[Issue 24828] New: Generic parameter type is constrained by the type of the default value

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24828

          Issue ID: 24828
           Summary: Generic parameter type is constrained by the type of
                    the default value
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: maxsamukha at gmail.com

I've been hit by this regularly, having to fallback to overloading every time:

void foo(T)(T v = 0)
{
}

void main()
{
    foo(); // ok
    foo(1); // ok
    foo("x"); // fail
}

Error: cannot implicitly convert expression `0` of type `int` to `string`.

For some reason, the compiler attempts to convert the default value to the type
of the argument and fails. This behavior renders default values of generically
typed parameters completely unusable.

Compare to C++, which does the right thing:

template<typename T = int> void foo(T a = 0)
{
} 

int main()
{
    foo();
    foo(1);
    foo("x");
    return 0;
}

Note that C++ also requires a default type, while it shouldn't - the type can
be infered from the type of the default value. Anyway, adding the default type
in D doesn't help.

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