compilers: add support for c++20/gnu++20

Fixes #8084.

docs/markdown/Builtin-options.md

@@ -172,7 +172,7 @@ compiler being used:
 | c_thread_count   | 4             | integer value ≥ 0                        | Number of threads to use with emcc when using threads |
 | cpp_args         |               | free-form comma-separated list           | C++ compile arguments to use |
 | cpp_link_args    |               | free-form comma-separated list           | C++ link arguments to use |
-| cpp_std          | none          | none, c++98, c++03, c++11, c++14, c++17, c++2a <br/>c++1z, gnu++03, gnu++11, gnu++14, gnu++17, gnu++1z, gnu++2a, <br/> vc++14, vc++17, vc++latest | C++ language standard to use |
+| cpp_std          | none          | none, c++98, c++03, c++11, c++14, c++17, c++20 <br/>c++2a, c++1z, gnu++03, gnu++11, gnu++14, gnu++17, gnu++1z, <br/> gnu++2a, gnu++20, vc++14, vc++17, vc++latest | C++ language standard to use |
 | cpp_debugstl     | false         | true, false                              | C++ STL debug mode |
 | cpp_eh           | default       | none, default, a, s, sc                  | C++ exception handling type |
 | cpp_rtti         | true          | true, false                              | Whether to enable RTTI (runtime type identification) |

docs/markdown/Configuring-a-build-directory.md

@@ -64,7 +64,7 @@ sample output for a simple project.
       cpp_args      []                                                                                                                          Extra arguments passed to the C++ compiler
       cpp_debugstl  false         [true, false]                                                                                                 STL debug mode
       cpp_link_args []                                                                                                                          Extra arguments passed to the C++ linker
-      cpp_std       c++11         [none, c++98, c++03, c++11, c++14, c++17, c++1z, c++2a, gnu++03, gnu++11, gnu++14, gnu++17, gnu++1z, gnu++2a] C++ language standard to use
+      cpp_std       c++11         [none, c++98, c++03, c++11, c++14, c++17, c++1z, c++2a, c++20, gnu++03, gnu++11, gnu++14, gnu++17, gnu++1z, gnu++2a, gnu++20] C++ language standard to use
       fortran_std   []            [none, legacy, f95, f2003, f2008, f2018]                                                                      language standard to use
 
     Directories:

mesonbuild/compilers/cpp.py

@@ -150,7 +150,9 @@ class CPPCompiler(CLikeCompiler, Compiler):
             'c++14': 'c++1y',
             'gnu++14': 'gnu++1y',
             'c++17': 'c++1z',
-            'gnu++17': 'gnu++1z'
+            'gnu++17': 'gnu++1z',
+            'c++20': 'c++2a',
+            'gnu++20': 'gnu++2a',
         }
 
         # Currently, remapping is only supported for Clang, Elbrus and GCC
@@ -206,7 +208,8 @@ class ClangCPPCompiler(ClangCompiler, CPPCompiler):
         })
         opts['std'].choices = [  # type: ignore
             'none', 'c++98', 'c++03', 'c++11', 'c++14', 'c++17', 'c++1z',
-            'c++2a', 'gnu++11', 'gnu++14', 'gnu++17', 'gnu++1z', 'gnu++2a',
+            'c++2a', 'c++20', 'gnu++11', 'gnu++14', 'gnu++17', 'gnu++1z',
+            'gnu++2a', 'gnu++20',
         ]
         if self.info.is_windows() or self.info.is_cygwin():
             opts.update({
@@ -344,8 +347,8 @@ class GnuCPPCompiler(GnuCompiler, CPPCompiler):
         })
         opts['std'].choices = [  # type: ignore
             'none', 'c++98', 'c++03', 'c++11', 'c++14', 'c++17', 'c++1z',
-            'c++2a', 'gnu++03', 'gnu++11', 'gnu++14', 'gnu++17', 'gnu++1z',
-            'gnu++2a',
+            'c++2a', 'c++20', 'gnu++03', 'gnu++11', 'gnu++14', 'gnu++17',
+            'gnu++1z', 'gnu++2a', 'gnu++20',
         ]
         if self.info.is_windows() or self.info.is_cygwin():
             opts.update({