feat: add _type-id_ production for decltype

regression-tests/mixed-bugfix-for-ufcs-non-local.cpp2

@@ -38,7 +38,7 @@ d: _ == t<o.f()>(); // Fails on Clang 12 (lambda in unevaluated context).
 
 u: @struct type = {
   b: bool == o.f();
-  c: bool == :(x: std::type_identity_t<decltype(o.f())>) x;(true); // Fails on Clang 12 (lambda in unevaluated context).
+  c: bool == :(x: decltype(o.f())) x;(true); // Fails on Clang 12 (lambda in unevaluated context).
   g: (s, sz) pre(s.sz() != 0) = { }
 }
 

regression-tests/pure2-bugfix-for-non-local-function-expression.cpp2

@@ -4,10 +4,10 @@
 
 v: <T> concept = :() -> bool = true;();
 
-u: type == std::type_identity_t<decltype(:() = {})>;
+u: type == decltype(:() = {});
 
 t: @struct type = {
-    this: std::type_identity_t<decltype(:() = {})>;
+    this: decltype(:() = {});
 }
 
 main: () = { }

regression-tests/pure2-deduction-1-error.cpp2

@@ -0,0 +1 @@
+f: () -> decltype(auto) = 0;

regression-tests/pure2-deduction-2-error.cpp2

@@ -0,0 +1 @@
+x: decltype(auto) = 0;

regression-tests/pure2-last-use.cpp2

@@ -42,7 +42,7 @@ issue_313: () = {
     _ = e is (e);
 
     f := new<int>(0);
-    _ = f is std::type_identity_t<decltype(f)>; // OK?
+    _ = f is decltype(f); // OK?
 
     g := new<int>(0);
     for (identity(g)* + identity(g)*)

regression-tests/pure2-print.cpp2

@@ -100,6 +100,8 @@ outer: @print type = {
     all: <Args...: type> (args...: Args) -> bool =
         (... && args);
 
+    y: (_: decltype(0)) = { }
+
 }
 
 main: () = {

regression-tests/test-results/mixed-bugfix-for-ufcs-non-local.cpp

@@ -95,7 +95,7 @@ auto g() -> void{
 
 #line 40 "mixed-bugfix-for-ufcs-non-local.cpp2"
   inline CPP2_CONSTEXPR bool u::b{ CPP2_UFCS_NONLOCAL(f)(o) };
-  inline CPP2_CONSTEXPR bool u::c{ [](cpp2::impl::in<std::type_identity_t<decltype(CPP2_UFCS_NONLOCAL(f)(o))>> x) -> auto { return x; }(true) };// Fails on Clang 12 (lambda in unevaluated context).
+  inline CPP2_CONSTEXPR bool u::c{ [](cpp2::impl::in<decltype(CPP2_UFCS_NONLOCAL(f)(o))> x) -> auto { return x; }(true) };// Fails on Clang 12 (lambda in unevaluated context).
   auto u::g(auto const& s, auto const& sz) -> void{
                                   if (cpp2::cpp2_default.is_active() && !(CPP2_UFCS(sz)(s) != 0) ) { cpp2::cpp2_default.report_violation(""); }}
 

regression-tests/test-results/pure2-bugfix-for-non-local-function-expression.cpp

@@ -22,9 +22,9 @@ class t;
 #line 5 "pure2-bugfix-for-non-local-function-expression.cpp2"
 template<typename T> concept v = []() -> bool { return true;  }(); 
 
-using u = std::type_identity_t<decltype([]() -> void{})>;
+using u = decltype([]() -> void{});
 
-class t: public std::type_identity_t<decltype([]() -> void{})> {
+class t: public decltype([]() -> void{}) {
 
 };
 

regression-tests/test-results/pure2-deduction-1-error.cpp2.output

@@ -0,0 +1,5 @@
+pure2-deduction-1-error.cpp2...
+pure2-deduction-1-error.cpp2(1,10): error: decltype(auto) is not needed in Cpp2 - use 'forward: _' parameters/returns/objects instead
+  (temporary alpha limitation) returns means auto&&
+  (temporary alpha limitation) objects are not yet supported
+

regression-tests/test-results/pure2-deduction-2-error.cpp2.output

@@ -0,0 +1,5 @@
+pure2-deduction-2-error.cpp2...
+pure2-deduction-2-error.cpp2(1,4): error: decltype(auto) is not needed in Cpp2 - use 'forward: _' parameters/returns/objects instead
+  (temporary alpha limitation) returns means auto&&
+  (temporary alpha limitation) objects are not yet supported
+

regression-tests/test-results/pure2-last-use.cpp

@@ -606,7 +606,7 @@ auto issue_313() -> void{
     static_cast<void>(cpp2::impl::is(e, (e)));
 
     auto f {cpp2_new<int>(0)}; 
-    static_cast<void>(cpp2::impl::is<std::type_identity_t<decltype(f)>>(f));// OK?
+    static_cast<void>(cpp2::impl::is<decltype(f)>(f));// OK?
 
     auto g {cpp2_new<int>(0)}; 
     for ( 

regression-tests/test-results/pure2-print.cpp

@@ -70,12 +70,15 @@ CPP2_REQUIRES_ (cpp2::impl::cmp_greater_eq(sizeof...(Args),0u)) ;
 
 #line 100 "pure2-print.cpp2"
     public: template<typename ...Args> [[nodiscard]] static auto all(Args const& ...args) -> bool;
+
+#line 103 "pure2-print.cpp2"
+    public: static auto y([[maybe_unused]] cpp2::impl::in<decltype(0)> unnamed_param_1) -> void;
     public: outer() = default;
     public: outer(outer const&) = delete; /* No 'that' constructor, suppress copy */
     public: auto operator=(outer const&) -> void = delete;
 
 
-#line 103 "pure2-print.cpp2"
+#line 105 "pure2-print.cpp2"
 };
 
 auto main() -> int;
@@ -199,7 +202,10 @@ requires (cpp2::impl::cmp_greater_eq(sizeof...(Args),0u)) {
     template<typename ...Args> [[nodiscard]] auto outer::all(Args const& ...args) -> bool { 
         return (... && args);  }
 
-#line 105 "pure2-print.cpp2"
+#line 103 "pure2-print.cpp2"
+    auto outer::y([[maybe_unused]] cpp2::impl::in<decltype(0)> unnamed_param_1) -> void{}
+
+#line 107 "pure2-print.cpp2"
 auto main() -> int{
     outer::test();
 }

regression-tests/test-results/pure2-print.cpp2.output

@@ -146,6 +146,10 @@ outer:/* @print */ type =
     }
 
     all: <Args...: type, >(in args...: Args, ) -> move bool = (... && args);
+
+    y:(in _: decltype(0), ) = 
+    {
+    }
 }
  ok (all Cpp2, passes safety checks)
 

source/parse.h

@@ -1365,9 +1365,10 @@ struct type_id_node
     int                       dereference_cnt           = {};
     token const*              suspicious_initialization = {};
 
-    enum active : u8 { empty=0, qualified, unqualified, function, keyword };
+    enum active : u8 { empty=0, decltype_, qualified, unqualified, function, keyword };
     std::variant<
         std::monostate,
+        std::unique_ptr<postfix_expression_node>,
         std::unique_ptr<qualified_id_node>,
         std::unique_ptr<unqualified_id_node>,
         std::unique_ptr<function_type_node>,
@@ -1419,6 +1420,9 @@ struct type_id_node
         if (id.index() == unqualified) {
             return std::get<unqualified>(id)->template_arguments();
         }
+        else if (id.index() != qualified) {
+            cpp2_default.report_violation("ICE: this type_id has no template arguments");
+        }
         // else
         return std::get<qualified>(id)->template_arguments();
     }
@@ -1432,6 +1436,8 @@ struct type_id_node
         switch (id.index()) {
         break;case empty:
             return {};
+        break;case decltype_:
+            return {};
         break;case qualified:
             return {};
         break;case unqualified:
@@ -1460,6 +1466,7 @@ struct type_id_node
         for (auto q : pc_qualifiers) {
             v.start(*q, depth+1);
         }
+        try_visit<decltype_  >(id, v, depth);
         try_visit<qualified  >(id, v, depth);
         try_visit<unqualified>(id, v, depth);
         try_visit<function   >(id, v, depth);
@@ -2770,6 +2777,8 @@ auto type_id_node::to_string() const
     switch (id.index()) {
     break;case empty:
         ret += "_";
+    break;case decltype_:
+        ret += std::get<decltype_>(id)->to_string();
     break;case qualified:
         ret += std::get<qualified>(id)->to_string();
     break;case unqualified:
@@ -5084,6 +5093,7 @@ auto pretty_print_visualize(type_id_node const& n, int indent)
     }
 
     if (n.id.index() == type_id_node::empty) { ret += "_"; }
+    ret += try_pretty_print_visualize<type_id_node::decltype_  >(n.id, indent);
     ret += try_pretty_print_visualize<type_id_node::qualified  >(n.id, indent);
     ret += try_pretty_print_visualize<type_id_node::unqualified>(n.id, indent);
     ret += try_pretty_print_visualize<type_id_node::function   >(n.id, indent);
@@ -6867,6 +6877,7 @@ class parser
 
 
     //G type-id:
+    //G     type-qualifier-seq? 'decltype' '(' expression ')' is-type-constraint?
     //G     type-qualifier-seq? qualified-id is-type-constraint?
     //G     type-qualifier-seq? unqualified-id is-type-constraint?
     //G     type-qualifier-seq? function-type is-type-constraint?
@@ -6909,7 +6920,41 @@ class parser
             next();
         }
 
-        if (auto id = qualified_id()) {
+        if (auto& c = curr();
+            c == "decltype"
+            )
+        {
+            if (
+                peek(1) && peek(1)->type() == lexeme::LeftParen
+                && peek(2) && *peek(2) == "auto"
+                && peek(3) && peek(3)->type() == lexeme::RightParen)
+            {
+                error(
+                    "decltype(auto) is not needed in Cpp2 - use 'forward: _' parameters/returns/objects instead"
+                    "\n  (temporary alpha limitation) returns means auto&&"
+                    "\n  (temporary alpha limitation) objects are not yet supported",
+                    false,
+                    c.position()
+                );
+            }
+            if (auto id = postfix_expression();
+                id
+                && id->ops.size() == 1
+                && id->ops[0].expr_list->expressions.size() == 1
+                && id->ops[0].expr_list->open_paren->type() == lexeme::LeftParen
+                )
+            {
+                n->pos = id->position();
+                n->id  = std::move(id);
+                assert (n->id.index() == type_id_node::decltype_);
+            }
+            else
+            {
+                error("'decltype' must be followed by a single parenthesized expression", false, c.position());
+                return {};
+            }
+        }
+        else if (auto id = qualified_id()) {
             n->pos = id->position();
             n->id  = std::move(id);
             assert (n->id.index() == type_id_node::qualified);

source/to_cpp1.h

@@ -1941,6 +1941,7 @@ class cppfront
         else {
             try_emit<type_id_node::unqualified>(n.id, 0, false);
             try_emit<type_id_node::qualified  >(n.id);
+            try_emit<type_id_node::decltype_  >(n.id);
             try_emit<type_id_node::keyword    >(n.id);
         }