Emit requires clause for variable template, closes #607

And update GCC 10 diagnostic

Author: hsutter

include/cpp2util.h

@@ -1755,10 +1755,10 @@ using cpp2::cpp2_new;
 // See commit 5a0d77f8e297902c0b9712c5aafb6208cfa4c139.
 #if !defined(__clang__) && defined(__GNUC__) && __GNUC__ == 10
     #define CPP2_REQUIRES(...) /* empty */
-    #define CPP2_REQUIRES_MEMFN(...) static_assert(false, "GCC 11 or higher is required to support type-scope functions that have a 'requires' clause. This includes a 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.")
+    #define CPP2_REQUIRES_(...) static_assert(false, "GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.")
 #else
     #define CPP2_REQUIRES(...) requires (__VA_ARGS__)
-    #define CPP2_REQUIRES_MEMFN(...) requires (__VA_ARGS__)
+    #define CPP2_REQUIRES_(...) requires (__VA_ARGS__)
 #endif
 
 #endif

regression-tests/pure2-requires-clauses.cpp2

@@ -15,6 +15,8 @@ f:  <T: type, U: type>
     return a * b;
 }
 
+v: <T> const T requires std::same_as<T, i32> = 0;
+
 main: () = {
     x: X<int,int> = ();
     std::cout << f<int,int>(2,5);

regression-tests/test-results/clang-12/pure2-bugfix-for-non-local-initialization.cpp.execution

@@ -1,3 +1,3 @@
 true
 false
-./test.exe
+"test.exe"

regression-tests/test-results/clang-12/pure2-bugfix-for-non-local-initialization.cpp.output

@@ -1,15 +1,15 @@
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:36: error: expected ‘;’ at end of member declaration
 In file included from pure2-bugfix-for-requires-clause-in-forward-declaration.cpp:7:
-../../../include/cpp2util.h:1758:52: error: static assertion failed: GCC 11 or higher is required to support type-scope functions that have a 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)' - if you must use GCC 10, use 'forward s: _' instead
- 1758 |     #define CPP2_REQUIRES_MEMFN(...) static_assert(false, "GCC 11 or higher is required to support type-scope functions that have a 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)' - if you must use GCC 10, use 'forward s: _' instead")
-      |                                                    ^~~~~
-pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:4:1: note: in expansion of macro ‘CPP2_REQUIRES_MEMFN’
+../../../include/cpp2util.h:1758:47: error: static assertion failed: GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.
+ 1758 |     #define CPP2_REQUIRES_(...) static_assert(false, "GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.")
+      |                                               ^~~~~
+pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:4:1: note: in expansion of macro ‘CPP2_REQUIRES_’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:46: error: expected ‘;’ at end of member declaration
 In file included from pure2-bugfix-for-requires-clause-in-forward-declaration.cpp:7:
-../../../include/cpp2util.h:1758:52: error: static assertion failed: GCC 11 or higher is required to support type-scope functions that have a 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)' - if you must use GCC 10, use 'forward s: _' instead
- 1758 |     #define CPP2_REQUIRES_MEMFN(...) static_assert(false, "GCC 11 or higher is required to support type-scope functions that have a 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)' - if you must use GCC 10, use 'forward s: _' instead")
-      |                                                    ^~~~~
-pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:4:1: note: in expansion of macro ‘CPP2_REQUIRES_MEMFN’
+../../../include/cpp2util.h:1758:47: error: static assertion failed: GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.
+ 1758 |     #define CPP2_REQUIRES_(...) static_assert(false, "GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.")
+      |                                               ^~~~~
+pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:4:1: note: in expansion of macro ‘CPP2_REQUIRES_’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:3: error: no declaration matches ‘element::element(auto:80&&) requires  is_same_v<typename std::remove_cv<typename std::remove_reference<decltype(element::__ct ::n)>::type>::type, std::__cxx11::string>’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:5:11: note: candidates are: ‘element::element(const element&)’
 pure2-bugfix-for-requires-clause-in-forward-declaration.cpp2:3:20: note:                 ‘template<class auto:78> element::element(auto:78&&)’

regression-tests/test-results/gcc-10/mixed-fixed-type-aliases.cpp.execution

@@ -0,0 +1,5 @@
+In file included from pure2-requires-clauses.cpp:7:
+../../../include/cpp2util.h:1758:33: error: expected unqualified-id before ‘static_assert’
+ 1758 |     #define CPP2_REQUIRES_(...) static_assert(false, "GCC 11 or higher is required to support variables and type-scope functions that have a 'requires' clause. This includes a type-scope 'forward' parameter of non-wildcard type, such as 'func: (this, forward s: std::string)', which relies on being able to add a 'requires' clause - in that case, use 'forward s: _' instead if you need the result to compile with GCC 10.")
+      |                                 ^~~~~~~~~~~~~
+pure2-requires-clauses.cpp2:19:1: note: in expansion of macro ‘CPP2_REQUIRES_’