[BUG] requires clause does not appear in "cpp2 type definitions and function declaration" causing out-of-line definition error

In the current implementation of cppfront (827ed79), the following code:

element: type = {
    name: std::string;

    operator=: (out this, forward n: std::string ) = {
        name = n;
    }
}

Generates:

//=== Cpp2 type declarations ====================================================


#include "cpp2util.h"

#line 1 "../tests/bug_assignement_operator_5.cpp2"
class element;

//=== Cpp2 type definitions and function declarations ===========================

#line 1 "../tests/bug_assignement_operator_5.cpp2"
class element {
    private: std::string name; 

    public: explicit element(auto&& n); // <--- notice lack of requires clause
#line 4 "../tests/bug_assignement_operator_5.cpp2"
    public: auto operator=(auto&& n) -> element& ; // <--- notice lack of requires clause

#line 7 "../tests/bug_assignement_operator_5.cpp2"
};

//=== Cpp2 function definitions =================================================


#line 4 "../tests/bug_assignement_operator_5.cpp2"
    element::element(auto&& n)
requires (std::is_same_v<CPP2_TYPEOF(n), std::string>) // <--- notice requires clause is present
        : name{ CPP2_FORWARD(n) }
#line 4 "../tests/bug_assignement_operator_5.cpp2"
    {

    }
#line 4 "../tests/bug_assignement_operator_5.cpp2"
    auto element::operator=(auto&& n) -> element& 
requires (std::is_same_v<CPP2_TYPEOF(n), std::string>) // <--- notice requires clause is present
#line 4 "../tests/bug_assignement_operator_5.cpp2"
    {
        name = CPP2_FORWARD(n);
        return *this;

#line 6 "../tests/bug_assignement_operator_5.cpp2"
    }

Trying to compile that with cpp1 compiler (Apple clang version 14.0.3 in my case):

../tests/bug_assignement_operator_5.cpp2... ok (all Cpp2, passes safety checks)

../tests/bug_assignement_operator_5.cpp2:4:14: error: out-of-line definition of 'element' does not match any declaration in 'element'
    element::element(auto&& n)
             ^~~~~~~
../tests/bug_assignement_operator_5.cpp2:4:19: error: out-of-line definition of 'operator=' does not match any declaration in 'element'
    auto element::operator=(auto&& n) -> element& 
                  ^~~~~~~~
2 errors generated.

The fix is pretty straightforward - requires clause must also appear in the Cpp2 type definitions and function declarations section (I have changed that manually, and it works).

While looking for a place on how to fix it, I have found comments:

cppfront/source/cppfront.cpp

Lines 5110 to 5129 in 827ed79

    
           //  *** NOTE ===================================================== 
        
           //  
        
           //      This branch to emit the requires-clause should maybe be 
        
           //      moved to location (A) above, so that it's also emitted 
        
           //      on the function declaration. But moving it to (A) triggers 
        
           //      a bug in GCC 10.x (that was fixed in 11.x), where it would 
        
           //      break using a 'forward' parameter of a concrete type and 
        
           //      also explicitly user-written requires-clauses that do 
        
           //      similar decltype tests. 
        
           //  
        
           //      I don't want to neednessly break compatibility with a 
        
           //      decently conforming C++20 compiler that works well for 
        
           //      everything else that Cpp2 needs from C++20. If the 
        
           //      'requires' down here doesn't cause a problem, I'll keep 
        
           //      it here for now... if we do encounter a reason it needs to 
        
           //      also be on the declaration, move this code to (A). 
        
           //  
        
           //  Handle requires clause - an explicit one the user wrote, 
        
           //  and/or any conditions we generated while processing the 
        
           //  parameters (i.e., forwarding a concrete type)

And the (A) location is here:

cppfront/source/cppfront.cpp

Line 5013 in 827ed79

// *** LOCATION (A) -- SEE NOTE REGARDING (A) BELOW

@hsutter

@hsutter regarding gcc-11 installation on Ubuntu 20.04 - have you tried #293 (comment) ?

TLDR: you can install gcc-11 and g++-11 with:

add-apt-repository -y ppa:ubuntu-toolchain-r/test && apt-get update && apt-get install -y --no-install-recommends gcc-11 g++-11

I think the test happens to work, despite #323, because the definition overloads with the declaration. So actually not emitting the requires on the declaration wouldn't be a fix even for GCC 10.
-- #477 (comment)

GCC not-10 is also broken: https://cpp2.godbolt.org/z/MsY89sMxh.
We shouldn't be deprived of requires clauses for GCC 10. Nerf it.

Proof that not emitting requires makes an overload set: https://cpp2.godbolt.org/z/PrEqz4nW6.

f: (forward x: std::string) = std::ignore = x;
main: () = f(0);

It wants to call the unconstrained forward declaration!

/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/14.0.0/../../../../x86_64-linux-gnu/bin/ld: CMakeFiles/test.dir/_cppfront/main.cpp.o: in function `main':
main.cpp:(.text+0x14): undefined reference to `void f<int>(int&&)'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

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[BUG] requires clause does not appear in "cpp2 type definitions and function declaration" causing out-of-line definition error #323

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	// *** NOTE =====================================================
	//
	// This branch to emit the requires-clause should maybe be
	// moved to location (A) above, so that it's also emitted
	// on the function declaration. But moving it to (A) triggers
	// a bug in GCC 10.x (that was fixed in 11.x), where it would
	// break using a 'forward' parameter of a concrete type and
	// also explicitly user-written requires-clauses that do
	// similar decltype tests.
	//
	// I don't want to neednessly break compatibility with a
	// decently conforming C++20 compiler that works well for
	// everything else that Cpp2 needs from C++20. If the
	// 'requires' down here doesn't cause a problem, I'll keep
	// it here for now... if we do encounter a reason it needs to
	// also be on the declaration, move this code to (A).
	//
	// Handle requires clause - an explicit one the user wrote,
	// and/or any conditions we generated while processing the
	// parameters (i.e., forwarding a concrete type)

[BUG] requires clause does not appear in "cpp2 type definitions and function declaration" causing out-of-line definition error #323

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filipsajdak commented on Apr 5, 2023

filipsajdak commented on Apr 5, 2023

JohelEGP commented on May 31, 2023

JohelEGP commented on Jun 1, 2023

JohelEGP commented on Jun 7, 2023

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