Enable natural use of single named return value

Given:

    f: () -> (ret: int = 42)
    = {
        // ...
    }

The call site `f().ret` has always worked, as it should for a named return.

The call site `f()` now also works -- we can omit the name if we don't need it. This lets callers have the same convenience of a single unnamed return value, while giving function authors the convenience of writing a named return value, including so the return value has a name for postconditions:

    f: () -> (ret: int = 42)
        post (ret > 0)
    = {
        // ...
    }

Author: hsutter

regression-tests/test-results/mixed-captures-in-expressions-and-postconditions.cpp

@@ -47,7 +47,7 @@ auto insert_at(cpp2::in<int> where, cpp2::in<int> val) -> void
 #line 22 "mixed-captures-in-expressions-and-postconditions.cpp2"
 {
     cpp2::Default.expects(cpp2::cmp_less_eq(0,where) && cpp2::cmp_less_eq(where,CPP2_UFCS_0(ssize, vec)), "");
-    auto post_21_5 = cpp2::finally_success([_0 = CPP2_UFCS_0(ssize, vec)]{cpp2::Default.expects(CPP2_UFCS_0(ssize, vec) == _0 + 1, "");} );
+    auto post_21_5 = cpp2::finally_success([&, _1 = CPP2_UFCS_0(ssize, vec)]{cpp2::Default.expects(CPP2_UFCS_0(ssize, vec) == _1 + 1, "");} );
 #line 23 "mixed-captures-in-expressions-and-postconditions.cpp2"
     static_cast<void>(CPP2_UFCS(insert, vec, CPP2_UFCS_0(begin, vec) + where, val));
 }

regression-tests/test-results/mixed-postexpression-with-capture.cpp

@@ -42,7 +42,7 @@ auto insert_at(cpp2::in<int> where, cpp2::in<int> val) -> void
 #line 17 "mixed-postexpression-with-capture.cpp2"
 {
     cpp2::Default.expects(cpp2::cmp_less_eq(0,where) && cpp2::cmp_less_eq(where,CPP2_UFCS_0(ssize, vec)), "");
-    auto post_16_5 = cpp2::finally_success([_0 = CPP2_UFCS_0(size, vec)]{cpp2::Default.expects(CPP2_UFCS_0(size, vec) == _0 + 1, "");} );
+    auto post_16_5 = cpp2::finally_success([&, _1 = CPP2_UFCS_0(size, vec)]{cpp2::Default.expects(CPP2_UFCS_0(size, vec) == _1 + 1, "");} );
 #line 18 "mixed-postexpression-with-capture.cpp2"
     CPP2_UFCS(push_back, vec, val);
 }

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

@@ -1,3 +1,3 @@
-Microsoft (R) C/C++ Optimizing Compiler Version 19.37.32825 for x86
+Microsoft (R) C/C++ Optimizing Compiler Version 19.37.32826.1 for x86
 Copyright (C) Microsoft Corporation.  All rights reserved.
 

regression-tests/test-results/pure2-bugfix-for-name-lookup-and-value-decoration.cpp

@@ -13,7 +13,8 @@
 
 #line 1 "pure2-bugfix-for-name-lookup-and-value-decoration.cpp2"
 
-struct vals_ret { int i; };
+struct vals_ret { int i; 
+operator auto() const { return i; }};
 
 [[nodiscard]] auto vals() -> vals_ret;
 

regression-tests/test-results/pure2-look-up-parameter-across-unnamed-function.cpp

@@ -13,11 +13,13 @@
 
 #line 1 "pure2-look-up-parameter-across-unnamed-function.cpp2"
 
-struct f_ret { int ri; };
+struct f_ret { int ri; 
+operator auto() const { return ri; }};
 
 #line 2 "pure2-look-up-parameter-across-unnamed-function.cpp2"
 [[nodiscard]] auto f() -> f_ret;
-struct g_ret { int ri; };
+struct g_ret { int ri; 
+operator auto() const { return ri; }};
 
 
 

regression-tests/test-results/pure2-ufcs-member-access-and-chaining.cpp

@@ -18,7 +18,8 @@
 auto no_return([[maybe_unused]] auto const& unnamed_param_1) -> void;
 
 [[nodiscard]] auto ufcs(cpp2::in<int> i) -> int;
-struct fun_ret { int i; };
+struct fun_ret { int i; 
+operator auto() const { return i; }};
 
 
 

regression-tests/test-results/version

@@ -1,5 +1,5 @@
 
-cppfront compiler v0.3.0   Build 8B15:0741
+cppfront compiler v0.3.0   Build 8B15:1925
 Copyright(c) Herb Sutter   All rights reserved
 
 SPDX-License-Identifier: CC-BY-NC-ND-4.0

source/build.info

@@ -1 +1 @@
-"8B15:0741"
+"8B15:1925"

source/to_cpp1.h

@@ -2460,7 +2460,8 @@ class cppfront
     //
     auto build_capture_lambda_intro_for(
         capture_group&  captures,
-        source_position pos
+        source_position pos,
+        bool            include_default_reference_capture = false
     )
         -> std::string
     {
@@ -2499,9 +2500,11 @@ class cppfront
         auto num_captures = 0;
 
         if (
-            !current_functions.empty()
-            && current_functions.back().decl->is_function_with_this()
-            && !current_functions.back().decl->parent_is_namespace()
+            (!current_functions.empty()
+                && current_functions.back().decl->is_function_with_this()
+                && !current_functions.back().decl->parent_is_namespace()
+                )
+            || include_default_reference_capture
             )
         {
             //  Note: & is needed (when allowed, not at namespace scope) because a
@@ -4239,7 +4242,8 @@ class cppfront
     auto emit(
         parameter_declaration_list_node const& n,
         bool                                   is_returns            = false,
-        bool                                   is_template_parameter = false
+        bool                                   is_template_parameter = false,
+        std::string                            extra_text            = ""
     )
         -> void
     {
@@ -4277,7 +4281,7 @@ class cppfront
         }
 
         if (is_returns) {
-            printer.print_extra( "};\n" );
+            printer.print_extra( extra_text + "};\n" );
         }
         else {
             //  Position heuristic (aka hack): Avoid emitting extra whitespace before )
@@ -4307,7 +4311,7 @@ class cppfront
         //  For a postcondition, we'll wrap it in a final_action_success lambda
         //
         if (*n.kind == "post") {
-            auto lambda_intro = build_capture_lambda_intro_for(n.captures, n.position());
+            auto lambda_intro = build_capture_lambda_intro_for(n.captures, n.position(), true);
             printer.print_cpp2(
                 "auto post_" + std::to_string(n.position().lineno) + "_" +
                     std::to_string(n.position().colno) + " = cpp2::finally_success(" +
@@ -5318,7 +5322,16 @@ class cppfront
                 printer.print_extra( "\nstruct " );
                 printer.print_extra( *n.name() );
                 printer.print_extra( "_ret " );
-                emit(*r, true);
+
+                auto extra = std::string{};
+                if (std::ssize(r->parameters) == 1) {
+                    assert (r->parameters[0]->declaration->name());
+                    extra += "\noperator auto() const { return "
+                        + r->parameters[0]->declaration->name()->to_string()
+                        + "; }";
+                }
+
+                emit(*r, true, false, extra);
                 printer.print_extra( "\n" );
             }
         }
@@ -6030,15 +6043,6 @@ class cppfront
                     function_returns.emplace_back(nullptr);        // no return type at all
                 }
 
-                for (auto&& c : func->contracts)
-                {
-                    auto print = std::string();
-                    printer.emit_to_string(&print);
-                    emit(*c);
-                    printer.emit_to_string();
-                    current_functions.back().prolog.statements.push_back(print);
-                }
-
                 if (func->returns.index() == function_type_node::list)
                 {
                     auto& r = std::get<function_type_node::list>(func->returns);
@@ -6093,6 +6097,15 @@ class cppfront
                     }
                 }
 
+                for (auto&& c : func->contracts)
+                {
+                    auto print = std::string();
+                    printer.emit_to_string(&print);
+                    emit(*c);
+                    printer.emit_to_string();
+                    current_functions.back().prolog.statements.push_back(print);
+                }
+
                 printer.preempt_position_push( n.equal_sign );
 
                 emit_requires_clause();