Description
As a regular user of the Swift programming language, I get frustrated when I see diagnostics for the following cases dealing with cast destination types:
let _ = 1 as Int16 < 7let _ = 1 as Int16 << 7
In both cases, I see error: expected type diagnostics, even though the context of the expression does not imply any presence of the type after <.
Proposed Solution:
I would like to see more helpful diagnostics. For example, here is what Rust does in the same situation:
error: `<` is interpreted as a start of generic arguments for `i16`, not a shift
--> <source>:3:16
|
3 | num as i16 << 7
| ^^ - interpreted as generic arguments
| |
| not interpreted as shift
|
help: try shifting the cast value
|
3 | (num as i16) << 7
| + +
Alternatives
The alternative would be silencing the expected error diagnostics, and this was already attempted in my PR #60088. However, @CodaFi provided a great feedback which says,
This embeds a form of unbounded lookahead in the Swift type grammar. We should be careful committing to additional lookahead like this to disambiguate parses. This is strictly a change to the language and should probably be discussed on evolution.
I tend to agree with that, so therefore I think it's best to change the diagnostics in this case instead of proceeding with the alternative solution in my PR.