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();