The map value is just a hint, so we don't panic in that case. In other words, there's something reasonable we can do and the program can continue (unlike the slice or chan situations). More discussion at #24308
I agree that the chan failure should have "runtime error:" prefixed, and probably be an errorString. I think we're allowed to change runtime errors without breaking go1 compatibility.

I see, thanks. Perhaps the fact that this is a hint as opposed to an actual size should also be documented in package builtin. The docs currently say (emphasis added):

Map: An empty map is allocated with enough space to hold the
specified number of elements. The size may be omitted, in which case
a small starting size is allocated.

The Spec calls the parameter “hint”, which is correct, I assume.

I think we decided in #14965 that we weren't going to add the "runtime error:" prefix to errors we were already generating, just to avoid breaking people that depended on the text. So I don't think there's anything to do here. Closing as unfortunate, but not worth fixing.

@randall77, should the documentation in package builtin mention that invalid hints are set to zero, or would that be unnecessary? If it should, I am ready to send a CL about that. If not, oh well.

I don't think it is necessary.

Map: An empty map is allocated with enough space to hold the specified number of elements.

That seems correct and complete. If you specify -3, then the map is allocated with enough space for -3 elements.

The "size" part in the next sentence (which you quoted above) is just the formal parameter name in the declaration.

If anything, we could document the panic behavior for the other kinds (slice, chan). But we generally don't get that formal (for instance, close of a nil channel panics, but we don't mention that in the builtin docs).