feat: support function types

Add _function-type_ as an alternative production of _type-id_.
A parameter's identifier must be named `_`.
`!throws` needs to be specified for Cpp1-`noexcept` function types.

Some examples:
```Cpp2
main: () = {
  f0: * () = :()      = {};
  f7: * (move _: i32) = :(move x: i32) = {};
  [[assert: inspect f0 -> bool {
    is () !throws = (std::terminate(), false);
    is * ()       = true;
    is _          = false;
  }]]
}

// Test case from hsutter#343.
f2: () -> std::function<(_: std::string) -> std::string> = {
  return :(s: std::string) -> std::string = { return s + " World!"; };
}

// Adapted from <https://github.com/hsutter/cppfront/wiki/Design-note%3A-Postfix-operators>.
            f:   (x: i32) -> * (_: i32) -> std::string = :(x: i32) -> std::string = "";
postfix_operators: () = {
  [[assert: f is (_: i32) -> * (_: i32) -> std::string]]
  //             /           |  |
  //            /            |  |
  [[assert: f(42)     is     * (_: i32) -> std::string]]
  //               _________/   |
  //              /             |
  [[assert: f(42)*     is      (_: i32) -> std::string]]
  //                   ________/
  //                  /
  [[assert: (f(42)*)(1)         is         std::string]]
}

include/cpp2util.h

@@ -191,7 +191,7 @@
         // in our -pure-cpp2 "import std;" simulation mode... if you need this,
         // use mixed mode (not -pure-cpp2) and #include all the headers you need
         // including this one
-        // 
+        //
         // #include <execution>
     #endif
 
@@ -277,6 +277,17 @@ using __schar   = signed char;      // normally use i8 instead
 using __uchar   = unsigned char;    // normally use u8 instead
 
 
+//-----------------------------------------------------------------------
+//
+//  fn_t<R(ArgTypes...)>       For emitted Cpp2 function types
+//
+//-----------------------------------------------------------------------
+//
+template<typename T>
+    requires std::is_function_v<T>
+using fn_t = T;
+
+
 //-----------------------------------------------------------------------
 //
 //  String: A helper workaround for passing a string literal as a
@@ -466,7 +477,7 @@ template<typename T>
 auto Typeid() -> decltype(auto) {
 #ifdef CPP2_NO_RTTI
     Type.expects(
-        !"'any' dynamic casting is disabled with -fno-rtti", // more likely to appear on console 
+        !"'any' dynamic casting is disabled with -fno-rtti", // more likely to appear on console
          "'any' dynamic casting is disabled with -fno-rtti"  // make message available to hooked handlers
     );
 #else
@@ -828,17 +839,17 @@ auto is( X const& ) -> bool {
 
 template< typename C, typename X >
     requires (
-        ( std::is_base_of_v<X, C> || 
-          ( std::is_polymorphic_v<C> && std::is_polymorphic_v<X>) 
+        ( std::is_base_of_v<X, C> ||
+          ( std::is_polymorphic_v<C> && std::is_polymorphic_v<X>)
         ) && !std::is_same_v<C,X>)
 auto is( X const& x ) -> bool {
     return Dynamic_cast<C const*>(&x) != nullptr;
 }
 
 template< typename C, typename X >
     requires (
-        ( std::is_base_of_v<X, C> || 
-          ( std::is_polymorphic_v<C> && std::is_polymorphic_v<X>) 
+        ( std::is_base_of_v<X, C> ||
+          ( std::is_polymorphic_v<C> && std::is_polymorphic_v<X>)
         ) && !std::is_same_v<C,X>)
 auto is( X const* x ) -> bool {
     return Dynamic_cast<C const*>(x) != nullptr;
@@ -1426,7 +1437,7 @@ inline auto to_string(std::string const& s) -> std::string const&
 
 template<typename T>
 inline auto to_string(T const& sv) -> std::string
-    requires (std::is_convertible_v<T, std::string_view> 
+    requires (std::is_convertible_v<T, std::string_view>
               && !std::is_convertible_v<T, const char*>)
 {
     return std::string{sv};

regression-tests/pure2-bugfix-for-multi-token-type-prvalue.cpp2

@@ -0,0 +1 @@
+main: (args) = { _ = :* int = args.argc&; }

regression-tests/pure2-function-type-id-2-error.cpp2

@@ -0,0 +1,3 @@
+main: () = {
+  f: * (x: i32) throws = :(_) = {};
+}

regression-tests/pure2-function-type-id-3-error.cpp2

@@ -0,0 +1,3 @@
+main: () = {
+  f: * () [[pre: true]] = :() = {};
+}

regression-tests/pure2-function-type-id-4-error.cpp2

@@ -0,0 +1 @@
+main: () = { f: * () -> (x: i32); }

regression-tests/pure2-function-type-id-5-error.cpp2

@@ -0,0 +1,3 @@
+main: () = {
+  f: * (_) throws = :(_) = {};
+}

regression-tests/pure2-function-type-id.cpp2

@@ -0,0 +1,112 @@
+main: () = {
+  postfix_operators();
+
+  // Variables with type of a mix of `*`/`const` to `() throws -> void`.
+  f0: * () throws = :()       = {};
+  f1: const * () throws       = f0;
+  f2: * const () throws       = f0;
+  f3: const * const () throws = f0;
+
+  // Uninitialized.
+  f4: * () throws;
+  f4 = f0;
+
+  f10: * * () throws             = f0&;
+  f11: const * * () throws       = f10;
+  f12: * const * () throws       = f10;
+  f13: const * const * () throws = f10;
+
+  i: i32                = 0;
+  i0: * i32             = i&;
+  i1: const * i32       = i0;
+  i2: * const i32       = i0;
+  i3: const * const i32 = i0;
+
+  // Assert consistent '*'/'const' with non-function type variables.
+  static_assert((std::is_const_v<decltype(f10)>) == std::is_const_v<decltype(i0)>);
+  static_assert((std::is_const_v<decltype(f11)>) == std::is_const_v<decltype(i1)>);
+  static_assert((std::is_const_v<decltype(f12)>) == std::is_const_v<decltype(i2)>);
+  static_assert((std::is_const_v<decltype(f13)>) == std::is_const_v<decltype(i3)>);
+  _ = f10;
+  _ = f11;
+  _ = f12;
+  _ = f13;
+  _ = i0;
+  _ = i1;
+  _ = i2;
+  _ = i3;
+
+  // Variables with various kinds of parameter.
+  f5: * (_: i32) throws = :(x: i32) = {};
+  f6: * (_: std::any) throws = :(x: std::any) = {};
+  f7: * (move _: i32) throws = :(move x: i32) = {};
+  f8: * (out _: i32) throws = :(copy x) = {};
+
+  // In alternative.
+  [[assert: inspect f0 -> bool {
+    is ()          = (std::terminate(), false);
+    is () throws   = (std::terminate(), false);
+    is * () throws = true;
+    is _           = false;
+  }]]
+  [[assert: inspect f0* -> bool {
+    is () throws = true;
+    is _         = false;
+  }]]
+
+  // As block variable.
+  (f: * () throws = f0) { }
+  (f: () throws   = f0*) { }
+
+  // As local function parameter.
+  _ = :(f: () throws) = {};
+  _ = :(f: * () throws) = {};
+  _ = :(f: * () throws -> * () throws) = {};
+  _ = :(f: * () throws -> * () throws -> * () throws) = {};
+
+  // In local function return type.
+  _ = :() -> forward() throws = f0$*;
+  _ = :() -> * () throws = nullptr;
+  _ = :() -> * () throws -> * () throws = nullptr;
+
+  // Without `throws`.
+  _ = :* (copy _: std::string_view, copy _: CPP2_MESSAGE_PARAM) = cpp2::report_and_terminate;
+
+  // As template argument.
+  _ = :std::type_identity_t<* () throws> = f0;
+  static_assert(std::is_function_v<() throws>);
+}
+
+// As non-local function parameter.
+g: (f: () throws)                  = { }
+g: (f: * () throws)                = { }
+g: (f: * () throws -> * () throws) = { }
+// As template parameter.
+g: <V: * () throws> ()                = { }
+g: <V: * () throws -> * () throws> () = { }
+
+// In non-local function return type.
+g1: () -> * () throws                      = nullptr;
+g1: <V: bool> () -> * () throws requires V = { return nullptr; }
+g2: () -> * () throws -> * () throws       = nullptr;
+
+// clang-format off
+// Test case from #343.
+f2: () -> std::function<(_: std::string) throws -> std::string> = {
+  return :(s: std::string) -> std::string = { return s + " World!"; };
+}
+
+// Adapted from <https://github.com/hsutter/cppfront/wiki/Design-note%3A-Postfix-operators>.
+            f:   (x: i32)        -> * (_: i32) throws -> std::string = +:(x: i32) -> std::string = "";
+postfix_operators: () = {
+  [[assert: f is (_: i32) throws -> * (_: i32) throws -> std::string]]
+  //             /                  |  |
+  //            /                   |  |
+  [[assert: f(42)        is         * (_: i32) throws -> std::string]]
+  //               ________________/   |
+  //              /                    |
+  [[assert: f(42)*         is         (_: i32) throws -> std::string]]
+  //                   _______________/
+  //                  /
+  [[assert: (f(42)*)(1)                is                std::string]]
+} // clang-format on

regression-tests/test-results/pure2-bugfix-for-multi-token-type-prvalue.cpp

@@ -0,0 +1,24 @@
+
+#define CPP2_USE_MODULES         Yes
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "pure2-bugfix-for-multi-token-type-prvalue.cpp2"
+auto main(int const argc_, char const* const* const argv_) -> int;
+
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "pure2-bugfix-for-multi-token-type-prvalue.cpp2"
+auto main(int const argc_, char const* const* const argv_) -> int{
+                 auto args = cpp2::make_args(argc_, argv_); 
+#line 1 "pure2-bugfix-for-multi-token-type-prvalue.cpp2"
+(void) std::type_identity_t<int*>{&args.argc}; }
+

regression-tests/test-results/pure2-bugfix-for-multi-token-type-prvalue.cpp2.output

@@ -0,0 +1,2 @@
+pure2-bugfix-for-multi-token-type-prvalue.cpp2... ok (all Cpp2, passes safety checks)
+

regression-tests/test-results/pure2-function-type-id-2-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-function-type-id-2-error.cpp2...
+pure2-function-type-id-2-error.cpp2(2,9): error: the parameter of a function type must be named '_'
+

regression-tests/test-results/pure2-function-type-id-3-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-function-type-id-3-error.cpp2...
+pure2-function-type-id-3-error.cpp2(2,11): error: a function type can't have contracts
+

regression-tests/test-results/pure2-function-type-id-4-error.cpp2.output

@@ -0,0 +1,5 @@
+pure2-function-type-id-4-error.cpp2...
+pure2-function-type-id-4-error.cpp2(1,25): error: a function type can't have an anonymous return type
+pure2-function-type-id-4-error.cpp2(1,14): error: f - variable must be initialized on every branch path
+  ==> program violates initialization safety guarantee - see previous errors
+

regression-tests/test-results/pure2-function-type-id-5-error.cpp2.output

@@ -0,0 +1,3 @@
+pure2-function-type-id-5-error.cpp2...
+pure2-function-type-id-5-error.cpp2(2,9): error: function type parameter must have a type
+