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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**)
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‎regression-tests/pure2-default-arguments.cpp2‎

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// Note: Using source_location requires GCC 11 or higher,
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// Clang 16 or higher, MSVC 2019 16.10 or higher.
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// Older compilers will emit failures for this test case.
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my_function_name: (
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fn: *const char = std::source_location::current().function_name()
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)
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= {
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std::cout << "calling: (fn)$\n";
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}
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f: (x: i32 = 0) = { std::cout << x; }
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main: (args) = {
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my_function_name();
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f();
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f(1);
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f(2);
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}

‎regression-tests/test-results/clang-12-c++20/pure2-default-arguments.cpp.output‎

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pure2-default-arguments.cpp2:6:61: error: no member named 'source_location' in namespace 'std'
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char const* fn = CPP2_UFCS_NONLOCAL(function_name)(std::source_location::current())
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~~~~~^
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1 error generated.

‎regression-tests/test-results/gcc-10-c++20/pure2-default-arguments.cpp.output‎

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In file included from pure2-default-arguments.cpp:7:
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../../../include/cpp2util.h:2086:28: error: local variable ‘obj’ may not appear in this context
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2086 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<17>(x)), T >) { if (x.index() == 17) return operator_as<17>(x); }
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| ^~~
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../../../include/cpp2util.h:2047:34: note: in definition of macro ‘CPP2_UFCS_IDENTITY’
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2047 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as< 3>(x)), T >) { if (x.index() == 3) return operator_as<3>(x); }
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| ^~~~~~~~~~~
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../../../include/cpp2util.h:2086:15: note: in expansion of macro ‘CPP2_FORWARD’
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2086 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<17>(x)), T >) { if (x.index() == 17) return operator_as<17>(x); }
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| ^~~~~~~~~~~~
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../../../include/cpp2util.h:2107:22: note: in expansion of macro ‘CPP2_UFCS_CONSTRAINT_ARG’
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2107 | { return !x.has_value(); }
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| ^~~~~~~~~
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../../../include/cpp2util.h:2137:59: note: in expansion of macro ‘CPP2_UFCS_’
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2137 | { return std::any_cast<T>( x ); }
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| ^
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pure2-default-arguments.cpp2:6:22: note: in expansion of macro ‘CPP2_UFCS_NONLOCAL’
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../../../include/cpp2util.h:2086:92: error: local variable ‘params’ may not appear in this context
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2086 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<17>(x)), T >) { if (x.index() == 17) return operator_as<17>(x); }
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| ^~~~~~
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../../../include/cpp2util.h:2047:34: note: in definition of macro ‘CPP2_UFCS_IDENTITY’
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2047 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as< 3>(x)), T >) { if (x.index() == 3) return operator_as<3>(x); }
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| ^~~~~~~~~~~
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../../../include/cpp2util.h:2086:79: note: in expansion of macro ‘CPP2_FORWARD’
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2086 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<17>(x)), T >) { if (x.index() == 17) return operator_as<17>(x); }
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| ^~~~~~~~~~~~
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../../../include/cpp2util.h:2107:22: note: in expansion of macro ‘CPP2_UFCS_CONSTRAINT_ARG’
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2107 | { return !x.has_value(); }
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| ^~~~~~~~~
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../../../include/cpp2util.h:2137:59: note: in expansion of macro ‘CPP2_UFCS_’
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2137 | { return std::any_cast<T>( x ); }
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| ^
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pure2-default-arguments.cpp2:6:22: note: in expansion of macro ‘CPP2_UFCS_NONLOCAL’
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../../../include/cpp2util.h:2087:74: error: local variable ‘obj’ may not appear in this context
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2087 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<18>(x)), T >) { if (x.index() == 18) return operator_as<18>(x); }
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| ^~~
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../../../include/cpp2util.h:2047:34: note: in definition of macro ‘CPP2_UFCS_IDENTITY’
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2047 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as< 3>(x)), T >) { if (x.index() == 3) return operator_as<3>(x); }
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| ^~~~~~~~~~~
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../../../include/cpp2util.h:2087:61: note: in expansion of macro ‘CPP2_FORWARD’
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2087 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<18>(x)), T >) { if (x.index() == 18) return operator_as<18>(x); }
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| ^~~~~~~~~~~~
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../../../include/cpp2util.h:2107:22: note: in expansion of macro ‘CPP2_UFCS_CONSTRAINT_ARG’
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2107 | { return !x.has_value(); }
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| ^~~~~~~~~
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../../../include/cpp2util.h:2137:59: note: in expansion of macro ‘CPP2_UFCS_’
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2137 | { return std::any_cast<T>( x ); }
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| ^
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pure2-default-arguments.cpp2:6:22: note: in expansion of macro ‘CPP2_UFCS_NONLOCAL’
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../../../include/cpp2util.h:2087:93: error: local variable ‘params’ may not appear in this context
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2087 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<18>(x)), T >) { if (x.index() == 18) return operator_as<18>(x); }
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| ^~~~~~
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../../../include/cpp2util.h:2047:34: note: in definition of macro ‘CPP2_UFCS_IDENTITY’
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2047 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as< 3>(x)), T >) { if (x.index() == 3) return operator_as<3>(x); }
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| ^~~~~~~~~~~
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../../../include/cpp2util.h:2087:80: note: in expansion of macro ‘CPP2_FORWARD’
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2087 | if constexpr (std::is_same_v< CPP2_TYPEOF(operator_as<18>(x)), T >) { if (x.index() == 18) return operator_as<18>(x); }
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| ^~~~~~~~~~~~
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../../../include/cpp2util.h:2107:22: note: in expansion of macro ‘CPP2_UFCS_CONSTRAINT_ARG’
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2107 | { return !x.has_value(); }
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| ^~~~~~~~~
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../../../include/cpp2util.h:2137:59: note: in expansion of macro ‘CPP2_UFCS_’
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2137 | { return std::any_cast<T>( x ); }
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| ^
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pure2-default-arguments.cpp2:6:22: note: in expansion of macro ‘CPP2_UFCS_NONLOCAL’
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pure2-default-arguments.cpp2:6:61: error: ‘std::source_location’ has not been declared

‎regression-tests/test-results/gcc-14-c++2b/pure2-default-arguments.cpp.execution‎

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calling: int main(int, char**)
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012

‎regression-tests/test-results/msvc-2022-c++latest/pure2-default-arguments.cpp.execution‎

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calling: int __cdecl main(const int,char **)
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‎regression-tests/test-results/msvc-2022-c++latest/pure2-default-arguments.cpp.output‎

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pure2-default-arguments.cpp

‎regression-tests/test-results/pure2-default-arguments.cpp‎

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#define CPP2_IMPORT_STD Yes
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//=== Cpp2 type declarations ====================================================
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#include "cpp2util.h"
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#line 1 "pure2-default-arguments.cpp2"
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//=== Cpp2 type definitions and function declarations ===========================
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#line 1 "pure2-default-arguments.cpp2"
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// Note: Using source_location requires GCC 11 or higher,
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// Clang 16 or higher, MSVC 2019 16.10 or higher.
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// Older compilers will emit failures for this test case.
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#line 5 "pure2-default-arguments.cpp2"
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auto my_function_name(
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char const* fn = CPP2_UFCS_NONLOCAL(function_name)(std::source_location::current())
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) -> void;
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#line 12 "pure2-default-arguments.cpp2"
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auto f(cpp2::impl::in<cpp2::i32> x = 0) -> void;
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auto main(int const argc_, char** argv_) -> int;
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//=== Cpp2 function definitions =================================================
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#line 1 "pure2-default-arguments.cpp2"
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#line 5 "pure2-default-arguments.cpp2"
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auto my_function_name(
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char const* fn
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) -> void
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{
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std::cout << "calling: " + cpp2::to_string(fn) + "\n";
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}
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#line 12 "pure2-default-arguments.cpp2"
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auto f(cpp2::impl::in<cpp2::i32> x) -> void{std::cout << x; }
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#line 14 "pure2-default-arguments.cpp2"
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auto main(int const argc_, char** argv_) -> int{
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auto const args = cpp2::make_args(argc_, argv_);
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#line 15 "pure2-default-arguments.cpp2"
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my_function_name();
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f();
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f(1);
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f(2);
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}
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‎regression-tests/test-results/pure2-default-arguments.cpp2.output‎

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pure2-default-arguments.cpp2... ok (all Cpp2, passes safety checks)
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‎regression-tests/test-results/version‎

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2-
cppfront compiler v0.7.2 Build 9804:1033
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cppfront compiler v0.7.2 Build 9809:1046
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Copyright(c) Herb Sutter All rights reserved
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SPDX-License-Identifier: CC-BY-NC-ND-4.0

‎source/build.info‎

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"9804:1033"
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"9809:1046"

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