AMReX C++20 in VS2026

[ax3l]

According to a non-descript bug report in WarpX, AMReX might not yet compile with the latest VS 2026 (MSVC) for Windows. VS 2026 uses C++20 by default, so I suspect it is somehow related to C++20 in MSVC.

We should update our CI runners to add a C++20 build in those and fix issues that we see.

[ax3l]

Problem is WarpX specific: BLAST-WarpX/warpx#6406 (comment)

[ax3l]

Ah, but the errors are in AMReX LinearSolvers MLMG:
BLAST-WarpX/warpx#6448

...
2025-12-09T18:27:19.9118795Z     10>D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(24,30): error C2039: 'fab_type': is not a member of 'std::array<amrex::MultiFab,3>' [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T18:27:19.9120581Z          (compiling source file '../Source/Initialization/DivCleaner/ProjectionDivCleaner.cpp')
2025-12-09T18:27:19.9121683Z              C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\array(402,7):
2025-12-09T18:27:19.9122634Z              see declaration of 'std::array<amrex::MultiFab,3>'
2025-12-09T18:27:19.9123488Z              D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(24,30):
2025-12-09T18:27:19.9124438Z              the template instantiation context (the oldest one first) is
2025-12-09T18:27:19.9125373Z                  D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCurlCurl.H(25,14):
2025-12-09T18:27:19.9126555Z                  see reference to class template instantiation 'amrex::MLLinOpT<std::array<amrex::MultiFab,3>>' being compiled
2025-12-09T18:27:19.9127771Z                  D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(522,1):
2025-12-09T18:27:19.9129199Z                  while compiling class template member function 'void amrex::MLLinOpT<std::array<amrex::MultiFab,3>>::setLevelBC(int,const MF *,const MF *,const MF *,const MF *)'
2025-12-09T18:27:19.9131962Z                  with
2025-12-09T18:27:19.9132345Z                  [
2025-12-09T18:27:19.9132740Z                      MF=std::array<amrex::MultiFab,3>
2025-12-09T18:27:19.9133257Z                  ]
2025-12-09T18:27:19.9133918Z                  D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(520,19):
2025-12-09T18:27:19.9135017Z                  see reference to class template instantiation 'amrex::MLCellLinOpT<MF>' being compiled
2025-12-09T18:27:19.9136320Z                  with
2025-12-09T18:27:19.9136702Z                  [
2025-12-09T18:27:19.9137138Z                      MF=std::array<amrex::MultiFab,3>
2025-12-09T18:27:19.9137589Z                  ]
2025-12-09T18:27:19.9137932Z          
2025-12-09T18:27:19.9139009Z     10>D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(24,30): error C2061: syntax error: identifier 'fab_type' [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T18:27:19.9140550Z          (compiling source file '../Source/Initialization/DivCleaner/ProjectionDivCleaner.cpp')
2025-12-09T18:27:19.9141255Z          
2025-12-09T18:27:19.9425450Z     10>D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(107,16): error C2555: 'amrex::MLCellLinOpT<MF>::getSolvabilityOffset': overriding virtual function return type differs and is not covariant from 'amrex::MLLinOpT<std::array<amrex::MultiFab,3>>::getSolvabilityOffset' [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T18:27:19.9427089Z D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(107,16): error C2555:         with [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T18:27:19.9428020Z D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(107,16): error C2555:         [ [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T18:27:19.9428990Z D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(107,16): error C2555:             MF=std::array<amrex::MultiFab,3> [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T18:27:19.9430053Z D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(107,16): error C2555:         ] [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T18:27:19.9430840Z          (compiling source file '../Source/Initialization/DivCleaner/ProjectionDivCleaner.cpp')
...

job-logs.txt

[ax3l]

Could be a missing include of #include <AMReX_MultiFab.H> in Src/LinearSolvers/MLMG/AMReX_MLCellLinOp.H?

[WeiqunZhang]

I think MSVC is confused by setLevelBC, which has a virtual function version and a template version.

[WeiqunZhang]

If you have a test set up, we can try

diff --git a/Src/LinearSolvers/MLMG/AMReX_MLCellLinOp.H b/Src/LinearSolvers/MLMG/AMReX_MLCellLinOp.H
index c63921ced1..a45d8ea198 100644
--- a/Src/LinearSolvers/MLMG/AMReX_MLCellLinOp.H
+++ b/Src/LinearSolvers/MLMG/AMReX_MLCellLinOp.H
@@ -21,8 +21,8 @@ class MLCellLinOpT  // NOLINT(cppcoreguidelines-virtual-class-destructor)
 {
 public:
 
-    using FAB = typename MF::fab_type;
-    using RT  = typename MF::value_type;
+    using FAB = typename FabDataType<MF>::fab_type;
+    using RT  = typename FabDataType<MF>::value_type;
 
     using BCType = LinOpBCType;
     using BCMode    = typename MLLinOpT<MF>::BCMode;

[ax3l]

Good idea, trying in BLAST-WarpX/warpx#6448

Branch: https://github.com/ax3l/amrex/tree/fix-linop-mlmg

[ax3l]

That changes the error somewhat, now:

...
2025-12-09T19:24:28.8177040Z     10>D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(533,14): error C2039: 'define': is not a member of 'std::array<amrex::MultiFab,3>' [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T19:24:28.8178335Z          (compiling source file '../Source/Initialization/DivCleaner/ProjectionDivCleaner.cpp')
2025-12-09T19:24:28.8179762Z              C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\array(402,7):
2025-12-09T19:24:28.8180765Z              see declaration of 'std::array<amrex::MultiFab,3>'
2025-12-09T19:24:28.8181610Z              D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(533,14):
2025-12-09T19:24:28.8182674Z              the template instantiation context (the oldest one first) is
2025-12-09T19:24:28.8183605Z                  D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCurlCurl.H(25,14):
2025-12-09T19:24:28.8184845Z                  see reference to class template instantiation 'amrex::MLLinOpT<std::array<amrex::MultiFab,3>>' being compiled
2025-12-09T19:24:28.8187644Z                  D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(522,1):
2025-12-09T19:24:28.8189106Z                  while compiling class template member function 'void amrex::MLLinOpT<std::array<amrex::MultiFab,3>>::setLevelBC(int,const MF *,const MF *,const MF *,const MF *)'
2025-12-09T19:24:28.8190273Z                  with
2025-12-09T19:24:28.8190634Z                  [
2025-12-09T19:24:28.8191003Z                      MF=std::array<amrex::MultiFab,3>
2025-12-09T19:24:28.8191469Z                  ]
2025-12-09T19:24:28.8191777Z          
2025-12-09T19:24:28.8193009Z     10>D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(534,14): error C2039: 'setVal': is not a member of 'std::array<amrex::MultiFab,3>' [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T19:24:28.8195059Z          (compiling source file '../Source/Initialization/DivCleaner/ProjectionDivCleaner.cpp')
2025-12-09T19:24:28.8196192Z              C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\array(402,7):
2025-12-09T19:24:28.8197140Z              see declaration of 'std::array<amrex::MultiFab,3>'
2025-12-09T19:24:28.8197625Z          
2025-12-09T19:24:28.8198875Z     10>D:\a\warpx\warpx\build\_deps\fetchedamrex-src\Src\LinearSolvers\MLMG\AMReX_MLCellLinOp.H(536,9): error C2039: 'nGrowVect': is not a member of 'std::array<amrex::MultiFab,3>' [D:\a\warpx\warpx\build\lib_3d.vcxproj]
2025-12-09T19:24:28.8200535Z          (compiling source file '../Source/Initialization/DivCleaner/ProjectionDivCleaner.cpp')
2025-12-09T19:24:28.8201679Z              C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\array(402,7):
2025-12-09T19:24:28.8202616Z              see declaration of 'std::array<amrex::MultiFab,3>'
...

job-logs.txt