Fix -Wuseless-cast compiler warnings (#1145)

**Issue:**
#973

This warning exists in GCC only (not Clang), and for C++ only (not C)

**Description of changes:**
- Add `-Wuseless-cast` warning to the "header checker"
- Remove useless casts in `array_list.inl`
- Ignore `-Wuseless-cast` warning in `math.inl`
    - The tricky part is that SOME platforms (Apple and BSD variants) define `size_t` as `unsigned long` and require casting to `uint64_t` (aka `unsigned long long`). While other platforms define `size_t` as `unsigned long long` and warn about useless casts if you cast it to `uint64_t`. My first commit attempted to only do casts on those platforms, but it was more complex than simply ignoring the warning.

cmake/AwsCheckHeaders.cmake

@@ -63,13 +63,22 @@ function(aws_check_headers_internal target std is_cxx)
         C_STANDARD 99
     )
 
-    # Ensure our headers can be included by an application with its warnings set very high
+    # Ensure our headers can be included by an application with its warnings set very high.
+    # Most compiler options are universal, but some are C++ only.
+    set(compiler_options_all "")
+    set(compiler_options_cxx_only "")
     if(MSVC)
-        # MSVC complains about windows' own header files. Use /W4 instead of /Wall 
-        target_compile_options(${HEADER_CHECKER_LIB} PRIVATE /W4 /WX)
+        # MSVC complains about windows' own header files. Use /W4 instead of /Wall
+        list(APPEND compiler_options_all /W4 /WX)
     else()
-        target_compile_options(${HEADER_CHECKER_LIB} PRIVATE -Wall -Wextra -Wpedantic -Werror)
+        list(APPEND compiler_options_all -Wall -Wextra -Wpedantic -Werror)
+
+        if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+            # -Wuseless-cast requested by https://github.com/awslabs/aws-c-common/issues/973
+            list(APPEND compiler_options_cxx_only -Wuseless-cast)
+        endif()
     endif()
+    target_compile_options(${HEADER_CHECKER_LIB} PRIVATE ${compiler_options_all})
 
     foreach(header IN LISTS ARGN)
         if (NOT ${header} MATCHES "\\.inl$")
@@ -78,6 +87,7 @@ function(aws_check_headers_internal target std is_cxx)
             file(RELATIVE_PATH include_path "${CMAKE_CURRENT_SOURCE_DIR}/include" ${header})
             # replace non-alphanumeric characters with underscores
             string(REGEX REPLACE "[^a-zA-Z0-9]" "_" unique_token ${include_path})
+            # test compiling header from a .cpp and .c file (or just .cpp if this header IS_CXX)
             set(c_file "${HEADER_CHECKER_ROOT}/headerchecker_${unique_token}.c")
             set(cpp_file "${HEADER_CHECKER_ROOT}/headerchecker_${unique_token}.cpp")
             # include header twice to check for include-guards
@@ -88,6 +98,11 @@ function(aws_check_headers_internal target std is_cxx)
                 file(WRITE "${c_file}" "#include <${include_path}>\n#include <${include_path}>\nint ${unique_token}_c;\n")
                 target_sources(${HEADER_CHECKER_LIB} PUBLIC "${c_file}")
             endif()
+
+            # for .cpp file, apply C++ only compiler options
+            if(compiler_options_cxx_only)
+                set_source_files_properties(${cpp_file} PROPERTIES COMPILE_OPTIONS ${compiler_options_cxx_only})
+            endif()
         endif()
     endforeach(header)
 endfunction()

include/aws/common/array_list.inl

@@ -123,7 +123,7 @@ AWS_STATIC_IMPL
 void aws_array_list_clean_up_secure(struct aws_array_list *AWS_RESTRICT list) {
     AWS_PRECONDITION(AWS_IS_ZEROED(*list) || aws_array_list_is_valid(list));
     if (list->alloc && list->data) {
-        aws_secure_zero((void *)list->data, list->current_size);
+        aws_secure_zero(list->data, list->current_size);
         aws_mem_release(list->alloc, list->data);
     }
 

include/aws/common/math.inl

@@ -51,7 +51,12 @@ AWS_EXTERN_C_BEGIN
 #ifdef _MSC_VER
 #    pragma warning(push)
 #    pragma warning(disable : 4127) /*Disable "conditional expression is constant" */
-#endif                              /* _MSC_VER */
+#elif defined(__GNUC__)
+#    pragma GCC diagnostic push
+#    if defined(__cplusplus) && !defined(__clang__)
+#        pragma GCC diagnostic ignored "-Wuseless-cast" /* Warning is C++ only (not C), and GCC only (not clang) */
+#    endif
+#endif
 
 AWS_STATIC_IMPL uint64_t aws_sub_u64_saturating(uint64_t a, uint64_t b) {
     return a <= b ? 0 : a - b;
@@ -190,6 +195,8 @@ AWS_STATIC_IMPL int aws_round_up_to_power_of_two(size_t n, size_t *result) {
 
 #ifdef _MSC_VER
 #    pragma warning(pop)
+#elif defined(__GNUC__)
+#    pragma GCC diagnostic pop
 #endif /* _MSC_VER */
 
 AWS_STATIC_IMPL uint8_t aws_min_u8(uint8_t a, uint8_t b) {