Repository navigation
Trait solver fails to see trait is implemented for all values of const generic param #145133
Description
I tried this code:
trait RandomTrait
{
type Item;
}
struct BoolWrapper<const VAL : bool>;
impl RandomTrait for BoolWrapper<true> { type Item = String; }
impl RandomTrait for BoolWrapper<false> { type Item = i32; }
// RandomTrait is now implemented for all possible values
// of the constant generic parameter of BoolWrapper. There
// exists no bool X for which BoolWrapper<X> does not
// implement RandomTrait.
//
// And yet...
trait BoolWrapperTypeConstructor
{
type Of<const VAL : bool> : RandomTrait;
}
struct BoolWrapperFamily;
impl BoolWrapperTypeConstructor for BoolWrapperFamily {
type Of<const VAL : bool> = BoolWrapper<VAL>;
}I expected the code to compile, as RandomTrait is implemented for all 'flavors' of BoolWrapper.
Instead, the trait solver does not recognize that true and false are the only possible values a bool can have, and assumes that RandomTrait is not implemented for all possible types constructable by BoolWrapper.
This surprises me since a match expression can, in fact, recognize at compile-time whether all possible variants/values of a type are handled, in even more sophisticated ways than possible with const generics.
Meta
rustc --version --verbose:
rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-pc-windows-msvc
release: 1.86.0
LLVM version: 19.1.7
Activity
fmease commentedon Aug 8, 2025
I'm not sure if we have a r-l/r issue for this but this is basically a duplicate of / tracked in rust-lang/project-const-generics#26.
CC #104803, rust-lang/a-mir-formality#132.
fmease commentedon Aug 8, 2025
I'm inclined to close this as a duplicate of rust-lang/project-const-generics#26 but on the other hand it'd be nice if we had a corresp. r-l/r issue, wouldn't it?