Feature/forced cast

[schorg]

Forced cast 'as!'.

[FriedrichGretz]

The semantics (or error messages) are not yet fully clear to me.

let c: int16 = 4200
var i2 = c as! int8

generate C code

(*blc_main_c) = 4200;
(*blc_main_i2) = (blc_int8)(*blc_main_c);

but making c a constant

const c: int16 = 4200
var i2 = c as! int8

produces an error

error: Value '4200' cannot be represented in type 'int8'.
  --> .\forcedCast.blc:6:14 [typing]

6  |     var i2 = c as! int8
   |              ^^^^^^^^^^ conversion not possible
6  |     var i2 = c as! int8
   |              - value not in 'int8'

Is this error meant to be an additional safety feature? What is not clear to me is what are the capabilities of as! and what its guarantees.

[schorg]

Semantics of 'as!'

Debug code: Traps if the value does not 'fit' into the target type, runtime check necessary. Release code: Unsafe behavior or saturation (still to be decided) Constant evaluation: The runtime check for debug code is evaluated at compile time.

[FriedrichGretz]

Since there is not debug/release code as of now, it means "unsafe behaviour" (or C defined behaviour) unless the value being converted is a compile time constant. In particular for all primitive types X and Y, we can as!-cast X into Y. Correct?

[schorg]

Even worse: Undefined behavior like in C

[FriedrichGretz]

I have briefly tested the code and it generates C code as expected. Do you plan to add more unit tests to this PR? For example, ensuring that complex types are not converted. Or code generation tests that ensure that particular values are the result of a cast?

You can merge your PR at your discretion. Do not forget to squash commits into a signed one.

[schorg]

Typechecking still needs more rework. Could be done then.