_CL_EXCEPTIONS_FLAGS}")
+  else()
+    set(ABSL_DEFAULT_COPTS "${ABSL_LLVM_FLAGS}")
+    set(ABSL_TEST_COPTS "${ABSL_LLVM_FLAGS};${ABSL_LLVM_TEST_FLAGS}")
+    set(ABSL_EXCEPTIONS_FLAG "${ABSL_LLVM_EXCEPTIONS_FLAGS}")
+    if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+      # AppleClang doesn't have lsan
+      # https://developer.apple.com/documentation/code_diagnostics
+      if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 3.5)
+        set(ABSL_LSAN_LINKOPTS "-fsanitize=leak")
+        set(ABSL_HAVE_LSAN ON)
+      endif()
     endif()
   endif()
 elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
-  set(ABSL_DEFAULT_COPTS "${MSVC_FLAGS}")
-  set(ABSL_TEST_COPTS "${MSVC_FLAGS};${MSVC_TEST_FLAGS}")
-  set(ABSL_EXCEPTIONS_FLAG "${MSVC_EXCEPTIONS_FLAGS}")
+  set(ABSL_DEFAULT_COPTS "${ABSL_MSVC_FLAGS}")
+  set(ABSL_TEST_COPTS "${ABSL_MSVC_FLAGS};${ABSL_MSVC_TEST_FLAGS}")
+  set(ABSL_EXCEPTIONS_FLAG "${ABSL_MSVC_EXCEPTIONS_FLAGS}")
 else()
   message(WARNING "Unknown compiler: ${CMAKE_CXX_COMPILER}.  Building with no default flags")
   set(ABSL_DEFAULT_COPTS "")
@@ -42,4 +49,4 @@ elseif(NOT "${CMAKE_CXX_STANDARD}")
   set(ABSL_CXX_STANDARD 11)
 else()
   set(ABSL_CXX_STANDARD "${CMAKE_CXX_STANDARD}")
-endif()
\ No newline at end of file
+endif()
diff --git a/absl/copts/GENERATED_AbseilCopts.cmake b/absl/copts/GENERATED_AbseilCopts.cmake
index d02ea193c1c1..9031bfa5daf7 100644
--- a/absl/copts/GENERATED_AbseilCopts.cmake
+++ b/absl/copts/GENERATED_AbseilCopts.cmake
@@ -3,11 +3,87 @@
 # (1) Edit absl/copts/copts.py.
 # (2) Run `python <path_to_absl>/copts/generate_copts.py`.
 
-list(APPEND GCC_EXCEPTIONS_FLAGS
+list(APPEND ABSL_CLANG_CL_EXCEPTIONS_FLAGS
+    "/U_HAS_EXCEPTIONS"
+    "/D_HAS_EXCEPTIONS=1"
+    "/EHsc"
+)
+
+list(APPEND ABSL_CLANG_CL_FLAGS
+    "/W3"
+    "-Wno-c++98-compat-pedantic"
+    "-Wno-conversion"
+    "-Wno-covered-switch-default"
+    "-Wno-deprecated"
+    "-Wno-disabled-macro-expansion"
+    "-Wno-double-promotion"
+    "-Wno-comma"
+    "-Wno-extra-semi"
+    "-Wno-extra-semi-stmt"
+    "-Wno-packed"
+    "-Wno-padded"
+    "-Wno-sign-compare"
+    "-Wno-float-conversion"
+    "-Wno-float-equal"
+    "-Wno-format-nonliteral"
+    "-Wno-gcc-compat"
+    "-Wno-global-constructors"
+    "-Wno-exit-time-destructors"
+    "-Wno-nested-anon-types"
+    "-Wno-non-modular-include-in-module"
+    "-Wno-old-style-cast"
+    "-Wno-range-loop-analysis"
+    "-Wno-reserved-id-macro"
+    "-Wno-shorten-64-to-32"
+    "-Wno-switch-enum"
+    "-Wno-thread-safety-negative"
+    "-Wno-undef"
+    "-Wno-unknown-warning-option"
+    "-Wno-unreachable-code"
+    "-Wno-unused-macros"
+    "-Wno-weak-vtables"
+    "-Wbitfield-enum-conversion"
+    "-Wbool-conversion"
+    "-Wconstant-conversion"
+    "-Wenum-conversion"
+    "-Wint-conversion"
+    "-Wliteral-conversion"
+    "-Wnon-literal-null-conversion"
+    "-Wnull-conversion"
+    "-Wobjc-literal-conversion"
+    "-Wno-sign-conversion"
+    "-Wstring-conversion"
+    "/DNOMINMAX"
+    "/DWIN32_LEAN_AND_MEAN"
+    "/D_CRT_SECURE_NO_WARNINGS"
+    "/D_SCL_SECURE_NO_WARNINGS"
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE"
+)
+
+list(APPEND ABSL_CLANG_CL_TEST_FLAGS
+    "-Wno-c99-extensions"
+    "-Wno-missing-noreturn"
+    "-Wno-missing-prototypes"
+    "-Wno-missing-variable-declarations"
+    "-Wno-null-conversion"
+    "-Wno-shadow"
+    "-Wno-shift-sign-overflow"
+    "-Wno-sign-compare"
+    "-Wno-unused-function"
+    "-Wno-unused-member-function"
+    "-Wno-unused-parameter"
+    "-Wno-unused-private-field"
+    "-Wno-unused-template"
+    "-Wno-used-but-marked-unused"
+    "-Wno-zero-as-null-pointer-constant"
+    "-Wno-gnu-zero-variadic-macro-arguments"
+)
+
+list(APPEND ABSL_GCC_EXCEPTIONS_FLAGS
     "-fexceptions"
 )
 
-list(APPEND GCC_FLAGS
+list(APPEND ABSL_GCC_FLAGS
     "-Wall"
     "-Wextra"
     "-Wcast-qual"
@@ -24,7 +100,7 @@ list(APPEND GCC_FLAGS
     "-Wno-sign-compare"
 )
 
-list(APPEND GCC_TEST_FLAGS
+list(APPEND ABSL_GCC_TEST_FLAGS
     "-Wno-conversion-null"
     "-Wno-missing-declarations"
     "-Wno-sign-compare"
@@ -33,11 +109,11 @@ list(APPEND GCC_TEST_FLAGS
     "-Wno-unused-private-field"
 )
 
-list(APPEND LLVM_EXCEPTIONS_FLAGS
+list(APPEND ABSL_LLVM_EXCEPTIONS_FLAGS
     "-fexceptions"
 )
 
-list(APPEND LLVM_FLAGS
+list(APPEND ABSL_LLVM_FLAGS
     "-Wall"
     "-Wextra"
     "-Weverything"
@@ -85,7 +161,7 @@ list(APPEND LLVM_FLAGS
     "-Wstring-conversion"
 )
 
-list(APPEND LLVM_TEST_FLAGS
+list(APPEND ABSL_LLVM_TEST_FLAGS
     "-Wno-c99-extensions"
     "-Wno-missing-noreturn"
     "-Wno-missing-prototypes"
@@ -104,28 +180,28 @@ list(APPEND LLVM_TEST_FLAGS
     "-Wno-gnu-zero-variadic-macro-arguments"
 )
 
-list(APPEND MSVC_EXCEPTIONS_FLAGS
+list(APPEND ABSL_MSVC_EXCEPTIONS_FLAGS
     "/U_HAS_EXCEPTIONS"
     "/D_HAS_EXCEPTIONS=1"
     "/EHsc"
 )
 
-list(APPEND MSVC_FLAGS
+list(APPEND ABSL_MSVC_FLAGS
     "/W3"
+    "/DNOMINMAX"
+    "/DWIN32_LEAN_AND_MEAN"
+    "/D_CRT_SECURE_NO_WARNINGS"
+    "/D_SCL_SECURE_NO_WARNINGS"
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE"
     "/wd4005"
     "/wd4068"
     "/wd4180"
     "/wd4244"
     "/wd4267"
     "/wd4800"
-    "/DNOMINMAX"
-    "/DWIN32_LEAN_AND_MEAN"
-    "/D_CRT_SECURE_NO_WARNINGS"
-    "/D_SCL_SECURE_NO_WARNINGS"
-    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE"
 )
 
-list(APPEND MSVC_TEST_FLAGS
+list(APPEND ABSL_MSVC_TEST_FLAGS
     "/wd4018"
     "/wd4101"
     "/wd4503"
diff --git a/absl/copts/GENERATED_copts.bzl b/absl/copts/GENERATED_copts.bzl
index d23f40698e93..e05a58e39b40 100644
--- a/absl/copts/GENERATED_copts.bzl
+++ b/absl/copts/GENERATED_copts.bzl
@@ -4,11 +4,87 @@
 (2) Run `python <path_to_absl>/copts/generate_copts.py`.
 """
 
-GCC_EXCEPTIONS_FLAGS = [
+ABSL_CLANG_CL_EXCEPTIONS_FLAGS = [
+    "/U_HAS_EXCEPTIONS",
+    "/D_HAS_EXCEPTIONS=1",
+    "/EHsc",
+]
+
+ABSL_CLANG_CL_FLAGS = [
+    "/W3",
+    "-Wno-c++98-compat-pedantic",
+    "-Wno-conversion",
+    "-Wno-covered-switch-default",
+    "-Wno-deprecated",
+    "-Wno-disabled-macro-expansion",
+    "-Wno-double-promotion",
+    "-Wno-comma",
+    "-Wno-extra-semi",
+    "-Wno-extra-semi-stmt",
+    "-Wno-packed",
+    "-Wno-padded",
+    "-Wno-sign-compare",
+    "-Wno-float-conversion",
+    "-Wno-float-equal",
+    "-Wno-format-nonliteral",
+    "-Wno-gcc-compat",
+    "-Wno-global-constructors",
+    "-Wno-exit-time-destructors",
+    "-Wno-nested-anon-types",
+    "-Wno-non-modular-include-in-module",
+    "-Wno-old-style-cast",
+    "-Wno-range-loop-analysis",
+    "-Wno-reserved-id-macro",
+    "-Wno-shorten-64-to-32",
+    "-Wno-switch-enum",
+    "-Wno-thread-safety-negative",
+    "-Wno-undef",
+    "-Wno-unknown-warning-option",
+    "-Wno-unreachable-code",
+    "-Wno-unused-macros",
+    "-Wno-weak-vtables",
+    "-Wbitfield-enum-conversion",
+    "-Wbool-conversion",
+    "-Wconstant-conversion",
+    "-Wenum-conversion",
+    "-Wint-conversion",
+    "-Wliteral-conversion",
+    "-Wnon-literal-null-conversion",
+    "-Wnull-conversion",
+    "-Wobjc-literal-conversion",
+    "-Wno-sign-conversion",
+    "-Wstring-conversion",
+    "/DNOMINMAX",
+    "/DWIN32_LEAN_AND_MEAN",
+    "/D_CRT_SECURE_NO_WARNINGS",
+    "/D_SCL_SECURE_NO_WARNINGS",
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
+]
+
+ABSL_CLANG_CL_TEST_FLAGS = [
+    "-Wno-c99-extensions",
+    "-Wno-missing-noreturn",
+    "-Wno-missing-prototypes",
+    "-Wno-missing-variable-declarations",
+    "-Wno-null-conversion",
+    "-Wno-shadow",
+    "-Wno-shift-sign-overflow",
+    "-Wno-sign-compare",
+    "-Wno-unused-function",
+    "-Wno-unused-member-function",
+    "-Wno-unused-parameter",
+    "-Wno-unused-private-field",
+    "-Wno-unused-template",
+    "-Wno-used-but-marked-unused",
+    "-Wno-zero-as-null-pointer-constant",
+    "-Wno-gnu-zero-variadic-macro-arguments",
+]
+
+ABSL_GCC_EXCEPTIONS_FLAGS = [
     "-fexceptions",
 ]
 
-GCC_FLAGS = [
+ABSL_GCC_FLAGS = [
     "-Wall",
     "-Wextra",
     "-Wcast-qual",
@@ -25,7 +101,7 @@ GCC_FLAGS = [
     "-Wno-sign-compare",
 ]
 
-GCC_TEST_FLAGS = [
+ABSL_GCC_TEST_FLAGS = [
     "-Wno-conversion-null",
     "-Wno-missing-declarations",
     "-Wno-sign-compare",
@@ -34,11 +110,11 @@ GCC_TEST_FLAGS = [
     "-Wno-unused-private-field",
 ]
 
-LLVM_EXCEPTIONS_FLAGS = [
+ABSL_LLVM_EXCEPTIONS_FLAGS = [
     "-fexceptions",
 ]
 
-LLVM_FLAGS = [
+ABSL_LLVM_FLAGS = [
     "-Wall",
     "-Wextra",
     "-Weverything",
@@ -86,7 +162,7 @@ LLVM_FLAGS = [
     "-Wstring-conversion",
 ]
 
-LLVM_TEST_FLAGS = [
+ABSL_LLVM_TEST_FLAGS = [
     "-Wno-c99-extensions",
     "-Wno-missing-noreturn",
     "-Wno-missing-prototypes",
@@ -105,28 +181,28 @@ LLVM_TEST_FLAGS = [
     "-Wno-gnu-zero-variadic-macro-arguments",
 ]
 
-MSVC_EXCEPTIONS_FLAGS = [
+ABSL_MSVC_EXCEPTIONS_FLAGS = [
     "/U_HAS_EXCEPTIONS",
     "/D_HAS_EXCEPTIONS=1",
     "/EHsc",
 ]
 
-MSVC_FLAGS = [
+ABSL_MSVC_FLAGS = [
     "/W3",
+    "/DNOMINMAX",
+    "/DWIN32_LEAN_AND_MEAN",
+    "/D_CRT_SECURE_NO_WARNINGS",
+    "/D_SCL_SECURE_NO_WARNINGS",
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
     "/wd4005",
     "/wd4068",
     "/wd4180",
     "/wd4244",
     "/wd4267",
     "/wd4800",
-    "/DNOMINMAX",
-    "/DWIN32_LEAN_AND_MEAN",
-    "/D_CRT_SECURE_NO_WARNINGS",
-    "/D_SCL_SECURE_NO_WARNINGS",
-    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
 ]
 
-MSVC_TEST_FLAGS = [
+ABSL_MSVC_TEST_FLAGS = [
     "/wd4018",
     "/wd4101",
     "/wd4503",
diff --git a/absl/copts/configure_copts.bzl b/absl/copts/configure_copts.bzl
index 57cd3f62998b..1655addd2461 100644
--- a/absl/copts/configure_copts.bzl
+++ b/absl/copts/configure_copts.bzl
@@ -6,35 +6,35 @@ change Abseil copts, edit absl/copts/copts.py
 
 load(
     "//absl:copts/GENERATED_copts.bzl",
-    "GCC_EXCEPTIONS_FLAGS",
-    "GCC_FLAGS",
-    "GCC_TEST_FLAGS",
-    "LLVM_EXCEPTIONS_FLAGS",
-    "LLVM_FLAGS",
-    "LLVM_TEST_FLAGS",
-    "MSVC_EXCEPTIONS_FLAGS",
-    "MSVC_FLAGS",
-    "MSVC_TEST_FLAGS",
+    "ABSL_GCC_EXCEPTIONS_FLAGS",
+    "ABSL_GCC_FLAGS",
+    "ABSL_GCC_TEST_FLAGS",
+    "ABSL_LLVM_EXCEPTIONS_FLAGS",
+    "ABSL_LLVM_FLAGS",
+    "ABSL_LLVM_TEST_FLAGS",
+    "ABSL_MSVC_EXCEPTIONS_FLAGS",
+    "ABSL_MSVC_FLAGS",
+    "ABSL_MSVC_TEST_FLAGS",
 )
 
 ABSL_DEFAULT_COPTS = select({
-    "//absl:windows": MSVC_FLAGS,
-    "//absl:llvm_compiler": LLVM_FLAGS,
-    "//conditions:default": GCC_FLAGS,
+    "//absl:windows": ABSL_MSVC_FLAGS,
+    "//absl:llvm_compiler": ABSL_LLVM_FLAGS,
+    "//conditions:default": ABSL_GCC_FLAGS,
 })
 
 # in absence of modules (--compiler=gcc or -c opt), cc_tests leak their copts
 # to their (included header) dependencies and fail to build outside absl
 ABSL_TEST_COPTS = ABSL_DEFAULT_COPTS + select({
-    "//absl:windows": MSVC_TEST_FLAGS,
-    "//absl:llvm_compiler": LLVM_TEST_FLAGS,
-    "//conditions:default": GCC_TEST_FLAGS,
+    "//absl:windows": ABSL_MSVC_TEST_FLAGS,
+    "//absl:llvm_compiler": ABSL_LLVM_TEST_FLAGS,
+    "//conditions:default": ABSL_GCC_TEST_FLAGS,
 })
 
 ABSL_EXCEPTIONS_FLAG = select({
-    "//absl:windows": MSVC_EXCEPTIONS_FLAGS,
-    "//absl:llvm_compiler": LLVM_EXCEPTIONS_FLAGS,
-    "//conditions:default": GCC_EXCEPTIONS_FLAGS,
+    "//absl:windows": ABSL_MSVC_EXCEPTIONS_FLAGS,
+    "//absl:llvm_compiler": ABSL_LLVM_EXCEPTIONS_FLAGS,
+    "//conditions:default": ABSL_GCC_EXCEPTIONS_FLAGS,
 })
 
 ABSL_EXCEPTIONS_FLAG_LINKOPTS = select({
diff --git a/absl/copts/copts.py b/absl/copts/copts.py
index 5a2d91a3aa6e..3c9d42945f1b 100644
--- a/absl/copts/copts.py
+++ b/absl/copts/copts.py
@@ -11,8 +11,119 @@ The generated copts are consumed by configure_copts.bzl and
 AbseilConfigureCopts.cmake.
 """
 
+# /Wall with msvc includes unhelpful warnings such as C4711, C4710, ...
+MSVC_BIG_WARNING_FLAGS = [
+    "/W3",
+]
+
+LLVM_BIG_WARNING_FLAGS = [
+    "-Wall",
+    "-Wextra",
+    "-Weverything",
+]
+
+# Docs on single flags is preceded by a comment.
+# Docs on groups of flags is preceded by ###.
+LLVM_DISABLE_WARNINGS_FLAGS = [
+    # Abseil does not support C++98
+    "-Wno-c++98-compat-pedantic",
+    # Turns off all implicit conversion warnings. Most are re-enabled below.
+    "-Wno-conversion",
+    "-Wno-covered-switch-default",
+    "-Wno-deprecated",
+    "-Wno-disabled-macro-expansion",
+    "-Wno-double-promotion",
+    ###
+    # Turned off as they include valid C++ code.
+    "-Wno-comma",
+    "-Wno-extra-semi",
+    "-Wno-extra-semi-stmt",
+    "-Wno-packed",
+    "-Wno-padded",
+    ###
+    # Google style does not use unsigned integers, though STL containers
+    # have unsigned types.
+    "-Wno-sign-compare",
+    ###
+    "-Wno-float-conversion",
+    "-Wno-float-equal",
+    "-Wno-format-nonliteral",
+    # Too aggressive: warns on Clang extensions enclosed in Clang-only
+    # compilation paths.
+    "-Wno-gcc-compat",
+    ###
+    # Some internal globals are necessary. Don't do this at home.
+    "-Wno-global-constructors",
+    "-Wno-exit-time-destructors",
+    ###
+    "-Wno-nested-anon-types",
+    "-Wno-non-modular-include-in-module",
+    "-Wno-old-style-cast",
+    # Warns on preferred usage of non-POD types such as string_view
+    "-Wno-range-loop-analysis",
+    "-Wno-reserved-id-macro",
+    "-Wno-shorten-64-to-32",
+    "-Wno-switch-enum",
+    "-Wno-thread-safety-negative",
+    "-Wno-undef",
+    "-Wno-unknown-warning-option",
+    "-Wno-unreachable-code",
+    # Causes warnings on include guards
+    "-Wno-unused-macros",
+    "-Wno-weak-vtables",
+    ###
+    # Implicit conversion warnings turned off by -Wno-conversion
+    # which are re-enabled below.
+    "-Wbitfield-enum-conversion",
+    "-Wbool-conversion",
+    "-Wconstant-conversion",
+    "-Wenum-conversion",
+    "-Wint-conversion",
+    "-Wliteral-conversion",
+    "-Wnon-literal-null-conversion",
+    "-Wnull-conversion",
+    "-Wobjc-literal-conversion",
+    "-Wno-sign-conversion",
+    "-Wstring-conversion",
+]
+
+LLVM_TEST_DISABLE_WARNINGS_FLAGS = [
+    "-Wno-c99-extensions",
+    "-Wno-missing-noreturn",
+    "-Wno-missing-prototypes",
+    "-Wno-missing-variable-declarations",
+    "-Wno-null-conversion",
+    "-Wno-shadow",
+    "-Wno-shift-sign-overflow",
+    "-Wno-sign-compare",
+    "-Wno-unused-function",
+    "-Wno-unused-member-function",
+    "-Wno-unused-parameter",
+    "-Wno-unused-private-field",
+    "-Wno-unused-template",
+    "-Wno-used-but-marked-unused",
+    "-Wno-zero-as-null-pointer-constant",
+    # gtest depends on this GNU extension being offered.
+    "-Wno-gnu-zero-variadic-macro-arguments",
+]
+
+MSVC_STYLE_EXCEPTIONS_FLAGS = [
+    "/U_HAS_EXCEPTIONS", "/D_HAS_EXCEPTIONS=1", "/EHsc"
+]
+
+MSVC_DEFINES = [
+    "/DNOMINMAX",  # Don't define min and max macros (windows.h)
+    # Don't bloat namespace with incompatible winsock versions.
+    "/DWIN32_LEAN_AND_MEAN",
+    # Don't warn about usage of insecure C functions.
+    "/D_CRT_SECURE_NO_WARNINGS",
+    "/D_SCL_SECURE_NO_WARNINGS",
+    # Introduced in VS 2017 15.8, allow overaligned types in aligned_storage
+    "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
+]
+
 COPT_VARS = {
-    "GCC_FLAGS": [
+    "ABSL_GCC_FLAGS": [
         "-Wall",
         "-Wextra",
         "-Wcast-qual",
@@ -33,7 +144,7 @@ COPT_VARS = {
         # have unsigned types.
         "-Wno-sign-compare",
     ],
-    "GCC_TEST_FLAGS": [
+    "ABSL_GCC_TEST_FLAGS": [
         "-Wno-conversion-null",
         "-Wno-missing-declarations",
         "-Wno-sign-compare",
@@ -41,98 +152,15 @@ COPT_VARS = {
         "-Wno-unused-parameter",
         "-Wno-unused-private-field",
     ],
-    "GCC_EXCEPTIONS_FLAGS": ["-fexceptions"],
-
-    # Docs on single flags is preceded by a comment.
-    # Docs on groups of flags is preceded by ###.
-    "LLVM_FLAGS": [
-        "-Wall",
-        "-Wextra",
-        "-Weverything",
-        # Abseil does not support C++98
-        "-Wno-c++98-compat-pedantic",
-        # Turns off all implicit conversion warnings. Most are re-enabled below.
-        "-Wno-conversion",
-        "-Wno-covered-switch-default",
-        "-Wno-deprecated",
-        "-Wno-disabled-macro-expansion",
-        "-Wno-double-promotion",
-        ###
-        # Turned off as they include valid C++ code.
-        "-Wno-comma",
-        "-Wno-extra-semi",
-        "-Wno-extra-semi-stmt",
-        "-Wno-packed",
-        "-Wno-padded",
-        ###
-        # Google style does not use unsigned integers, though STL containers
-        # have unsigned types.
-        "-Wno-sign-compare",
-        ###
-        "-Wno-float-conversion",
-        "-Wno-float-equal",
-        "-Wno-format-nonliteral",
-        # Too aggressive: warns on Clang extensions enclosed in Clang-only
-        # compilation paths.
-        "-Wno-gcc-compat",
-        ###
-        # Some internal globals are necessary. Don't do this at home.
-        "-Wno-global-constructors",
-        "-Wno-exit-time-destructors",
-        ###
-        "-Wno-nested-anon-types",
-        "-Wno-non-modular-include-in-module",
-        "-Wno-old-style-cast",
-        # Warns on preferred usage of non-POD types such as string_view
-        "-Wno-range-loop-analysis",
-        "-Wno-reserved-id-macro",
-        "-Wno-shorten-64-to-32",
-        "-Wno-switch-enum",
-        "-Wno-thread-safety-negative",
-        "-Wno-undef",
-        "-Wno-unknown-warning-option",
-        "-Wno-unreachable-code",
-        # Causes warnings on include guards
-        "-Wno-unused-macros",
-        "-Wno-weak-vtables",
-        ###
-        # Implicit conversion warnings turned off by -Wno-conversion
-        # which are re-enabled below.
-        "-Wbitfield-enum-conversion",
-        "-Wbool-conversion",
-        "-Wconstant-conversion",
-        "-Wenum-conversion",
-        "-Wint-conversion",
-        "-Wliteral-conversion",
-        "-Wnon-literal-null-conversion",
-        "-Wnull-conversion",
-        "-Wobjc-literal-conversion",
-        "-Wno-sign-conversion",
-        "-Wstring-conversion",
-    ],
-    "LLVM_TEST_FLAGS": [
-        "-Wno-c99-extensions",
-        "-Wno-missing-noreturn",
-        "-Wno-missing-prototypes",
-        "-Wno-missing-variable-declarations",
-        "-Wno-null-conversion",
-        "-Wno-shadow",
-        "-Wno-shift-sign-overflow",
-        "-Wno-sign-compare",
-        "-Wno-unused-function",
-        "-Wno-unused-member-function",
-        "-Wno-unused-parameter",
-        "-Wno-unused-private-field",
-        "-Wno-unused-template",
-        "-Wno-used-but-marked-unused",
-        "-Wno-zero-as-null-pointer-constant",
-        # gtest depends on this GNU extension being offered.
-        "-Wno-gnu-zero-variadic-macro-arguments",
-    ],
-    "LLVM_EXCEPTIONS_FLAGS": ["-fexceptions"],
-    # /Wall with msvc includes unhelpful warnings such as C4711, C4710, ...
-    "MSVC_FLAGS": [
-        "/W3",
+    "ABSL_GCC_EXCEPTIONS_FLAGS": ["-fexceptions"],
+    "ABSL_LLVM_FLAGS": LLVM_BIG_WARNING_FLAGS + LLVM_DISABLE_WARNINGS_FLAGS,
+    "ABSL_LLVM_TEST_FLAGS": LLVM_TEST_DISABLE_WARNINGS_FLAGS,
+    "ABSL_LLVM_EXCEPTIONS_FLAGS": ["-fexceptions"],
+    "ABSL_CLANG_CL_FLAGS": (MSVC_BIG_WARNING_FLAGS +
+                            LLVM_DISABLE_WARNINGS_FLAGS + MSVC_DEFINES),
+    "ABSL_CLANG_CL_TEST_FLAGS": LLVM_TEST_DISABLE_WARNINGS_FLAGS,
+    "ABSL_CLANG_CL_EXCEPTIONS_FLAGS": MSVC_STYLE_EXCEPTIONS_FLAGS,
+    "ABSL_MSVC_FLAGS": MSVC_BIG_WARNING_FLAGS + MSVC_DEFINES + [
         "/wd4005",  # macro-redefinition
         "/wd4068",  # unknown pragma
         "/wd4180",  # qualifier applied to function type has no meaning; ignored
@@ -140,21 +168,11 @@ COPT_VARS = {
         "/wd4267",  # conversion from 'size_t' to 'type', possible loss of data
         # forcing value to bool 'true' or 'false' (performance warning)
         "/wd4800",
-        "/DNOMINMAX",  # Don't define min and max macros (windows.h)
-        # Don't bloat namespace with incompatible winsock versions.
-        "/DWIN32_LEAN_AND_MEAN",
-        # Don't warn about usage of insecure C functions.
-        "/D_CRT_SECURE_NO_WARNINGS",
-        "/D_SCL_SECURE_NO_WARNINGS",
-        # Introduced in VS 2017 15.8, allow overaligned types in aligned_storage
-        "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
     ],
-    "MSVC_TEST_FLAGS": [
+    "ABSL_MSVC_TEST_FLAGS": [
         "/wd4018",  # signed/unsigned mismatch
         "/wd4101",  # unreferenced local variable
         "/wd4503",  # decorated name length exceeded, name was truncated
     ],
-    "MSVC_EXCEPTIONS_FLAGS": [
-        "/U_HAS_EXCEPTIONS", "/D_HAS_EXCEPTIONS=1", "/EHsc"
-    ]
+    "ABSL_MSVC_EXCEPTIONS_FLAGS": MSVC_STYLE_EXCEPTIONS_FLAGS,
 }