clang: support cxx20_flag and cxx23_flag (#43438)

* clang: support cxx20_flag and cxx23_flag

* clang: coverage test cxx{}_flag and c{}_flag additions

lib/spack/spack/compilers/clang.py

@@ -96,6 +96,8 @@ def verbose_flag(self):
 
     openmp_flag = "-fopenmp"
 
+    # C++ flags based on CMake Modules/Compiler/Clang.cmake
+
     @property
     def cxx11_flag(self):
         if self.real_version < Version("3.3"):
@@ -120,6 +122,24 @@ def cxx17_flag(self):
 
         return "-std=c++17"
 
+    @property
+    def cxx20_flag(self):
+        if self.real_version < Version("5.0"):
+            raise UnsupportedCompilerFlag(self, "the C++20 standard", "cxx20_flag", "< 5.0")
+        elif self.real_version < Version("11.0"):
+            return "-std=c++2a"
+        else:
+            return "-std=c++20"
+
+    @property
+    def cxx23_flag(self):
+        if self.real_version < Version("12.0"):
+            raise UnsupportedCompilerFlag(self, "the C++23 standard", "cxx23_flag", "< 12.0")
+        elif self.real_version < Version("17.0"):
+            return "-std=c++2b"
+        else:
+            return "-std=c++23"
+
     @property
     def c99_flag(self):
         return "-std=c99"
@@ -142,7 +162,10 @@ def c17_flag(self):
     def c23_flag(self):
         if self.real_version < Version("9.0"):
             raise UnsupportedCompilerFlag(self, "the C23 standard", "c23_flag", "< 9.0")
-        return "-std=c2x"
+        elif self.real_version < Version("18.0"):
+            return "-std=c2x"
+        else:
+            return "-std=c23"
 
     @property
     def cc_pic_flag(self):

lib/spack/spack/test/compilers/basics.py

@@ -384,9 +384,18 @@ def test_clang_flags():
     unsupported_flag_test("cxx17_flag", "clang@3.4")
     supported_flag_test("cxx17_flag", "-std=c++1z", "clang@3.5")
     supported_flag_test("cxx17_flag", "-std=c++17", "clang@5.0")
+    unsupported_flag_test("cxx20_flag", "clang@4.0")
+    supported_flag_test("cxx20_flag", "-std=c++2a", "clang@5.0")
+    supported_flag_test("cxx20_flag", "-std=c++20", "clang@11.0")
+    unsupported_flag_test("cxx23_flag", "clang@11.0")
+    supported_flag_test("cxx23_flag", "-std=c++2b", "clang@12.0")
+    supported_flag_test("cxx23_flag", "-std=c++23", "clang@17.0")
     supported_flag_test("c99_flag", "-std=c99", "clang@3.3")
     unsupported_flag_test("c11_flag", "clang@2.0")
     supported_flag_test("c11_flag", "-std=c11", "clang@6.1.0")
+    unsupported_flag_test("c23_flag", "clang@8.0")
+    supported_flag_test("c23_flag", "-std=c2x", "clang@9.0")
+    supported_flag_test("c23_flag", "-std=c23", "clang@18.0")
     supported_flag_test("cc_pic_flag", "-fPIC", "clang@3.3")
     supported_flag_test("cxx_pic_flag", "-fPIC", "clang@3.3")
     supported_flag_test("f77_pic_flag", "-fPIC", "clang@3.3")