Try out explicit forward expressions, see hsutter#77 comment thread

Author: hsutter

regression-tests/mixed-forwarding.cpp2

@@ -0,0 +1,29 @@
+#include <iostream>
+#include <utility>
+
+struct X {
+    int i;
+    X(int i)         : i{     i} { std::cout << "+X "     << i << "\n"; }
+    X(X const& that) : i{that.i} { std::cout << "copy X " << i << "\n"; }
+    X(X &&     that) : i{that.i} { std::cout << "move X " << i << "\n"; }
+};
+
+discard: (copy x:_) = { }
+
+apply_implicit_forward: (forward t: std::pair<X, X>) = {
+    discard(t.first);
+    discard(t.second);
+}
+
+apply_explicit_forward: (forward t: std::pair<X, X>) = {
+    discard(forward t.first);
+    discard(forward t.second);
+}
+
+main: ()->int = {
+    t1: std::pair<X,X> = (1,2);
+    apply_implicit_forward(t1);
+
+    t2: std::pair<X,X> = (3,4);
+    apply_explicit_forward(t2);
+}

regression-tests/test-results/clang-12/mixed-forwarding.cpp.execution

@@ -0,0 +1,8 @@
++X 1
++X 2
+copy X 1
+move X 2
++X 3
++X 4
+move X 3
+move X 4

regression-tests/test-results/clang-12/mixed-forwarding.cpp.output

@@ -0,0 +1,2 @@
+rain
+rain

regression-tests/test-results/clang-12/mixed-ufcs-multiple-template-arguments.cpp.execution

@@ -0,0 +1,8 @@
++X 1
++X 2
+copy X 1
+move X 2
++X 3
++X 4
+move X 3
+move X 4

regression-tests/test-results/clang-12/mixed-ufcs-multiple-template-arguments.cpp.output

@@ -0,0 +1,2 @@
+rain
+rain

regression-tests/test-results/gcc-10/mixed-forwarding.cpp.execution

@@ -0,0 +1,50 @@
+// ----- Cpp2 support -----
+#include "cpp2util.h"
+
+#line 1 "mixed-forwarding.cpp2"
+#include <iostream>
+#include <utility>
+
+struct X {
+    int i;
+    X(int i)         : i{     i} { std::cout << "+X "     << i << "\n"; }
+    X(X const& that) : i{that.i} { std::cout << "copy X " << i << "\n"; }
+    X(X &&     that) : i{that.i} { std::cout << "move X " << i << "\n"; }
+};
+
+auto discard(auto x) -> void;
+#line 13 "mixed-forwarding.cpp2"
+auto apply_implicit_forward(auto&& t) -> void;
+#line 18 "mixed-forwarding.cpp2"
+auto apply_explicit_forward(auto&& t) -> void;
+#line 23 "mixed-forwarding.cpp2"
+[[nodiscard]] auto main() -> int;
+
+//=== Cpp2 definitions ==========================================================
+
+#line 10 "mixed-forwarding.cpp2"
+
+auto discard(auto x) -> void{}
+
+auto apply_implicit_forward(auto&& t) -> void
+requires std::is_same_v<CPP2_TYPEOF(t), std::pair<X,X>>
+#line 14 "mixed-forwarding.cpp2"
+{   discard(t.first);
+    discard(CPP2_FORWARD(t).second);
+}
+
+auto apply_explicit_forward(auto&& t) -> void
+requires std::is_same_v<CPP2_TYPEOF(t), std::pair<X,X>>
+#line 19 "mixed-forwarding.cpp2"
+{   discard(CPP2_FORWARD(t.first));
+    discard(CPP2_FORWARD(CPP2_FORWARD(t).second));
+}
+
+[[nodiscard]] auto main() -> int{
+    std::pair<X,X> t1 { 1, 2 }; 
+    apply_implicit_forward(std::move(t1));
+
+    std::pair<X,X> t2 { 3, 4 }; 
+    apply_explicit_forward(std::move(t2));
+}
+

regression-tests/test-results/gcc-10/mixed-forwarding.cpp.output

@@ -0,0 +1,2 @@
+mixed-forwarding.cpp2... ok (mixed Cpp1/Cpp2, Cpp2 code passes safety checks)
+

regression-tests/test-results/gcc-10/mixed-ufcs-multiple-template-arguments.cpp.execution

@@ -0,0 +1,8 @@
++X 1
++X 2
+copy X 1
+move X 2
++X 3
++X 4
+move X 3
+move X 4

regression-tests/test-results/gcc-10/mixed-ufcs-multiple-template-arguments.cpp.output

@@ -0,0 +1 @@
+mixed-forwarding.cpp

regression-tests/test-results/mixed-forwarding.cpp

@@ -0,0 +1,2 @@
+rain
+rain

regression-tests/test-results/mixed-forwarding.cpp2.output

@@ -0,0 +1 @@
+mixed-ufcs-multiple-template-arguments.cpp

regression-tests/test-results/msvc-2022/mixed-forwarding.cpp.execution

@@ -2065,7 +2065,11 @@ class cppfront
             auto is_out = false;
 
             if (x.pass != passing_style::in) {
-                assert(to_string_view(x.pass) == "out" || to_string_view(x.pass) == "move");
+                assert(
+                    to_string_view(x.pass) == "out"  ||
+                    to_string_view(x.pass) == "move" ||
+                    to_string_view(x.pass) == "forward"
+                );
                 if (to_string_view(x.pass) == "out") {
                     is_out = true;
                     printer.print_cpp2("&", n.position());
@@ -2075,6 +2079,10 @@ class cppfront
                     printer.print_cpp2("std::move(", n.position());
                     offset = 6;    // because we're replacing "move " (followed by at least one space) with "std::move("
                 }
+                else if (to_string_view(x.pass) == "forward") {
+                    printer.print_cpp2("CPP2_FORWARD(", n.position());
+                    offset = 10;   // because we're replacing "forward " (followed by at least one space) with "CPP2_FORWARD("
+                }
             }
 
             if (is_out) {
@@ -2090,7 +2098,7 @@ class cppfront
                 in_non_rvalue_context.pop_back();
             }
 
-            if (to_string_view(x.pass) == "move") {
+            if (to_string_view(x.pass) == "move" || to_string_view(x.pass) == "forward") {
                 printer.print_cpp2(")", n.position());
             }
         }

regression-tests/test-results/msvc-2022/mixed-forwarding.cpp.output

@@ -1802,6 +1802,10 @@ class parser
             pass = passing_style::move;
             next();
         }
+        else if (curr().type() == lexeme::Identifier && curr() == "forward") {
+            pass = passing_style::forward;
+            next();
+        }
         auto x = expression();
 
         //  If this is an empty expression_list, we're done