Clang cl (#762)

* Added c++17 test configurations for clang5.0 and clang6.0

* Fix #739 correct cppcorecheck warnings for clang-cl

* Add clang-cl  configurations

* Corrections Appveyor;
Temporarily disable msvc compilation for faster testing (#741)

* Add path to clang-cl.exe (#741)

* Escape backslash in path (#741)

* Update vcpkg (#741)

* Check vcpkg version; try without building vcpkg; use latest clang-cl from path (#741)

* Fix blocks in ps script (#741)

* Try accessing APPVEYOR_BUILD_FOLDER variable (#471)

* Update span size() bug confirmation test for GCC 6.5 (#741)

* MSVC flags to Clang-cl; disable c++98-compat and undefined macro warnings (#741)

* Suppress clang warning on tests (missing-prototypes) (#741)

* Fix clang warning -Wnewline-eof (#741)

* Fix clang warning -Wdouble-promotion (#741)

* Set linker explicitly

* Clean condition statement

* For Clang, fallback to the AppVeyor installed version of CMake

* Fix clang warning -Wmissing-variable-declarations

* Fallback to the MSVC linker until vcpkg has been updated

* Revert "Fallback to the MSVC linker until vcpkg has been updated"

This reverts commit 7263f3289e6e835c32fc7800d0c2d4e58bd21178.

* Fix clang warning -Wunused-member-function

* Fix clang warning -Wmissing-noreturn

* Fix clang warning -Winvalid-noreturn on Windows

* Add macro block end comment on large #if blocks

* Workaround: fallback to mscv link.exe

* Workaround: get msvc paths into PowerShell through intermediate file

* Workaround: fix, remove "PATH=" from text

* Workaround: try with full-path; and return user PATH

* Workaround: fix, escape backslashes

* Revert all since "Workaround: fallback to mscv link.exe" did not work on AppVeyor

This reverts the commits:
bda3d6a428e5d19e97375b2e575b0f51ff1b4bc0
97062933acfb6428535c0fdcab5d94371ce72bfc
0f4fb04bac9c2d091005b791294237250dbe0668
1b0c19afd154f4ffc5ef793014c1bc2324534fd0
a5739ea5f0cd7633527b5e5d3b16a9e3ade7149f

* Suppress output of git pull; remove vcpkg from cache

* Re-enable AppVeyor builds for all platforms

* Correct typo

Co-Authored-By: Farwaykorse <Farwaykorse@users.noreply.github.com>

* Add Clang-cl 7.0.0 to the supported platforms

* Revert "Fix clang warning -Wunused-member-function"

This reverts commit 6fe1a4203501145ab4350b9152ecc11f3a30e49a.

* Fix or locally suppress clang warning -Wunused-member-function

* format touched code and correct comment

* git pull --quiet

* fix logic error in workaround

* fix missing bracket

* Suppress output of mkdir

* Replace MSBuild with Ninja

* Suppress output of 7z

* Add architecture flags for Clang

* Drop workaround for lld-link

* 7-zip Overwrite and Alternative output suppression without suppressing errors

Replaces 3c1c0794dd9a29908cc7c38f591bd3c7b4929677

* AppVeyor setup and CMake before build

* reorder compiler configuration

* remove unnecessary

* remove -fno-strict-aliasing

* remove -Wsign-conversion, since before v4.0 part of -Wconversion

* -Wctor-dtor-privacy is GCC only

* remove -Woverloaded-virtual part of -Wmost, part of -Wall

* add -Wmissing-noreturn

* remove the pragmas for -Wunused-member-function

* Re-add MSBuild generator on AppVeyor

* Print CMake commands

* Add MSBuild toolset selection

* Separate Architecture setting

* clang-cl: add -Weverything

* clang-cl -Wno-c++98-compat

* clang-cl -Wno-c++98-compat-pedantic

* clang-cl -Wno-missing-prototypes

* clang-cl C++14 -Wno-unused-member-function

* clang-cl -Wundef __GNUC__

* clang++: add -Weverything

* clang++ -Wno-c++98-compat

* clang++ -Wno-c++98-compat-pedantic

* clang++ -Wno-missing-prototypes

* clang++ -Wno-weak-vtables

* clang++ C++14 -Wno-unused-member-function

* clang++ fix -Wundef _MSC_VER

* clang++ -Wno-padded

* clang++ solve -Wdeprecated

* Add AppleClang compiler target
Since CMake v3.0 use of Clang for both is deprecated

* clang++ v5.0 C++17 -Wno-undefined-func-template

* Add VS2015 + LLVM/clang-cl to AppVeyor

* Do not disable constexpr when compiling with clang-cl on Windows

* Clean-up clang-only warnings (now under -Weverything)

* Revert "Fix clang warning -Winvalid-noreturn on Windows"

This reverts commit 2238c4760e86feebb2a18620b77312bd01055f61.

* Suppress -Winvalid-noreturn for the MS STL noexception workaround

* CMake: put preprocessor definition in target_compile_definitions

* Solve compiler warning C4668: __GNUC__ not defined

17 files changed, 213 insertions, 106 deletions

diff --git a/README.md b/README.md
index f6fde29..db7d406 100644
--- a/README.md
+++ b/README.md
@@ -27,6 +27,7 @@ The test suite that exercises GSL has been built and passes successfully on the
 * Windows using Visual Studio 2015
 * Windows using Visual Studio 2017
 * Windows using Clang/LLVM 3.6
+* Windows using Clang/LLVM 7.0.0
 * Windows using GCC 5.1
 * Windows using Intel C++ Compiler 18.0
 * GNU/Linux using Clang/LLVM 3.6
diff --git a/appveyor.yml b/appveyor.yml
index 2fc8a16..c3e3c87 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -13,44 +13,108 @@ image:
   - Visual Studio 2017
 
 environment:
+  NINJA_TAG: v1.8.2
+  NINJA_SHA512: 9B9CE248240665FCD6404B989F3B3C27ED9682838225E6DC9B67B551774F251E4FF8A207504F941E7C811E7A8BE1945E7BCB94472A335EF15E23A0200A32E6D5
+  NINJA_PATH: C:\Tools\ninja\ninja-%NINJA_TAG%
+  VCVAR2015: 'C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat'
+  VCVAR2017: 'C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat'
   matrix:
     - GSL_CXX_STANDARD: 14
+      USE_TOOLSET: MSVC
+      USE_GENERATOR: MSBuild
     - GSL_CXX_STANDARD: 17
+      USE_TOOLSET: MSVC
+      USE_GENERATOR: MSBuild
+    - GSL_CXX_STANDARD: 14
+      USE_TOOLSET: LLVM
+      USE_GENERATOR: Ninja
+    - GSL_CXX_STANDARD: 17
+      USE_TOOLSET: LLVM
+      USE_GENERATOR: Ninja
 
 matrix:
     exclude:
     - image: Visual Studio 2015
       GSL_CXX_STANDARD: 17
+    - image: Visual Studio 2015
+      USE_TOOLSET: LLVM
+      USE_GENERATOR: MSBuild
 
 cache:
   - C:\cmake-3.8.0-win32-x86
+  - C:\Tools\ninja
 
 install:
   - ps: |
-      if (![IO.File]::Exists("C:\cmake-3.8.0-win32-x86\bin\cmake.exe")) {
-        Start-FileDownload 'https://cmake.org/files/v3.8/cmake-3.8.0-win32-x86.zip'
-        7z x -y cmake-3.8.0-win32-x86.zip -oC:\
+      if (![IO.File]::Exists("$env:NINJA_PATH\ninja.exe")) {
+        Start-FileDownload `
+          "https://github.com/ninja-build/ninja/releases/download/$env:NINJA_TAG/ninja-win.zip"
+        $hash = (Get-FileHash ninja-win.zip -Algorithm SHA512).Hash
+        if ($env:NINJA_SHA512 -eq $hash) {
+          7z e -y -bso0 ninja-win.zip -o"$env:NINJA_PATH"
+        } else { Write-Warning "Ninja download hash changed!"; Write-Output "$hash" }
+      }
+      if ([IO.File]::Exists("$env:NINJA_PATH\ninja.exe")) {
+        $env:PATH = "$env:NINJA_PATH;$env:PATH"
+      } else { Write-Warning "Failed to find ninja.exe in expected location." }
+      if ($env:USE_TOOLSET -ne "LLVM") {
+        if (![IO.File]::Exists("C:\cmake-3.8.0-win32-x86\bin\cmake.exe")) {
+          Start-FileDownload 'https://cmake.org/files/v3.8/cmake-3.8.0-win32-x86.zip'
+          7z x -y -bso0 cmake-3.8.0-win32-x86.zip -oC:\
+        }
+        $env:PATH="C:\cmake-3.8.0-win32-x86\bin;$env:PATH"
       }
-      $env:PATH="C:\cmake-3.8.0-win32-x86\bin;$env:PATH"
 
 before_build:
   - ps: |
-      mkdir build
-      cd build
-
-      if ("$env:APPVEYOR_JOB_NAME" -match "Image: Visual Studio 2015") {
-          $env:generator="Visual Studio 14 2015"
+      if ("$env:USE_GENERATOR" -eq "Ninja") {
+        $GeneratorFlags = '-k 10'
+        $Architecture = $env:PLATFORM
+        if ("$env:APPVEYOR_BUILD_WORKER_IMAGE" -eq "Visual Studio 2015") {
+          $env:VCVARSALL = "`"$env:VCVAR2015`" $Architecture"
+        } else {
+          $env:VCVARSALL = "`"$env:VCVAR2017`" $Architecture"
+        }
+        $env:CMakeGenFlags = "-G Ninja -DGSL_CXX_STANDARD=$env:GSL_CXX_STANDARD"
       } else {
-          $env:generator="Visual Studio 15 2017"
+        $GeneratorFlags = '/m /v:minimal'
+        if ("$env:APPVEYOR_BUILD_WORKER_IMAGE" -eq "Visual Studio 2015") {
+          $Generator = 'Visual Studio 14 2015'
+        } else {
+          $Generator = 'Visual Studio 15 2017'
+        }
+        if ("$env:PLATFORM" -eq "x86") { 
+            $Architecture = "Win32"
+        } else { 
+            $Architecture = "x64" 
+        }
+        if ("$env:USE_TOOLSET" -eq "LLVM") {
+          $env:CMakeGenFlags = "-G `"$Generator`" -A $Architecture -T llvm -DGSL_CXX_STANDARD=$env:GSL_CXX_STANDARD"
+        } else {
+          $env:CMakeGenFlags = "-G `"$Generator`" -A $Architecture -DGSL_CXX_STANDARD=$env:GSL_CXX_STANDARD"
+        }
       }
-      if ($env:PLATFORM -eq "x64") {
-          $env:generator="$env:generator Win64"
+      if ("$env:USE_TOOLSET" -eq "LLVM") {
+        $env:CC  = "clang-cl"
+        $env:CXX = "clang-cl"
+        if ("$env:PLATFORM" -eq "x86") {
+          $env:CFLAGS   = "-m32";
+          $env:CXXFLAGS = "-m32";
+        } else {
+          $env:CFLAGS   = "-m64";
+          $env:CXXFLAGS = "-m64";
+        }
       }
-      echo generator="$env:generator"
-      cmake .. -G "$env:generator" -DGSL_CXX_STANDARD="$env:GSL_CXX_STANDARD"
+      $env:CMakeBuildFlags = "--config $env:CONFIGURATION -- $GeneratorFlags"
+  - mkdir build
+  - cd build
+  - if %USE_GENERATOR%==Ninja (call %VCVARSALL%)
+  - echo %CMakeGenFlags%
+  - cmake .. %CMakeGenFlags%
 
 build_script:
-  - cmake --build . --config %CONFIGURATION% -- /m /v:minimal
+  - echo %CMakeBuildFlags%
+  - cmake --build . %CMakeBuildFlags%
 
 test_script:
   - ctest -j2
diff --git a/include/gsl/gsl_assert b/include/gsl/gsl_assert
index 3c952e6..c704633 100644
--- a/include/gsl/gsl_assert
+++ b/include/gsl/gsl_assert
@@ -22,17 +22,17 @@
 
 //
 // make suppress attributes parse for some compilers
-// Hopefully temporary until suppresion standardization occurs
+// Hopefully temporary until suppression standardization occurs
 //
-#if defined (_MSC_VER)
-#define GSL_SUPPRESS(x) [[gsl::suppress(x)]]
-#else
 #if defined(__clang__)
 #define GSL_SUPPRESS(x) [[gsl::suppress("x")]]
 #else
+#if defined(_MSC_VER)
+#define GSL_SUPPRESS(x) [[gsl::suppress(x)]]
+#else
 #define GSL_SUPPRESS(x)
-#endif // __clang__
 #endif // _MSC_VER
+#endif // __clang__
 
 //
 // Temporary until MSVC STL supports no-exceptions mode.
@@ -42,6 +42,12 @@
 #define GSL_MSVC_USE_STL_NOEXCEPTION_WORKAROUND
 #include <intrin.h>
 #define RANGE_CHECKS_FAILURE 0
+
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Winvalid-noreturn"
+#endif
+
 #endif
 
 //
@@ -98,7 +104,7 @@ namespace details
 #if defined(GSL_MSVC_USE_STL_NOEXCEPTION_WORKAROUND)
 
     typedef void  (__cdecl *terminate_handler)();
-    
+
     GSL_SUPPRESS(f.6) // NO-FORMAT: attribute
     [[noreturn]] inline void __cdecl default_terminate_handler()
     {
@@ -138,7 +144,7 @@ namespace details
         throw std::forward<Exception>(exception);
     }
 
-#endif
+#endif // GSL_TERMINATE_ON_CONTRACT_VIOLATION
 
 } // namespace details
 } // namespace gsl
@@ -159,9 +165,13 @@ namespace details
 
 #define GSL_CONTRACT_CHECK(type, cond) GSL_ASSUME(cond)
 
-#endif
+#endif // GSL_THROW_ON_CONTRACT_VIOLATION
 
 #define Expects(cond) GSL_CONTRACT_CHECK("Precondition", cond)
 #define Ensures(cond) GSL_CONTRACT_CHECK("Postcondition", cond)
 
+#if defined(GSL_MSVC_USE_STL_NOEXCEPTION_WORKAROUND) && defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
 #endif // GSL_CONTRACTS_H
diff --git a/include/gsl/gsl_byte b/include/gsl/gsl_byte
index bc18886..861446d 100644
--- a/include/gsl/gsl_byte
+++ b/include/gsl/gsl_byte
@@ -19,17 +19,17 @@
 
 //
 // make suppress attributes work for some compilers
-// Hopefully temporary until suppresion standardization occurs
+// Hopefully temporary until suppression standardization occurs
 //
-#if defined(_MSC_VER)
-#define GSL_SUPPRESS(x) [[gsl::suppress(x)]]
-#else
 #if defined(__clang__)
 #define GSL_SUPPRESS(x) [[gsl::suppress("x")]]
 #else
+#if defined(_MSC_VER)
+#define GSL_SUPPRESS(x) [[gsl::suppress(x)]]
+#else
 #define GSL_SUPPRESS(x)
-#endif // __clang__
 #endif // _MSC_VER
+#endif // __clang__
 
 #include <type_traits>
 
@@ -79,7 +79,7 @@
 #endif // _MSC_VER
 
 // Use __may_alias__ attribute on gcc and clang
-#if defined __clang__ || (__GNUC__ > 5)
+#if defined __clang__ || (defined(__GNUC__) && __GNUC__ > 5)
 #define byte_may_alias __attribute__((__may_alias__))
 #else // defined __clang__ || defined __GNUC__
 #define byte_may_alias
diff --git a/include/gsl/gsl_util b/include/gsl/gsl_util
index 1558044..4addde6 100644
--- a/include/gsl/gsl_util
+++ b/include/gsl/gsl_util
@@ -26,7 +26,7 @@
 #include <type_traits>      // for is_signed, integral_constant
 #include <utility>          // for forward
 
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) && !defined(__clang__)
 
 #pragma warning(push)
 #pragma warning(disable : 4127) // conditional expression is constant
@@ -161,7 +161,7 @@ constexpr T at(const std::initializer_list<T> cont, const index i)
 
 } // namespace gsl
 
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) && !defined(__clang__)
 #if _MSC_VER < 1910
 #undef constexpr
 #pragma pop_macro("constexpr")
diff --git a/include/gsl/multi_span b/include/gsl/multi_span
index 9995808..f1a909e 100644
--- a/include/gsl/multi_span
+++ b/include/gsl/multi_span
@@ -37,7 +37,7 @@
 #include <type_traits> // for enable_if_t, remove_cv_t, is_same, is_co...
 #include <utility>
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 
 // turn off some warnings that are noisy about our Expects statements
 #pragma warning(push)
@@ -62,7 +62,7 @@
 // compiletime, so the compiler wouldn't have to warn indiscriminently, but
 // could check if the source value actually doesn't fit into the target type
 // and only warn in those cases.
-#if __GNUC__ > 6
+#if defined(__GNUC__) && __GNUC__ > 6
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wsign-conversion"
 #endif
@@ -596,7 +596,6 @@ public:
     using sliced_type = static_bounds<RestRanges...>;
     using mapping_type = contiguous_mapping_tag;
 
-    constexpr static_bounds(const static_bounds&) noexcept = default;
     constexpr static_bounds() /*noexcept*/ = default;
 
     template <typename SourceType, typename TargetType, std::size_t Rank>
@@ -1887,6 +1886,8 @@ public:
         auto d = narrow_cast<size_type>(sizeof(OtherValueType) / sizeof(value_type));
 
         const size_type size = this->bounds().total_size() / d;
+
+        GSL_SUPPRESS(type.3) // NO-FORMAT: attribute
         return {const_cast<OtherValueType*>(reinterpret_cast<const OtherValueType*>(this->data())),
                 size,
                 bounds_type{resize_extent(this->bounds().index_bounds(), d),
@@ -2274,7 +2275,7 @@ general_span_iterator<Span> operator+(typename general_span_iterator<Span>::diff
 
 } // namespace gsl
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 #if _MSC_VER < 1910
 
 #undef constexpr
@@ -2285,7 +2286,7 @@ general_span_iterator<Span> operator+(typename general_span_iterator<Span>::diff
 
 #endif // _MSC_VER
 
-#if __GNUC__ > 6
+#if defined(__GNUC__) && __GNUC__ > 6
 #pragma GCC diagnostic pop
 #endif // __GNUC__ > 6
 
diff --git a/include/gsl/pointers b/include/gsl/pointers
index 52c2bfd..0f2987a 100644
--- a/include/gsl/pointers
+++ b/include/gsl/pointers
@@ -25,7 +25,7 @@
 #include <system_error> // for hash
 #include <type_traits>  // for enable_if_t, is_convertible, is_assignable
 
-#if defined(_MSC_VER) && _MSC_VER < 1910
+#if defined(_MSC_VER) && _MSC_VER < 1910 && !defined(__clang__)
 #pragma push_macro("constexpr")
 #define constexpr /*constexpr*/
 
@@ -283,10 +283,12 @@ struct hash<gsl::strict_not_null<T>>
 };
 
 } // namespace std
-#if defined(_MSC_VER) && _MSC_VER < 1910
+
+#if defined(_MSC_VER) && _MSC_VER < 1910 && !defined(__clang__)
+
 #undef constexpr
 #pragma pop_macro("constexpr")
 
-#endif // defined(_MSC_VER) && _MSC_VER < 1910
+#endif // defined(_MSC_VER) && _MSC_VER < 1910 && !defined(__clang__)
 
 #endif // GSL_POINTERS_H
diff --git a/include/gsl/span b/include/gsl/span
index fae1522..b4da532 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -31,7 +31,7 @@
 #include <utility>
 #include <memory> // for std::addressof
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 #pragma warning(push)
 
 // turn off some warnings that are noisy about our Expects statements
@@ -61,7 +61,7 @@
 // compiletime, so the compiler wouldn't have to warn indiscriminently, but
 // could check if the source value actually doesn't fit into the target type
 // and only warn in those cases.
-#if __GNUC__ > 6
+#if defined(__GNUC__) && __GNUC__ > 6
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wsign-conversion"
 #endif
@@ -776,7 +776,7 @@ constexpr ElementType& at(span<ElementType, Extent> s, index i)
 
 } // namespace gsl
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 #if _MSC_VER < 1910
 #undef constexpr
 #pragma pop_macro("constexpr")
@@ -786,7 +786,7 @@ constexpr ElementType& at(span<ElementType, Extent> s, index i)
 #pragma warning(pop)
 #endif // _MSC_VER
 
-#if __GNUC__ > 6
+#if defined(__GNUC__) && __GNUC__ > 6
 #pragma GCC diagnostic pop
 #endif // __GNUC__ > 6
 
diff --git a/include/gsl/string_span b/include/gsl/string_span
index d298039..b5dfd18 100644
--- a/include/gsl/string_span
+++ b/include/gsl/string_span
@@ -30,7 +30,7 @@
 #include <string>      // for basic_string, allocator, char_traits
 #include <type_traits> // for declval, is_convertible, enable_if_t, add_...
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 #pragma warning(push)
 
 // Turn MSVC /analyze rules that generate too much noise. TODO: fix in the tool.
@@ -706,7 +706,7 @@ bool operator>=(const T& one, gsl::basic_string_span<CharT, Extent> other)
 #endif
 } // namespace gsl
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 #pragma warning(pop)
 
 #if _MSC_VER < 1910
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 6edb8a0..832513f 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8.7)
+cmake_minimum_required(VERSION 3.0.2)
 
 project(GSLTests CXX)
 
@@ -33,14 +33,24 @@ endif()
 # this interface adds compile options to how the tests are run
 # please try to keep entries ordered =)
 add_library(gsl_tests_config INTERFACE)
-target_compile_options(gsl_tests_config INTERFACE
-    $<$<CXX_COMPILER_ID:MSVC>:
+if(MSVC) # MSVC or simulating MSVC
+    target_compile_options(gsl_tests_config INTERFACE
+        ${GSL_CPLUSPLUS_OPT}
         /EHsc
         /W4
         /WX
-    >
-    ${GSL_CPLUSPLUS_OPT}
-    $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:
+        $<$<CXX_COMPILER_ID:Clang>:
+          -Weverything
+          -Wno-c++98-compat
+          -Wno-c++98-compat-pedantic
+          -Wno-missing-braces
+          -Wno-missing-prototypes
+          -Wno-unknown-attributes
+          $<$<EQUAL:${GSL_CXX_STANDARD},14>:-Wno-unused-member-function>
+        >
+    )
+else()
+    target_compile_options(gsl_tests_config INTERFACE
         -fno-strict-aliasing
         -Wall
         -Wcast-align
@@ -48,16 +58,25 @@ target_compile_options(gsl_tests_config INTERFACE
         -Wctor-dtor-privacy
         -Werror
         -Wextra
-        -Wno-missing-braces
-        -Wno-unknown-attributes
-        -Wnon-virtual-dtor
-        -Wold-style-cast
-        -Woverloaded-virtual
         -Wpedantic
         -Wshadow
         -Wsign-conversion
-    >
-)
+        $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:
+          -Weverything
+          -Wno-c++98-compat
+          -Wno-c++98-compat-pedantic
+          -Wno-missing-braces
+          -Wno-missing-prototypes
+          -Wno-padded
+          -Wno-unknown-attributes
+          $<$<EQUAL:${GSL_CXX_STANDARD},14>:-Wno-unused-member-function>
+          -Wno-weak-vtables
+        >
+        $<$<CXX_COMPILER_ID:Clang>:
+          $<$<CXX_COMPILER_VERSION:5.0.2>:-Wno-undefined-func-template>
+        >
+    )
+endif(MSVC)
 
 # for tests to find the catch header
 target_include_directories(gsl_tests_config INTERFACE
@@ -121,34 +140,49 @@ endforeach(flag_var)
 # this interface adds compile options to how the tests are run
 # please try to keep entries ordered =)
 add_library(gsl_tests_config_noexcept INTERFACE)
-target_compile_options(gsl_tests_config_noexcept INTERFACE
-    $<$<CXX_COMPILER_ID:MSVC>:
-        /D_HAS_EXCEPTIONS=0
-        /wd4702
-        /wd4577
+if(MSVC) # MSVC or simulating MSVC
+    target_compile_definitions(gsl_tests_config_noexcept INTERFACE
+        _HAS_EXCEPTIONS=0
+    )
+    target_compile_options(gsl_tests_config_noexcept INTERFACE
+        ${GSL_CPLUSPLUS_OPT}
         /W4
         /WX
-    >
-    ${GSL_CPLUSPLUS_OPT}
-    $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:
-        -fno-strict-aliasing
+        $<$<CXX_COMPILER_ID:MSVC>:
+          /wd4577
+          /wd4702
+        >
+        $<$<CXX_COMPILER_ID:Clang>:
+          -Weverything
+          -Wno-c++98-compat
+          -Wno-c++98-compat-pedantic
+          -Wno-missing-prototypes
+          -Wno-unknown-attributes
+        >
+    )
+else()
+    target_compile_options(gsl_tests_config_noexcept INTERFACE
         -fno-exceptions
+        -fno-strict-aliasing
         -Wall
         -Wcast-align
         -Wconversion
         -Wctor-dtor-privacy
         -Werror
         -Wextra
-        -Wno-missing-braces
-        -Wno-unknown-attributes
-        -Wnon-virtual-dtor
-        -Wold-style-cast
-        -Woverloaded-virtual
         -Wpedantic
         -Wshadow
         -Wsign-conversion
-    >
-)
+        $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:
+          -Weverything
+          -Wno-c++98-compat
+          -Wno-c++98-compat-pedantic
+          -Wno-missing-prototypes
+          -Wno-unknown-attributes
+          -Wno-weak-vtables
+        >
+    )
+endif(MSVC)
 
 # set definitions for tests
 target_compile_definitions(gsl_tests_config_noexcept INTERFACE
diff --git a/tests/multi_span_tests.cpp b/tests/multi_span_tests.cpp
index 9c05cb4..7e3ec89 100644
--- a/tests/multi_span_tests.cpp
+++ b/tests/multi_span_tests.cpp
@@ -1782,4 +1782,4 @@ TEST_CASE("iterator")
 
 #ifdef CONFIRM_COMPILATION_ERRORS
 copy(src_span_static, dst_span_static);
-#endif
\ No newline at end of file
+#endif
diff --git a/tests/no_exception_ensure_tests.cpp b/tests/no_exception_ensure_tests.cpp
index 3a7f007..b1ae15b 100644
--- a/tests/no_exception_ensure_tests.cpp
+++ b/tests/no_exception_ensure_tests.cpp
@@ -14,21 +14,17 @@
 //

 ///////////////////////////////////////////////////////////////////////////////

 

-#include <cstdlib>      // for std::exit

-#include <gsl/span>     // for span

+#include <cstdlib>  // for std::exit

+#include <gsl/span> // for span

 

 int operator_subscript_no_throw() noexcept

 {

-    int arr[10]; 

-    const gsl::span<int> sp { arr };

+    int arr[10];

+    const gsl::span<int> sp{arr};

     return sp[11];

 }

 

-

-void test_terminate()

-{ 

-    std::exit(0);

-}

+[[noreturn]] void test_terminate() { std::exit(0); }

 

 void setup_termination_handler() noexcept

 {

@@ -44,7 +40,6 @@ void setup_termination_handler() noexcept
 #endif

 }

 

-

 int main() noexcept

 {

     setup_termination_handler();

diff --git a/tests/no_exception_throw_tests.cpp b/tests/no_exception_throw_tests.cpp
index e9f4dac..b28ad00 100644
--- a/tests/no_exception_throw_tests.cpp
+++ b/tests/no_exception_throw_tests.cpp
@@ -14,20 +14,17 @@
 //

 ///////////////////////////////////////////////////////////////////////////////

 

-#include <cstdlib>      // for std::exit

-#include <gsl/gsl_util> // for narrow

+#include <cstdlib>        // for std::exit

 #include <gsl/gsl_assert> // for get_terminate

+#include <gsl/gsl_util>   // for narrow

 

 int narrow_no_throw()

 {

     const long long bigNumber = 0x0fffffffffffffff;

-    return gsl::narrow<int>(bigNumber); 

+    return gsl::narrow<int>(bigNumber);

 }

 

-void test_terminate()

-{ 

-    std::exit(0);

-}

+[[noreturn]] void test_terminate() { std::exit(0); }

 

 void setup_termination_handler() noexcept

 {

@@ -43,7 +40,6 @@ void setup_termination_handler() noexcept
 #endif

 }

 

-

 int main()

 {

     setup_termination_handler();

diff --git a/tests/notnull_tests.cpp b/tests/notnull_tests.cpp
index ef767c6..ce5a123 100644
--- a/tests/notnull_tests.cpp
+++ b/tests/notnull_tests.cpp
@@ -251,7 +251,7 @@ TEST_CASE("TestNotNullostream")
 {
     ostream_helper<int>(17);
     ostream_helper<float>(21.5f);
-    ostream_helper<double>(3.4566e-7f);
+    ostream_helper<double>(3.4566e-7);
     ostream_helper<char>('c');
     ostream_helper<uint16_t>(0x0123u);
     ostream_helper<const char*>("cstring");
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index 9637a0e..d22dc08 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -37,9 +37,10 @@
 #include <type_traits> // for integral_constant<>::value, is_default_co...
 #include <vector>      // for vector
 
-namespace gsl {
+namespace gsl
+{
 struct fail_fast;
-}  // namespace gsl
+} // namespace gsl
 
 using namespace std;
 using namespace gsl;
@@ -54,9 +55,12 @@ struct DerivedClass : BaseClass
 };
 struct AddressOverloaded
 {
+#if (__cplusplus > 201402L)
+    [[maybe_unused]]
+#endif
     AddressOverloaded operator&() const { return {}; }
 };
-}
+} // namespace
 
 GSL_SUPPRESS(con.4) // NO-FORMAT: attribute
 TEST_CASE("default_constructor")
@@ -513,8 +517,8 @@ TEST_CASE("from_std_array_constructor")
         static span<int> s2;
         s2 = s1;
 
-    #if __GNUC__ == 6 && (__GNUC_MINOR__ == 4 || __GNUC_MINOR__ == 5) && __GNUC_PATCHLEVEL__ == 0 &&   \
-    defined(__OPTIMIZE__)
+    #if defined(__GNUC__) && __GNUC__ == 6 && (__GNUC_MINOR__ == 4 || __GNUC_MINOR__ == 5) &&      \
+    __GNUC_PATCHLEVEL__ == 0 && defined(__OPTIMIZE__)
         // Known to be broken in gcc 6.4 and 6.5 with optimizations
         // Issue in gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83116
         CHECK(s1.size() == 4);
diff --git a/tests/strided_span_tests.cpp b/tests/strided_span_tests.cpp
index 2310f5b..1563d90 100644
--- a/tests/strided_span_tests.cpp
+++ b/tests/strided_span_tests.cpp
@@ -117,7 +117,7 @@ TEST_CASE("strided_span_constructors")
             CHECK(sav.bounds().strides() == multi_span_index<1>{1});
             CHECK(sav[1] == 2);
 
-#if _MSC_VER > 1800
+#if defined(_MSC_VER) && _MSC_VER > 1800
             // strided_span<const int, 1> sav_c{ {src}, {2, 1} };
             strided_span<const int, 1> sav_c{multi_span<const int>{src},
                                              strided_bounds<1>{2, 1}};
@@ -129,7 +129,7 @@ TEST_CASE("strided_span_constructors")
             CHECK(sav_c.bounds().strides() == multi_span_index<1>{1});
             CHECK(sav_c[1] == 2);
 
-#if _MSC_VER > 1800
+#if defined(_MSC_VER) && _MSC_VER > 1800
             strided_span<volatile int, 1> sav_v{src, {2, 1}};
 #else
             strided_span<volatile int, 1> sav_v{multi_span<volatile int>{src},
@@ -139,7 +139,7 @@ TEST_CASE("strided_span_constructors")
             CHECK(sav_v.bounds().strides() == multi_span_index<1>{1});
             CHECK(sav_v[1] == 2);
 
-#if _MSC_VER > 1800
+#if defined(_MSC_VER) && _MSC_VER > 1800
             strided_span<const volatile int, 1> sav_cv{src, {2, 1}};
 #else
             strided_span<const volatile int, 1> sav_cv{multi_span<const volatile int>{src},
@@ -159,7 +159,7 @@ TEST_CASE("strided_span_constructors")
             CHECK(sav_c.bounds().strides() == multi_span_index<1>{1});
             CHECK(sav_c[1] == 2);
 
-#if _MSC_VER > 1800
+#if defined(_MSC_VER) && _MSC_VER > 1800
             strided_span<const volatile int, 1> sav_cv{src, {2, 1}};
 #else
             strided_span<const volatile int, 1> sav_cv{multi_span<const volatile int>{src},
@@ -180,7 +180,7 @@ TEST_CASE("strided_span_constructors")
             CHECK(sav_v.bounds().strides() == multi_span_index<1>{1});
             CHECK(sav_v[1] == 2);
 
-#if _MSC_VER > 1800
+#if defined(_MSC_VER) && _MSC_VER > 1800
             strided_span<const volatile int, 1> sav_cv{src, {2, 1}};
 #else
             strided_span<const volatile int, 1> sav_cv{multi_span<const volatile int>{src},
@@ -598,7 +598,7 @@ void iterate_every_other_element(multi_span<int, dynamic_range> av)
     // pick every other element
 
     auto length = av.size() / 2;
-#if _MSC_VER > 1800
+#if defined(_MSC_VER) && _MSC_VER > 1800
     auto bounds = strided_bounds<1>({length}, {2});
 #else
     auto bounds = strided_bounds<1>(multi_span_index<1>{length}, multi_span_index<1>{2});
diff --git a/tests/utils_tests.cpp b/tests/utils_tests.cpp
index 5cacd97..3d54ce6 100644
--- a/tests/utils_tests.cpp
+++ b/tests/utils_tests.cpp
@@ -80,7 +80,7 @@ TEST_CASE("finally_function_with_bind")
     CHECK(i == 1);
 }
 
-int j = 0;
+static int j = 0;
 void g() { j += 1; }
 TEST_CASE("finally_function_ptr")
 {