On Mon, Dec 18, 2023 at 5:32 PM Ian Lance Taylor <iant@golang.org> wrote:
>
> This Go frontend patch moves the lowering pass after the type
> determination and the type checking passes.  This lets us simplify
> some of the code that determines the type of an expression, which
> previously had to work correctly both before and after type
> determination.
>
> I'm doing this to help with future generic support.  For example, with
> generics, we can see code like
>
>     func ident[T any](v T) T { return v }
>
>     func F() int32 {
>         s := int32(1)
>         return ident(s)
>     }
>
> Before this change, we would type check return statements in the
> lowering pass (see Return_statement::do_lower).  With a generic
> example like the above, that means we have to determine the type of s,
> and use that to infer the type arguments passed to ident, and use that
> to determine the result type of ident.  That is too much to do at
> lowering time.  Of course we can change the way that return statements
> work, but similar issues arise with index expressions, the types of
> closures for function literals, and probably other cases as well.
>
> Rather than try to deal with all those cases, we move the lowering
> pass after type checking.  This requires a bunch of changes, notably
> for determining constant types.  We have to add type checking for
> various constructs that formerly disappeared in the lowering pass. So
> it's a lot of shuffling.  Sorry for the size of the patch.
>
> Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
> to mainline.

Sorry, I forgot to commit the changes to some of the test files.  I've
committed this patch to fix them.  This fixes PR 113447.

Ian