Allow single-argument global main that gets a vector<string_view>, closes #262

Author: hsutter

include/cpp2util.h

@@ -203,6 +203,7 @@
     #include <memory>
     #include <string>
     #include <string_view>
+    #include <vector>
     #include <iostream>
     #include <variant>
     #include <any>
@@ -1408,6 +1409,21 @@ inline auto to_string(std::tuple<Ts...> const& t) -> std::string
 }
 
 
+//-----------------------------------------------------------------------
+//
+//  args: see main() arguments as vector<string_view>
+//
+//-----------------------------------------------------------------------
+//
+auto args(int argc, char **argv) -> std::vector<std::string_view> {
+    auto ret = std::vector<std::string_view>{(size_t)argc};
+    for (auto i = 0; i < argc; ++i) {
+        ret[i] = std::string_view{argv[i]};
+    }
+    return ret;
+}
+
+
 //-----------------------------------------------------------------------
 //
 //  Speculative: RAII wrapping for the C standard library

regression-tests/mixed-fixed-type-aliases.cpp2

@@ -12,10 +12,13 @@ test: (x:_) = {
         << "\n";
 }
 
-main: (argc: int, argv: **char) -> int = {
+main: (args) -> int = {
     y: my::u16 = 42;
     test(y);
 
     z: u16 = 42;
     test(z);
+
+    for args do :(arg) =
+        std::cout << arg << "\n";
 }

regression-tests/test-results/clang-12/mixed-fixed-type-aliases.cpp.execution

@@ -1,2 +1,3 @@
 true
 false
+./test.exe

regression-tests/test-results/gcc-10/mixed-fixed-type-aliases.cpp.execution

@@ -1,2 +1,3 @@
 true
 false
+./test.exe

regression-tests/test-results/mixed-fixed-type-aliases.cpp

@@ -13,7 +13,7 @@ namespace my {
 #line 8 "mixed-fixed-type-aliases.cpp2"
 auto test(auto const& x) -> void;
 #line 15 "mixed-fixed-type-aliases.cpp2"
-[[nodiscard]] auto main(cpp2::in<int> argc, cpp2::in<char**> argv) -> int;
+[[nodiscard]] auto main(int argc, char **argv) -> int;
 
 //=== Cpp2 definitions ==========================================================
 
@@ -26,11 +26,16 @@ auto test(auto const& x) -> void{
         << "\n";
 }
 
-[[nodiscard]] auto main(cpp2::in<int> argc, cpp2::in<char**> argv) -> int{
+[[nodiscard]] auto main(int argc, char **argv) -> int{
+    auto args = cpp2::args(argc, argv); 
+#line 16 "mixed-fixed-type-aliases.cpp2"
     my::u16 y {42}; 
     test(std::move(y));
 
     cpp2::u16 z {42}; 
     test(std::move(z));
+
+    for ( auto const& cpp2_range = args;  auto const& arg : cpp2_range ) 
+        std::cout << arg << "\n";
 }
 

regression-tests/test-results/msvc-2022/mixed-fixed-type-aliases.cpp.execution

@@ -1,2 +1,3 @@
 true
 false
+test.exe

source/cppfront.cpp

@@ -2962,7 +2962,34 @@ class cppfront
     auto emit(function_type_node const& n, token const* ident, bool is_main) -> void
     {
         assert(n.parameters);
-        emit(*n.parameters);
+        if (is_main && n.parameters->parameters.size() == 1) {
+            auto& param1 = *n.parameters->parameters[0];
+            auto  param1_name = param1.declaration->identifier->get_token()->to_string(true);
+            if (param1.pass != passing_style::in ||
+                param1.declaration->type.index() != declaration_node::object ||
+                !std::get<declaration_node::object>(param1.declaration->type)->is_wildcard()
+                )
+            {
+                // we don't currently bail right after emitting the declarations (because
+                // we don't necessarily want to hide other interesting errors), so this
+                // avoids emitting the error a second time when we emit the definitions
+                static auto once = true;
+                if (std::exchange(once, false)) {
+                    errors.emplace_back(
+                        param1.position(),
+                        "when 'main' has a single parameter, that parameter should be declared as just:  main: (" + param1_name + ")  - the type 'std::vector<std::string_view>' will be deduced"
+                    );
+                }
+                return;
+            }
+            printer.print_cpp2( "(int argc, char **argv)", n.parameters->position() );
+            current_function.back().prolog.push_back(
+                "auto " + param1_name + " = cpp2::args(argc, argv); "
+            );
+        }
+        else {
+            emit(*n.parameters);
+        }
 
         //  Add implicit noexcept when we implement proper EH
         //  to handle calling Cpp1 code that throws