[BUG] Bad operator= error message for @value types

[dutkalex]

Hi folks!
I ran into a very confusing bug while experimenting with cppfront. Consider the following type:

dummy: @value type = {
    name_: std::string;
    operator=: (out this, in name: std::string) = { name_ = name; }
}

Cppfront refuses to compile this code snippet and produces the following error message:

reproducer.cpp2: error: in operator=, expected 'name_ = ...' initialization statement (because type scope 
object 'name_' does not have a default initializer)
reproducer.cpp2(2,5): error: see declaration for 'name_' here
reproducer.cpp2: error: an operator= body must start with a series of 'member = value;' initialization statements 
for each of the type-scope objects in the same order they are declared, or the member must have a default 
initializer (in type 'dummy')

However, there is nothing wrong with the operator= here. The problem is actually that a value type must be default constructible, which is not the case in this example. This error message got me very confused at first, and I had to look into the sources to understand what was going on. Maybe the missing piece to get a better diagnostic is to require all data members of @value types to be declared with a default initializer. What do you think of this?
I'd be happy to submit a PR to fix this in the definition of the @value metaclass, but I'd rather make sure that my analysis is correct first.

Best regards,
Alex

[bluetarpmedia]

I'm definitely in favour of clear, friendly error messages. But instead of requiring all data members to have default initializers, I'd prefer the error to be something like: A value type must be default constructible. Then the user can choose to either add a default constructor or to add data member initializers.

[dutkalex]

Hi there!
I think I know how to resolve this problem, but I'm struggling with the actual implementation.

Basically, the root cause here is that the @basic_value metafunction adds a default constructor operator=: (out this) = { } if none is provided, even if such a constructor is not valid.

AFAIU, in order to tell if adding such a constructor is valid, one just has to iterate over all the member variables and check that either:

• a default initializer is provided, or
• if the type is defined in cpp2, it is default constructible, or
• if the type is defined in cpp1, defer somehow the check to the cpp1 compiler.

In terms of code, I don't know how to implement some parts of these checks:

basic_value: (inout t: meta::type_declaration) =
{
    t.copyable();

    for t.get_member_functions() do (inout mf) {
        mf.require( !mf.is_protected() && !mf.is_virtual(),
                    "a value type may not have a protected or virtual function" );
        mf.require( !mf.is_destructor() || mf.is_public() || mf.is_default_access(),
                    "a value type may not have a non-public destructor" );
    }

    if !t.has_default_ctor() {
        for t.get_member_objects() do (in mo) {
            mo.require( 
                mo.has_initializer() || /* if cpp2 type: */ mo./* get the type declaration */.has_default_ctor()
                                        /* else: defer check to cpp1 compiler using std::is_default_constructible */, 
                "a value type must be default constructible" 
            );
        }
        t.add_member( "operator=: (out this) = { }");
    }
}

If some of you have ideas on how to proceed, I'd be very interested to discuss!

[dutkalex]

For reference: #1259

[JohelEGP]

I strongly recommend reading #821.
See also #453.

@basic_value gives you a default constructor if you didn't define one.
See the difference in the error message when you comment out its definition (https://cpp2.godbolt.org/z/crnPzzK9r):

example.cpp2(3,3): error: in operator=, expected 'x = ...' initialization statement (because type scope object 'x' does not have a default initializer)
example.cpp2(2,3): error: see declaration for 'x' here
example.cpp2(3,3): error: an operator= body must start with a series of 'member = value;' initialization statements for each of the type-scope objects in the same order they are declared, or the member must have a default initializer (in type 'u')

example.cpp2: error: in operator=, expected 'x = ...' initialization statement (because type scope object 'x' does not have a default initializer)
example.cpp2(2,3): error: see declaration for 'x' here
example.cpp2: error: an operator= body must start with a series of 'member = value;' initialization statements for each of the type-scope objects in the same order they are declared, or the member must have a default initializer (in type 'u')

I suppose this ties back to #844.