Eigen failed with error C2440 with MSVC on windows

Hi All,

I tried to build Eigen on Windows with VS2019. It failed to build due to error C2440: 'return': cannot convert from 'const Packet' to 'Eigen::internal::unpacket_traitsEigen::internal::Packet16b::type'. It can be reproduced on latest reversion 8d1302f5. Could you please help take a look at this? Thanks!

Steps to reproduce:

1. git clone https://gitlab.com/libeigen/eigen F:\gitP\libeigen\eigen
2. Open a VS 2019 x64 command prompt and browse to F:\gitP\libeigen\eigen
3. mkdir build_amd64 && pushd build_amd64
4. cmake -G "Visual Studio 16 2019" -A x64 -DCMAKE_SYSTEM_VERSION=10.0.18362.0 -DEIGEN_BUILD_BTL=ON ..
5. msbuild /m /p:Platform=x64 /p:Configuration=Release Eigen3.sln /t:Rebuild /p:PreferredToolArchitecture=x64

Actual result: "F:\gitP\libeigen\eigen\build_amd64\Eigen3.sln" (Rebuild target) (1) -> "F:\gitP\libeigen\eigen\build_amd64\buildtests.vcxproj.metaproj" (Rebuild target) (148) -> "F:\gitP\libeigen\eigen\build_amd64\test\packetmath_14.vcxproj.metaproj" (Rebuild target) (510) -> "F:\gitP\libeigen\eigen\build_amd64\test\packetmath_14.vcxproj" (Rebuild target) (1225) -> (ClCompile target) -> F:\gitP\libeigen\eigen\Eigen\src/Core/GenericPacketMath.h(482,1): error C2440: 'return': cannot convert from ' const Packet' to 'Eigen::internal::unpacket_traitsEigen::internal::Packet16b::type' [F:\gitP\libeigen\eigen\bu ild_amd64\test\packetmath_14.vcxproj]

615 Warning(s)
1 Error(s)

Time Elapsed 00:58:29.69