Allow function parameter default arguments

Closes #1189 I had been experimenting with not allowing default arguments for function parameters, in part because of the potential for creating order-dependent code; the way to find out whether they're necessary is to not support them and see if that leaves a usability hole. The result of the experiment is that it does leave a hole: There's persistent feedback that default arguments are often useful, and are actually necessary for a few cases including particularly `std::source_location` parameters. As for order independence, there are already ways to opt into creating potentially order-dependent code (such as by deduced return types which depend on function bodies). So I think it's time to enable default arguments, and Cpp2 is still order-independent by default. This example now works: my_function_name: ( fn: *const char = std::source_location::current().function_name() ) = { std::cout << "calling: (fn)$\n"; } main: (args) = { my_function_name(); } // On MSVC 2022, prints: // calling: int __cdecl main(const int,char **) // On GCC 14, prints: // calling: int main(int, char**)
hsutter · Aug 9, 2024 · 873b760 · 873b760
1 parent 658d307
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diff --git a/regression-tests/pure2-default-arguments.cpp2 b/regression-tests/pure2-default-arguments.cpp2
@@ -0,0 +1,19 @@
+
+//  Note: Using source_location requires GCC 11 or higher,
+//        Clang 16 or higher, MSVC 2019 16.10 or higher.
+//        Older compilers will emit failures for this test case.
+my_function_name: (
+    fn: *const char = std::source_location::current().function_name()
+    )
+= {
+    std::cout << "calling: (fn)$\n";
+}
+
+f: (x: i32 = 0) = { std::cout << x; }
+
+main: (args) = {
+    my_function_name();
+    f();
+    f(1);
+    f(2);
+}
diff --git a/regression-tests/test-results/clang-12-c++20/pure2-default-arguments.cpp.output b/regression-tests/test-results/clang-12-c++20/pure2-default-arguments.cpp.output
@@ -0,0 +1,4 @@
+pure2-default-arguments.cpp2:6:61: error: no member named 'source_location' in namespace 'std'
+    char const* fn = CPP2_UFCS_NONLOCAL(function_name)(std::source_location::current())
+                                                       ~~~~~^
+1 error generated.
diff --git a/regression-tests/test-results/gcc-10-c++20/pure2-default-arguments.cpp.output b/regression-tests/test-results/gcc-10-c++20/pure2-default-arguments.cpp.output
@@ -0,0 +1,66 @@
+In file included from pure2-default-arguments.cpp:7:
+../../../include/cpp2util.h:2086:28: error: local variable ‘obj’ may not appear in this context
+ 2086 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<17>(x)), T >) { if (x.index() == 17) return operator_as<17>(x); }
+      |                            ^~~
+../../../include/cpp2util.h:2047:34: note: in definition of macro ‘CPP2_UFCS_IDENTITY’
+ 2047 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as< 3>(x)), T >) { if (x.index() ==  3) return operator_as<3>(x); }
+      |                                  ^~~~~~~~~~~
+../../../include/cpp2util.h:2086:15: note: in expansion of macro ‘CPP2_FORWARD’
+ 2086 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<17>(x)), T >) { if (x.index() == 17) return operator_as<17>(x); }
+      |               ^~~~~~~~~~~~
+../../../include/cpp2util.h:2107:22: note: in expansion of macro ‘CPP2_UFCS_CONSTRAINT_ARG’
+ 2107 |     { return !x.has_value(); }
+      |                      ^~~~~~~~~               
+../../../include/cpp2util.h:2137:59: note: in expansion of macro ‘CPP2_UFCS_’
+ 2137 |     { return std::any_cast<T>( x ); }
+      |                                                           ^         
+pure2-default-arguments.cpp2:6:22: note: in expansion of macro ‘CPP2_UFCS_NONLOCAL’
+../../../include/cpp2util.h:2086:92: error: local variable ‘params’ may not appear in this context
+ 2086 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<17>(x)), T >) { if (x.index() == 17) return operator_as<17>(x); }
+      |                                                                                            ^~~~~~
+../../../include/cpp2util.h:2047:34: note: in definition of macro ‘CPP2_UFCS_IDENTITY’
+ 2047 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as< 3>(x)), T >) { if (x.index() ==  3) return operator_as<3>(x); }
+      |                                  ^~~~~~~~~~~
+../../../include/cpp2util.h:2086:79: note: in expansion of macro ‘CPP2_FORWARD’
+ 2086 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<17>(x)), T >) { if (x.index() == 17) return operator_as<17>(x); }
+      |                                                                               ^~~~~~~~~~~~
+../../../include/cpp2util.h:2107:22: note: in expansion of macro ‘CPP2_UFCS_CONSTRAINT_ARG’
+ 2107 |     { return !x.has_value(); }
+      |                      ^~~~~~~~~               
+../../../include/cpp2util.h:2137:59: note: in expansion of macro ‘CPP2_UFCS_’
+ 2137 |     { return std::any_cast<T>( x ); }
+      |                                                           ^         
+pure2-default-arguments.cpp2:6:22: note: in expansion of macro ‘CPP2_UFCS_NONLOCAL’
+../../../include/cpp2util.h:2087:74: error: local variable ‘obj’ may not appear in this context
+ 2087 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<18>(x)), T >) { if (x.index() == 18) return operator_as<18>(x); }
+      |                                                                          ^~~
+../../../include/cpp2util.h:2047:34: note: in definition of macro ‘CPP2_UFCS_IDENTITY’
+ 2047 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as< 3>(x)), T >) { if (x.index() ==  3) return operator_as<3>(x); }
+      |                                  ^~~~~~~~~~~
+../../../include/cpp2util.h:2087:61: note: in expansion of macro ‘CPP2_FORWARD’
+ 2087 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<18>(x)), T >) { if (x.index() == 18) return operator_as<18>(x); }
+      |                                                             ^~~~~~~~~~~~
+../../../include/cpp2util.h:2107:22: note: in expansion of macro ‘CPP2_UFCS_CONSTRAINT_ARG’
+ 2107 |     { return !x.has_value(); }
+      |                      ^~~~~~~~~               
+../../../include/cpp2util.h:2137:59: note: in expansion of macro ‘CPP2_UFCS_’
+ 2137 |     { return std::any_cast<T>( x ); }
+      |                                                           ^         
+pure2-default-arguments.cpp2:6:22: note: in expansion of macro ‘CPP2_UFCS_NONLOCAL’
+../../../include/cpp2util.h:2087:93: error: local variable ‘params’ may not appear in this context
+ 2087 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<18>(x)), T >) { if (x.index() == 18) return operator_as<18>(x); }
+      |                                                                                             ^~~~~~
+../../../include/cpp2util.h:2047:34: note: in definition of macro ‘CPP2_UFCS_IDENTITY’
+ 2047 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as< 3>(x)), T >) { if (x.index() ==  3) return operator_as<3>(x); }
+      |                                  ^~~~~~~~~~~
+../../../include/cpp2util.h:2087:80: note: in expansion of macro ‘CPP2_FORWARD’
+ 2087 |     if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<18>(x)), T >) { if (x.index() == 18) return operator_as<18>(x); }
+      |                                                                                ^~~~~~~~~~~~
+../../../include/cpp2util.h:2107:22: note: in expansion of macro ‘CPP2_UFCS_CONSTRAINT_ARG’
+ 2107 |     { return !x.has_value(); }
+      |                      ^~~~~~~~~               
+../../../include/cpp2util.h:2137:59: note: in expansion of macro ‘CPP2_UFCS_’
+ 2137 |     { return std::any_cast<T>( x ); }
+      |                                                           ^         
+pure2-default-arguments.cpp2:6:22: note: in expansion of macro ‘CPP2_UFCS_NONLOCAL’
+pure2-default-arguments.cpp2:6:61: error: ‘std::source_location’ has not been declared
diff --git a/regression-tests/test-results/gcc-14-c++2b/pure2-default-arguments.cpp.execution b/regression-tests/test-results/gcc-14-c++2b/pure2-default-arguments.cpp.execution
@@ -0,0 +1,2 @@
+calling: int main(int, char**)
+012
diff --git a/regression-tests/test-results/msvc-2022-c++latest/pure2-default-arguments.cpp.execution b/regression-tests/test-results/msvc-2022-c++latest/pure2-default-arguments.cpp.execution
@@ -0,0 +1,2 @@
+calling: int __cdecl main(const int,char **)
+012
diff --git a/regression-tests/test-results/msvc-2022-c++latest/pure2-default-arguments.cpp.output b/regression-tests/test-results/msvc-2022-c++latest/pure2-default-arguments.cpp.output
@@ -0,0 +1 @@
+pure2-default-arguments.cpp
diff --git a/regression-tests/test-results/pure2-default-arguments.cpp b/regression-tests/test-results/pure2-default-arguments.cpp
@@ -0,0 +1,53 @@
+
+#define CPP2_IMPORT_STD          Yes
+
+//=== Cpp2 type declarations ====================================================
+
+
+#include "cpp2util.h"
+
+#line 1 "pure2-default-arguments.cpp2"
+
+
+//=== Cpp2 type definitions and function declarations ===========================
+
+#line 1 "pure2-default-arguments.cpp2"
+
+//  Note: Using source_location requires GCC 11 or higher,
+//        Clang 16 or higher, MSVC 2019 16.10 or higher.
+//        Older compilers will emit failures for this test case.
+#line 5 "pure2-default-arguments.cpp2"
+auto my_function_name(
+    char const* fn = CPP2_UFCS_NONLOCAL(function_name)(std::source_location::current())
+    ) -> void;
+
+#line 12 "pure2-default-arguments.cpp2"
+auto f(cpp2::impl::in<cpp2::i32> x = 0) -> void;
+
+auto main(int const argc_, char** argv_) -> int;
+
+//=== Cpp2 function definitions =================================================
+
+#line 1 "pure2-default-arguments.cpp2"
+
+#line 5 "pure2-default-arguments.cpp2"
+auto my_function_name(
+    char const* fn
+    ) -> void
+{
+    std::cout << "calling: " + cpp2::to_string(fn) + "\n";
+}
+
+#line 12 "pure2-default-arguments.cpp2"
+auto f(cpp2::impl::in<cpp2::i32> x) -> void{std::cout << x; }
+
+#line 14 "pure2-default-arguments.cpp2"
+auto main(int const argc_, char** argv_) -> int{
+    auto const args = cpp2::make_args(argc_, argv_); 
+#line 15 "pure2-default-arguments.cpp2"
+    my_function_name();
+    f();
+    f(1);
+    f(2);
+}
+
diff --git a/regression-tests/test-results/pure2-default-arguments.cpp2.output b/regression-tests/test-results/pure2-default-arguments.cpp2.output
@@ -0,0 +1,2 @@
+pure2-default-arguments.cpp2... ok (all Cpp2, passes safety checks)
+
diff --git a/regression-tests/test-results/version b/regression-tests/test-results/version
@@ -1,5 +1,5 @@
 
-cppfront compiler v0.7.2   Build 9804:1033
+cppfront compiler v0.7.2   Build 9809:1046
 Copyright(c) Herb Sutter   All rights reserved
 
 SPDX-License-Identifier: CC-BY-NC-ND-4.0

diff --git a/source/build.info b/source/build.info
@@ -1 +1 @@
-"9804:1033"
+"9809:1046"
diff --git a/source/parse.h b/source/parse.h
@@ -2857,8 +2857,9 @@ struct declaration_node
     bool member_function_generation = true;
 
     //  Cache some context
-    bool is_a_template_parameter = false;
-    bool is_a_parameter          = false;
+    bool is_a_template_parameter  = false;
+    bool is_a_parameter           = false;
+    bool is_a_statement_parameter = false;
 
     //  Constructor
     //
@@ -2885,6 +2886,12 @@ struct declaration_node
         return is_a_parameter;
     }
 
+    auto is_statement_parameter() const
+        -> bool
+    {
+        return is_a_statement_parameter;
+    }
+
     auto type_member_mark_for_removal()
         -> bool
     {
@@ -8041,6 +8048,7 @@ class parser
             pos = start_pos;    // backtrack
             return {};
         }
+        n->declaration->is_a_statement_parameter = is_statement;
 
         //  And some error checks
         //
@@ -8107,15 +8115,6 @@ class parser
             return {};
         }
 
-        if (
-            !is_returns
-            && !is_statement
-            && n->declaration->initializer
-            )
-        {
-            error("Cpp2 is currently exploring the path of not allowing default arguments - use overloading instead", false);
-            return {};
-        }
         if (is_named && is_returns) {
             auto tok = n->name();
             assert(tok);

diff --git a/source/to_cpp1.h b/source/to_cpp1.h
@@ -3291,14 +3291,18 @@ class cppfront
                     ufcs_string += "_TEMPLATE";
                 }
 
-                //  If we're in an object declaration (i.e., initializer)
-                //  at namespace scope, use the _NONLOCAL version
+                //  If we're in a namespace-scope object declaration (i.e., initializer)
+                //  or in a default function argument, use the _NONLOCAL version
                 //
                 //  Note: If there are other cases where code could execute
                 //  in a non-local scope where a capture-default for the UFCS
                 //  lambda would not be allowed, then add them here
                 if (
                     current_declarations.back()->is_namespace()
+                    || (
+                        current_declarations.back()->is_parameter()
+                        && !current_declarations.back()->is_statement_parameter()
+                        )
                     || (
                         current_declarations.back()->is_object()
                         && current_declarations.back()->parent_is_namespace()
@@ -4680,6 +4684,10 @@ class cppfront
         if (
             !is_returns
             && n.declaration->initializer
+            && (
+                is_statement 
+                || printer.get_phase() != printer.phase2_func_defs
+                )
             )
         {
             auto guard = stack_element(current_declarations, &*n.declaration);
@@ -4689,7 +4697,7 @@ class cppfront
             else {
                 printer.print_cpp2( " = ", n.declaration->initializer->position() );
             }
-            emit(*n.declaration->initializer, !is_statement);
+            emit(*n.declaration->initializer, false);
             if (is_statement) {
                 printer.print_cpp2( "};", n.declaration->initializer->position() );
             }
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		calling: int __cdecl main(const int,char **)
		012
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		pure2-default-arguments.cpp2... ok (all Cpp2, passes safety checks)