gcc: do not allow version skew when cross-building gcc (#352821)

Author: emilazy

pkgs/development/compilers/gcc/all.nix

@@ -1,7 +1,6 @@
 { lib
 , stdenv
-, gccStdenv
-, gcc9Stdenv
+, pkgs
 , callPackage
 , isl_0_20
 , libcCross
@@ -15,6 +14,7 @@ let
   versions = import ./versions.nix;
   gccForMajorMinorVersion = majorMinorVersion:
     let
+      majorVersion = lib.versions.major majorMinorVersion;
       atLeast = lib.versionAtLeast majorMinorVersion;
       attrName = "gcc${lib.replaceStrings ["."] [""] majorMinorVersion}";
       pkg = lowPrio (wrapCC (callPackage ./default.nix {
@@ -24,7 +24,25 @@ let
         profiledCompiler = false;
         libcCross = if stdenv.targetPlatform != stdenv.buildPlatform then args.libcCross else null;
         threadsCross = if stdenv.targetPlatform != stdenv.buildPlatform then threadsCross else { };
-        isl = if stdenv.hostPlatform.isDarwin then null else isl_0_20;
+        # do not allow version skew when cross-building gcc
+        #
+        # When `gcc` is cross-built (`build` != `target` && `host` == `target`)
+        # `gcc` assumes that it has a compatible cross-compiler in the environment
+        # that can build target libraries. Version of a cross-compiler has to
+        # match the compiler being cross-built as libraries frequently use fresh
+        # compiler features, like `-std=c++26` or target-specific types like
+        # `_Bfloat16`.
+        # Version mismatch causes build failures like:
+        #     https://github.com/NixOS/nixpkgs/issues/351905
+        #
+        # Similar problems (but on a smaller scale) happen when a `gcc`
+        # cross-compiler is built (`build` == `host` && `host` != `target`) built
+        # by a mismatching version of a native compiler (`build` == `host` &&
+        # `host` == `target`).
+        #
+        # Let's fix both problems by requiring the same compiler version for
+        # cross-case.
+        stdenv = if (stdenv.targetPlatform != stdenv.buildPlatform || stdenv.hostPlatform != stdenv.targetPlatform) && stdenv.cc.isGNU then pkgs."gcc${majorVersion}Stdenv" else stdenv;
       }));
     in
       lib.nameValuePair attrName pkg;