cgal: update depends versions for 6.0.1 (spack#47516)

* This extends PR spack#47285 to properly include some of the required version constrains of cgal 6 incl C++ standard. It also adds the new no-gmp backend as a variant.

* fix style

* disable cgal@6 +demo variant as the demos require qt6 which is not in spack

* disable the gmp variant until clarity on how its supposed to work is provided. bound shared and header_only variants to relevant versions

* Fix missing msvc compiler limit, fix variant left in

* Add more comments. Better describe the gmp variant. Remove testing code

* fix style

var/spack/repos/builtin/packages/cgal/package.py

@@ -36,14 +36,21 @@ class Cgal(CMakePackage):
 
     depends_on("cxx", type="build")  # generated
 
-    variant("shared", default=True, description="Enables the build of shared libraries")
+    # @5: is header only and doesn't build shared libs
+    variant(
+        "shared", default=True, description="Enables the build of shared libraries", when="@:4.14"
+    )
+
     variant(
         "build_type",
         default="Release",
         description="The build type to build",
         values=("Debug", "Release"),
     )
-    variant("header_only", default=False, description="Install in header only mode")
+
+    # header only is the default and only option for 5+
+    # https://doc.cgal.org/latest/Manual/installation.html
+    variant("header_only", default=False, description="Install in header only mode", when="@:4.13")
 
     # ---- See "7 CGAL Libraries" at:
     # https://doc.cgal.org/latest/Manual/installation.html
@@ -53,24 +60,43 @@ class Cgal(CMakePackage):
     #      https://cs.nyu.edu/exact/core_pages/svn-core.html
     variant("core", default=False, description="Build the CORE library for algebraic numbers")
     variant("imageio", default=False, description="Build utilities to read/write image files")
-    variant("demos", default=False, description="Build CGAL demos")
+    variant("demos", default=False, description="Build CGAL demos", when="@:5")
     variant("eigen", default=True, description="Build with Eigen support")
 
-    depends_on("cmake@2.8.11:", type="build")
+    # Starting with cgal 6, GMP/MPFR are no longer mandatory and Core library
+    # is based on on Boost.Multiprecision. However, either GMP backend or Boost
+    # backend can be used. Downstream cmake users must also set -DCGAL_DISABLE_GMP=1
+    # or the macro CMAKE_OVERRIDDEN_DEFAULT_ENT_BACKEND if GMP is disabled.
+    # This variant doesn't change how cgal is installed, but it does change spack to
+    # not depend on gmp & mpfr.
+    # More details here https://github.com/CGAL/cgal/issues/8606
+    variant("gmp", default=True, description="Enable the GMP backend", when="@6:")
+
+    # Upper bound follows CGAL's @6: CMakeLists.txt
+    depends_on("cmake@3.12:3.29", type="build", when="@6:")
+    depends_on("cmake@2.8.11:", type="build", when="@:5")
 
     # Essential Third Party Libraries
     depends_on("boost+exception+math+random+container", when="@5.0:")
+    depends_on("boost@1.72.0:+exception+math+random+container", when="@6:")
     depends_on("boost+thread+system", when="@:5.0")
-    depends_on("gmp")
-    depends_on("mpfr")
+
+    depends_on("gmp", when="@:5")
+    depends_on("mpfr", when="@:5")
+
+    depends_on("gmp", when="@6: +gmp")
+    depends_on("mpfr", when="@6: +gmp")
 
     # Required for CGAL_ImageIO
     # depends_on('opengl', when='+imageio') # not yet in Spack
     depends_on("zlib-api")
 
     # Optional to build CGAL_Qt5 (demos)
     # depends_on('opengl', when='+demos')   # not yet in Spack
-    depends_on("qt@5:", when="+demos")
+    depends_on("qt@5:", when="@:5 +demos")
+
+    # Demos are now based on qt6, but at the moment qt6 is not in spack
+    # depends_on("qt@6:", when="@6: +demos")
 
     # Optional Third Party Libraries
     depends_on("eigen", when="+eigen")
@@ -84,6 +110,24 @@ class Cgal(CMakePackage):
     # depends_on('esbtl')
     # depends_on('intel-tbb')
 
+    # @6: requires C++17 or later. The table gives tested
+    # compilers, so use the lwoer limit of that as the bounds
+    # https://www.cgal.org/2024/10/22/cgal601/
+    with when("@6:"):
+        # Gnu g++ 11.4.0 or later (on Linux or macOS)
+        conflicts("%gcc @:11.3.0", when="platform=darwin")
+        conflicts("%gcc @:11.3.0", when="platform=linux")
+
+        # LLVM Clang version 15.0.7 or later (on Linux)
+        conflicts("%clang @:15.0.6", when="platform=linux")
+
+        # Apple Clang compiler versions 10.0.1, 12.0.5, and 15.0.0 (on macOS)
+        # (10+ has C++17 support)
+        conflicts("%apple-clang @:10.0.0", when="platform=darwin")
+
+        # Visual C++ 15.9 or later
+        conflicts("%msvc @:15.8", when="platform=windows")
+
     conflicts(
         "~header_only",
         when="@:4.9",
@@ -120,7 +164,13 @@ def cmake_args(self):
         cmake_args.append("-DWITH_CGAL_ImageIO:BOOL=%s" % variant_bool("+imageio"))
         cmake_args.append("-DWITH_CGAL_Qt5:BOOL=%s" % variant_bool("+demos"))
 
+        if spec.satisfies("@6:"):
+            cmake_args.append("-DCXX_STANDARD=17")
+
         if spec.satisfies("@4.9:"):
             cmake_args.append("-DCGAL_HEADER_ONLY:BOOL=%s" % variant_bool("+header_only"))
 
+        if spec.satisfies("~gmp"):
+            cmake_args.append("-DCGAL_DISABLE_GMP:BOOL=1")
+
         return cmake_args