Improve explicit forward

Apply to first identifier
Diagnose attempt to `forward` an expression that doesn't start with a `forward` parameter name

That's roughed-in enough to experiment with and get feedback on whether we want to keep this feature... if we do, then we can add some more diagnostics for bad source (though this commit does make some attempt to check it's for a forwarding parameter), and consider changing the precedence of `forward` in parsing (the current parsing feels pretty good where `out` and `move` and `forward` apply to the whole argument, but maybe we'll see good reason to adjust that based on usage)

Author: hsutter

regression-tests/mixed-forwarding.cpp2

@@ -8,22 +8,28 @@ struct X {
     X(X &&     that) : i{that.i} { std::cout << "move X " << i << "\n"; }
 };
 
-discard: (copy x:_) = { }
+copy_from: (copy x:_) = { }
 
+use: (x:_) = {}
+
+// invoking each of these with an rvalue std::pair argument ...
 apply_implicit_forward: (forward t: std::pair<X, X>) = {
-    discard(t.first);
-    discard(t.second);
+    copy_from(t.first);             // copies
+    copy_from(t.second);            // moves
 }
-
 apply_explicit_forward: (forward t: std::pair<X, X>) = {
-    discard(forward t.first);
-    discard(forward t.second);
+    copy_from(forward t.first);     // moves
+    copy_from(forward t.second);    // moves
 }
 
 main: ()->int = {
     t1: std::pair<X,X> = (1,2);
     apply_implicit_forward(t1);
+    use(t1);
+    apply_implicit_forward(t1);
 
     t2: std::pair<X,X> = (3,4);
     apply_explicit_forward(t2);
+    use(t2);
+    apply_explicit_forward(t2);
 }

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

@@ -1,8 +1,12 @@
 +X 1
 +X 2
 copy X 1
+copy X 2
+copy X 1
 move X 2
 +X 3
 +X 4
+copy X 3
+copy X 4
 move X 3
 move X 4

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

@@ -1,8 +1,12 @@
 +X 1
 +X 2
 copy X 1
+copy X 2
+copy X 1
 move X 2
 +X 3
 +X 4
+copy X 3
+copy X 4
 move X 3
 move X 4

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

@@ -12,39 +12,47 @@ struct X {
     X(X &&     that) : i{that.i} { std::cout << "move X " << i << "\n"; }
 };
 
-auto discard(auto x) -> void;
+auto copy_from(auto x) -> void;
 #line 13 "mixed-forwarding.cpp2"
+auto use(auto const& x) -> void;
+#line 16 "mixed-forwarding.cpp2"
 auto apply_implicit_forward(auto&& t) -> void;
-#line 18 "mixed-forwarding.cpp2"
+#line 20 "mixed-forwarding.cpp2"
 auto apply_explicit_forward(auto&& t) -> void;
-#line 23 "mixed-forwarding.cpp2"
+#line 25 "mixed-forwarding.cpp2"
 [[nodiscard]] auto main() -> int;
 
 //=== Cpp2 definitions ==========================================================
 
 #line 10 "mixed-forwarding.cpp2"
 
-auto discard(auto x) -> void{}
+auto copy_from(auto x) -> void{}
 
+auto use(auto const& x) -> void{}
+
+// invoking each of these with an rvalue std::pair argument ...
 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);
+#line 17 "mixed-forwarding.cpp2"
+{   copy_from(t.first);             // copies
+    copy_from(CPP2_FORWARD(t).second);// moves
 }
-
 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));
+#line 21 "mixed-forwarding.cpp2"
+{   copy_from(CPP2_FORWARD(t).first);// moves
+    copy_from(CPP2_FORWARD(t).second);// moves
 }
 
 [[nodiscard]] auto main() -> int{
     std::pair<X,X> t1 { 1, 2 }; 
+    apply_implicit_forward(t1);
+    use(t1);
     apply_implicit_forward(std::move(t1));
 
     std::pair<X,X> t2 { 3, 4 }; 
+    apply_explicit_forward(t2);
+    use(t2);
     apply_explicit_forward(std::move(t2));
 }
 

regression-tests/test-results/msvc-2022/MSVC-version.output

@@ -1,3 +1,3 @@
-Microsoft (R) C/C++ Optimizing Compiler Version 19.34.31933 for x86
+Microsoft (R) C/C++ Optimizing Compiler Version 19.34.31937 for x86
 Copyright (C) Microsoft Corporation.  All rights reserved.
 

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

@@ -1,8 +1,12 @@
 +X 1
 +X 2
 copy X 1
+copy X 2
+copy X 1
 move X 2
 +X 3
 +X 4
+copy X 3
+copy X 4
 move X 3
 move X 4

source/cppfront.cpp

@@ -645,11 +645,17 @@ class cppfront
     cpp2::parser parser;
     cpp2::sema   sema;
 
-    bool source_loaded                   = true;
-    bool last_postfix_expr_was_pointer   = false;
-    bool violates_bounds_safety          = false;
-    bool violates_initialization_safety  = false;
-    bool suppress_move_from_last_use     = false;
+    bool source_loaded                  = true;
+    bool last_postfix_expr_was_pointer  = false;
+    bool violates_bounds_safety         = false;
+    bool violates_initialization_safety = false;
+    bool suppress_move_from_last_use    = false;
+
+    struct arg_info {
+        passing_style pass  = passing_style::in;
+        token const*  token = nullptr;
+    };
+    std::vector<arg_info> current_args  = { {} };
 
     //  For lowering
     //
@@ -1090,11 +1096,17 @@ class cppfront
             (in_synthesized_multi_return || (last_use && !suppress_move_from_last_use)) &&
             !in_non_rvalue_context.back();
 
+        //  For an explicit 'forward' apply forwarding to correct identifier
+        assert (!current_args.empty());
+        if (current_args.back().pass == passing_style::forward) {
+            add_std_forward = current_args.back().token == n.identifier;
+        }
+
         if (add_std_move) {
             printer.print_cpp2("std::move(", n.position());
         }
         if (add_std_forward) {
-            printer.print_cpp2("CPP2_FORWARD(", n.position());
+            printer.print_cpp2("CPP2_FORWARD(", {n.position().lineno, n.position().colno - 8});
         }
 
         assert(n.identifier);
@@ -1610,6 +1622,23 @@ class cppfront
         assert(n.expr);
         last_postfix_expr_was_pointer = false;
 
+        //  Ensure that forwarding postfix-expressions start with a forwarded parameter name
+        //
+        assert (!current_args.empty());
+        if (current_args.back().pass == passing_style::forward)
+        {
+            assert (n.expr->get_token());
+            assert (!current_args.back().token);
+            current_args.back().token = n.expr->get_token();
+            auto decl = sema.get_local_declaration_of(*current_args.back().token);
+            if (!(decl && decl->parameter && decl->parameter->pass == passing_style::forward)) {
+                errors.emplace_back(
+                    n.position(),
+                    n.expr->get_token()->to_string(true) + " is not a forwarding parameter name"
+                );
+            }
+        }
+
         //  Check that this isn't pointer arithmentic
         //      (initial partial implementation)
         if (n.expr->expr.index() == primary_expression_node::id_expression)
@@ -2066,23 +2095,19 @@ class cppfront
 
             if (x.pass != passing_style::in) {
                 assert(
-                    to_string_view(x.pass) == "out"  ||
-                    to_string_view(x.pass) == "move" ||
-                    to_string_view(x.pass) == "forward"
+                    x.pass == passing_style::out ||
+                    x.pass == passing_style::move ||
+                    x.pass == passing_style::forward
                 );
-                if (to_string_view(x.pass) == "out") {
+                if (x.pass == passing_style::out) {
                     is_out = true;
                     printer.print_cpp2("&", n.position());
                     offset = -3;   // because we're replacing "out " (followed by at least one space) with "&"
                 }
-                else if (to_string_view(x.pass) == "move") {
+                else if (x.pass == passing_style::move) {
                     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) {
@@ -2091,14 +2116,16 @@ class cppfront
 
             assert(x.expr);
             adjust_remaining_token_columns_on_this_line_visitor v(x.expr->position(), offset);
+            current_args.push_back( {x.pass} );
             x.expr->visit(v, 0);
             emit(*x.expr);
+            current_args.pop_back();
 
             if (is_out) {
                 in_non_rvalue_context.pop_back();
             }
 
-            if (to_string_view(x.pass) == "move" || to_string_view(x.pass) == "forward") {
+            if (x.pass == passing_style::move) {
                 printer.print_cpp2(")", n.position());
             }
         }

source/sema.h

@@ -34,7 +34,7 @@ struct declaration_sym {
     declaration_node const*           declaration = nullptr;
     token const*                      identifier  = nullptr;
     statement_node const*             initializer = nullptr;
-    parameter_declaration_node const* parameter = nullptr;
+    parameter_declaration_node const* parameter   = nullptr;
 
     declaration_sym(
         bool s = false,