[xscott]

I see your point, and it's a nice example, but not completely parallel to the Rust/StandardML thing. Here, your RHS is an initializer, not a value.

    // I don't think this flies in C or C++,
    // even with "designated initializers":
    f({ .flag = true, .value = 123, .stuff=0.456});

    // Both of these "probably" do work:
    f((some_struct){ .flag = true, ... });
    f(some_struct{ .flag = true, ... });

    // So this should work too:
    auto a = (some_struct){ .flag = true, ... };

Take all that with a grain of salt. I didn't try to compile any of it for this reply.

Anyways, I only touched SML briefly 30 some years ago, and my reaction to this level of type inference sophistication in Rust went through phases of initial astonishment, quickly embracing it, and eventually being annoyed at it. Just like data flows from expressions calculating values, I like it when the type inference flows in similarly obvious ways.