feat: add _type-id_ production for decltype (#921)

* feat: add _type-id_ production for `decltype`

* Update error message and local test run results

---------

Co-authored-by: Herb Sutter <herb.sutter@gmail.com>

Author: JohelEGP

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/clang-12-c++20/mixed-bugfix-for-ufcs-non-local.cpp.output

@@ -106,9 +106,9 @@ mixed-bugfix-for-ufcs-non-local.cpp2:27:29: error: a lambda expression cannot ap
 ../../../include/cpp2util.h:2099:66: note: expanded from macro 'CPP2_UFCS_'
 #define CPP2_UFCS_(LAMBDADEFCAPT,SFINAE,MVFWD,QUALID,TEMPKW,...) \
                                                                  ^
-mixed-bugfix-for-ufcs-non-local.cpp2:41:84: error: lambda expression in an unevaluated operand
-  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).
-                                                                                   ^
+mixed-bugfix-for-ufcs-non-local.cpp2:41:63: error: lambda expression in an unevaluated operand
+  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).
+                                                              ^
 ../../../include/cpp2util.h:2137:59: note: expanded from macro 'CPP2_UFCS_NONLOCAL'
 #define CPP2_UFCS_NONLOCAL(...)                           CPP2_UFCS_(,CPP2_UFCS_IDENTITY,CPP2_UFCS_IDENTITY,(),,__VA_ARGS__)
                                                           ^

regression-tests/test-results/clang-12-c++20/pure2-bugfix-for-non-local-function-expression.cpp.output

@@ -1,10 +1,10 @@
 pure2-bugfix-for-non-local-function-expression.cpp2:5:34: error: lambda expression in an unevaluated operand
 template<typename T> concept v = []() -> bool { return true;  }(); 
                                  ^
-pure2-bugfix-for-non-local-function-expression.cpp2:7:41: error: lambda expression in an unevaluated operand
-using u = std::type_identity_t<decltype([]() -> void{})>;
-                                        ^
-pure2-bugfix-for-non-local-function-expression.cpp2:9:47: error: lambda expression in an unevaluated operand
-class t: public std::type_identity_t<decltype([]() -> void{})> {
-                                              ^
+pure2-bugfix-for-non-local-function-expression.cpp2:7:20: error: lambda expression in an unevaluated operand
+using u = decltype([]() -> void{});
+                   ^
+pure2-bugfix-for-non-local-function-expression.cpp2:9:26: error: lambda expression in an unevaluated operand
+class t: public decltype([]() -> void{}) {
+                         ^
 3 errors generated.

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{}) {
 
 };