[Issue 11754] New: Disallow changing the default parameters of overridden inherited functions

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=11754

           Summary: Disallow changing the default parameters of overridden
                    inherited functions
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2013-12-16 14:58:11 PST ---
Perhaps it's a good idea to statically disallow code like this (returning a
compilation error), that changes the default parameters of overridden inherited
functions:


import std.stdio;
class Base {
    /*virtual*/ void foo(int i = 10) {
        writeln("Base.foo() ", i);
    }
}
class Derived: Base {
    override /*virtual*/ void foo(int i = 20) { // line 8
        writeln("Derived.foo() ", i);
    }
}
void main() {
    Base c1 = new Base;
    c1.foo();  // Output: Base.foo() 10
    Derived c2 = new Derived;
    c2.foo();  // Output: Derived.foo() 20
    Base c3 = new Derived;
    c3.foo(); // Output: Derived.foo() 10
}



The explanation:
http://www.gotw.ca/gotw/005.htm

> Like overloads, default parameters are taken from the static type (here Base) of the object,
> hence the default value of 10 is taken. However, the function happens to be virtual,
> and so the function actually called is based on the dynamic type (here Derived) of the object.


So I suggest that D code to give an error message like:

test.d(8): Error: changing default arguments of overridden inherited functions
is not allowed


Code like this is OK:

void foo(int i) {
...
override void foo(int i) {


Just as:

void foo(int i = 10) {
...
override void foo(int i = 10) {

------------------

Alternative design: just give a warning instead of an error.

------------------

Alternative design: the error message could be shown only if there is one
actual function call that doesn't specify the missing argument. This means in
this alternative design this main() doesn't generate the error message:

void main() {
    Base c1 = new Base;
    c1.foo(10);
    Derived c2 = new Derived;
    c2.foo(10);
    Base c3 = new Derived;
    c3.foo(10);
}

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