fix: preserve haskell.nix UCRT pthread flags

scripts/nix/README.md

@@ -279,6 +279,8 @@ The Windows CRT overlay backports upstream MinGW commit `e7e21db45eaa` to the
 verified 13.0.0/14.0.0 sources. It fixes ISO `swprintf` rejecting strings that
 exactly fit with their terminator; the regression also requires a negative
 return for a too-small buffer and intact outside-buffer sentinels.
+The overlay preserves haskell.nix's existing UCRT pthread flags when rebuilding
+the Windows scope; the current SimpleX Windows targets use MSVCRT.
 It requires real file opens, descriptor/stat information and fixture bytes for
 short, long and archive-shaped paths, including absolute and extended variants.
 Helper results and a separate direct Win32/CRT replay remain diagnostic output.

scripts/nix/check-mingw-stdio-scope.nix

@@ -3,7 +3,7 @@ let
   overlay = import ./mingw-stdio-overlay.nix;
   existing = ./ghc914-rts-windows-dll-search-path.patch;
   fix = ./mingw-w64-swprintf-exact-fit.patch;
-  apply = { windows ? true, version ? "14.0.0", patches ? [ existing ] }:
+  apply = { windows ? true, libc ? "msvcrt", version ? "14.0.0", patches ? [ existing ] }:
     let
       runtime = {
         inherit version patches;
@@ -12,14 +12,20 @@ let
       };
       scope = {
         mingw_w64 = runtime;
+        mingw_w64_pthreads.CPPFLAGS = "";
         unchanged = "other-Windows-packages";
         overrideScope = f: scope // f scope scope;
       };
+      # haskell.nix adds UCRT's flags with a shallow `windows // { ...; }`.
+      # overrideScope still closes over the original scope and drops that field.
+      customized = scope // (if libc == "ucrt" then {
+        mingw_w64_pthreads = { CPPFLAGS = "-D_UCRT"; marker = "existing-UCRT-customization"; };
+      } else {});
       prev = {
         lib.optionalAttrs = condition: attrs: if condition then attrs else {};
-        stdenv.hostPlatform.isWindows = windows;
+        stdenv.hostPlatform = { isWindows = windows; inherit libc; };
         # Non-Windows evaluation must not force any Windows dependencies.
-        windows = if windows then scope else throw "Windows scope was forced";
+        windows = if windows then customized else throw "Windows scope was forced";
       };
     in overlay prev prev;
   selected = apply {};
@@ -27,6 +33,12 @@ in
 assert selected.windows.mingw_w64.patches == [ existing fix ];
 assert selected.windows.mingw_w64.marker == "existing-runtime-attributes";
 assert selected.windows.unchanged == "other-Windows-packages";
+assert selected.windows.mingw_w64_pthreads.CPPFLAGS == "";
+assert (apply { libc = "ucrt"; }).windows.mingw_w64_pthreads == {
+  CPPFLAGS = "-D_UCRT";
+  marker = "existing-UCRT-customization";
+};
+assert (apply { libc = "ucrt"; }).windows.mingw_w64.patches == [ existing fix ];
 assert (apply { patches = []; }).windows.mingw_w64.patches == [ fix ];
 assert (apply { version = "13.0.0"; }).windows.mingw_w64.patches == [ existing fix ];
 assert (apply { version = "12.0.0"; }).windows.mingw_w64.patches == [ existing ];
@@ -35,6 +47,7 @@ assert apply { windows = false; } == {};
 {
   verifiedMingw13And14RuntimePatched = true;
   existingPatchOrderAndWindowsScopePreserved = true;
+  shallowUcrtPthreadCustomizationPreserved = true;
   unrelatedPlatformsAndVersionsUnchanged = true;
   nonWindowsDependenciesRemainLazy = true;
 }

scripts/nix/mingw-stdio-overlay.nix

@@ -2,12 +2,16 @@ final: prev:
 prev.lib.optionalAttrs prev.stdenv.hostPlatform.isWindows {
   # The CRT owns libmingwex.a. Patching headers or GHC's callers alone leaves
   # ISO swprintf rejecting successful exact-fit strings at runtime.
-  windows = prev.windows.overrideScope (_self: super: {
+  windows = (prev.windows.overrideScope (_self: super: {
     # Inspect the version only after constructing the Windows scope: forcing
     # the CRT in the outer condition recurses through the cross libc bootstrap.
     mingw_w64 = super.mingw_w64.overrideAttrs (old:
       prev.lib.optionalAttrs (builtins.elem old.version [ "13.0.0" "14.0.0" ]) {
         patches = (old.patches or []) ++ [ ./mingw-w64-swprintf-exact-fit.patch ];
       });
-  });
+  })) // prev.lib.optionalAttrs (prev.stdenv.hostPlatform.libc == "ucrt") {
+    # haskell.nix adds this alias with a shallow scope extension. overrideScope
+    # rebuilds the original scope, so retain its existing UCRT CPPFLAGS.
+    inherit (prev.windows) mingw_w64_pthreads;
+  };
 }