Try ucrt64

ci.nix

@@ -102,6 +102,10 @@
       && ((system == "x86_64-linux"  && builtins.elem compiler-nix-name ["ghc8107" "ghc902" "ghc926" "ghc927" "ghc928" "ghc947" "ghc963" (ghc980X nixpkgs) (ghc99X nixpkgs)])
        || (system == "x86_64-darwin" && builtins.elem compiler-nix-name []))) { # TODO add ghc versions when we have more darwin build capacity
     inherit (lib.systems.examples) mingwW64;
+  } // lib.optionalAttrs (nixpkgsName == "unstable"
+      && ((system == "x86_64-linux"  && builtins.elem compiler-nix-name ["ghc947" "ghc963" (ghc980X nixpkgs) (ghc99X nixpkgs)])
+       || (system == "x86_64-darwin" && builtins.elem compiler-nix-name []))) { # TODO add ghc versions when we have more darwin build capacity
+    inherit (lib.systems.examples) ucrt64;
   } // lib.optionalAttrs (system == "x86_64-linux" && nixpkgsName == "unstable" && builtins.elem compiler-nix-name ["ghc8107" "ghc902" "ghc922" "ghc923" "ghc924" "ghc926" "ghc927" "ghc928" "ghc947" "ghc963" (ghc980X nixpkgs) (ghc99X nixpkgs)]) {
     # Musl cross only works on linux
     # aarch64 cross only works on linux

overlays/bootstrap.nix

@@ -206,13 +206,15 @@ in {
                 # if the host system provides ucrt (e.g. wine with ucrtbase.dll), we may end up linking against symbols from ucrtbase, instead of msvcrt,
                 # thus leading to broken code.  E.g. the handles we create and hand to wine will all be busted, because they come from one and are processed
                 # by another crt.
-                ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "8.10"   "9.8"   ./patches/ghc/win-linker-no-ucrt.patch)
-                # Nixos/nixpkgs is mscvrt for now, thus we must disable ucrt in ghc, otherwise we end up with broken linking.
-                ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.4.1"  "9.6"    ./patches/ghc/no-ucrt-9.4.patch)
-                ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.6.1"  "9.6.3"  ./patches/ghc/no-ucrt-9.6.patch)
-                ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.6.3"  "9.8"    ./patches/ghc/no-ucrt-9.6.3.patch)
-                ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.8"    "9.9"    ./patches/ghc/no-ucrt-9.8.patch)
-                ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.9"    "9.10"   ./patches/ghc/no-ucrt-9.9.patch)
+                ++ final.lib.optionals (final.stdenv.targetPlatform.libc != "ucrt") (
+                    final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "8.10"   "9.8"   ./patches/ghc/win-linker-no-ucrt.patch)
+                  # Nixos/nixpkgs is mscvrt for now, thus we must disable ucrt in ghc, otherwise we end up with broken linking.
+                  ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.4.1"  "9.6"    ./patches/ghc/no-ucrt-9.4.patch)
+                  ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.6.1"  "9.6.3"  ./patches/ghc/no-ucrt-9.6.patch)
+                  ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.6.3"  "9.8"    ./patches/ghc/no-ucrt-9.6.3.patch)
+                  ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.8"    "9.9"    ./patches/ghc/no-ucrt-9.8.patch)
+                  ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.9"    "9.10"   ./patches/ghc/no-ucrt-9.9.patch)
+                )
                 ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.4.7"  "9.5"    ./patches/ghc/revert-289547580b6f2808ee123f106c3118b716486d5b.patch)
                 ++ final.lib.optionals (final.stdenv.targetPlatform.isWindows) (fromUntil "9.6.3"  "9.10"   ./patches/ghc/revert-289547580b6f2808ee123f106c3118b716486d5b.patch)
                 # the following is needed for cardano-prelude as it uses closure_sizeW :-/
@@ -1019,8 +1021,9 @@ in {
 
                 bootPkgs = bootPkgsGhc94 // {
                   ghc = if final.stdenv.buildPlatform != final.stdenv.targetPlatform
-                    then final.buildPackages.buildPackages.haskell-nix.compiler.ghc962
-                    else final.buildPackages.buildPackages.haskell.compiler.ghc962
+                    then final.buildPackages.buildPackages.haskell-nix.compiler.ghc963
+                    else final.buildPackages.buildPackages.haskell.compiler.ghc963
+                          or final.buildPackages.buildPackages.haskell.compiler.ghc962
                           or final.buildPackages.buildPackages.haskell.compiler.ghc945
                           or final.buildPackages.buildPackages.haskell.compiler.ghc944
                           or final.buildPackages.buildPackages.haskell.compiler.ghc943;

overlays/mingw_w64.nix

@@ -69,14 +69,14 @@ let
   ################################################################################
   # Build logic (TH support via remote iserv via wine)
   #
-  setupBuildFlags = map (opt: "--ghc-option=" + opt) (lib.optionals hostPlatform.isWindows [
+  setupBuildFlags = map (opt: "--ghc-option=" + opt) (lib.optionals hostPlatform.isWindows ([
     "-fexternal-interpreter"
     "-pgmi" "${wineIservWrapper}/bin/iserv-wrapper"
     # TODO: this should be automatically injected based on the extraLibrary.
     "-L${mingw_w64_pthreads}/lib"
     "-L${mingw_w64_pthreads}/bin"
     "-L${gmp}/lib"
-    ]);
+    ]));
 
   ################################################################################
   # Test logic via wine

overlays/patches/ghc/win-add-tzset-to-rtssyms.patch

@@ -2,12 +2,14 @@ diff --git a/rts/RtsSymbols.c b/rts/RtsSymbols.c
 index 10efb2a..d8ea070 100644
 --- a/rts/RtsSymbols.c
 +++ b/rts/RtsSymbols.c
-@@ -163,7 +163,9 @@ extern char **environ;
+@@ -163,7 +163,11 @@ extern char **environ;
        SymI_HasProto(__mingw_vfprintf)                    \
        /* ^^ Need to figure out why this is needed.  */   \
        SymI_HasProto(closure_sizeW_)                      \
 -      /* ^^ This one needed for cardano-prelude m(  */
 +      /* ^^ This one needed for cardano-prelude m(  */   \
++      SymI_NeedsProto(_tzset)                            \
++      /* ^^ This one needed for time, tzset deprecated */\
 +      SymI_NeedsProto(tzset)                             \
 +      /* ^^ This one needed for unix-time           */
  #else

overlays/windows.nix

@@ -17,6 +17,11 @@ final: prev:
    mfpr = if !prev.stdenv.hostPlatform.isWindows then prev.mpfr else prev.mfpr.overrideAttrs (drv: {
      configureFlags = (drv.configureFlags or []) ++ [ "--enable-static --disable-shared" ];
    });
+} // prev.lib.optionalAttrs (prev.stdenv.hostPlatform.isWindows && prev.stdenv.hostPlatform.libc == "ucrt") {
+  windows = prev.windows // {
+    # TODO update stdenv.cc so that the wrapper adds -D_UCRT for libc=="ucrt"
+    mingw_w64_pthreads = prev.windows.mingw_w64_pthreads.overrideAttrs { CPPFLAGS = "-D_UCRT"; };
+  };
 } // {
    libmpc = if !prev.stdenv.hostPlatform.isWindows then prev.libmpc else prev.libmpc.overrideAttrs (drv: {
      configureFlags = (drv.configureFlags or []) ++ [ "--enable-static --disable-shared" ];

test/th-dlls/default.nix

@@ -24,7 +24,10 @@ in recurseIntoAttrs {
     # the macOS linker tries to load `clang++` :facepalm:
     (stdenv.hostPlatform.isDarwin && compareGhc "9.4.0" >= 0) ||
     # On aarch64 this test also breaks form musl builds (including cross compiles on x86_64-linux)
-    (stdenv.hostPlatform.isAarch64 && stdenv.hostPlatform.isMusl);
+    (stdenv.hostPlatform.isAarch64 && stdenv.hostPlatform.isMusl) ||
+    # broken on ucrt64 windows
+    (stdenv.hostPlatform.libc == "ucrt")
+    ;
 
   ifdInputs = {
     inherit (project true) plan-nix;