[Issue 21285] New: Delegate covariance broken

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21285

          Issue ID: 21285
           Summary: Delegate covariance broken
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: regression
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: hsteoh at quickfur.ath.cx

Reduced code:
--------
void main()
{
    string path;
    int bank;
    auto aa = [
        "path": (string arg) {
            path = arg;
        },

        "bank": (string arg) {
            bank = 1;
            throw new Exception("");
        },
    ];
}
--------

DMD 2.094 output:
--------
/tmp/test.d(6): Error: cannot implicitly convert expression __lambda1 of type
void delegate(string arg) pure nothrow @nogc @safe to void delegate(string)
pure @safe
--------

Surely `void delegate(string) pure nothrow @nogc @safe` is implicitly
convertible to `void delegate(string) pure @safe`?  Since nothrow is covariant
to (implicit) throwing, and @nogc is covariant to (implicit) allocating. In any
case, it's a bug that the compiler is unable to find a common value type for
the above AA literal, since the common type `void delegate(string)` exists and
works for both cases.

Note that the same problem persists even if the AA's type is explicitly spelled
out `void delegate(string)[string]`, so this is not directly related to type
deduction of the AA, but rather is a failure to find a common type between the
two values of the AA literal.


This code successfully compiled with DMD 2.092, so this represents a recent
regression.

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