Fix issue with uncurried function with 1 arg being a variable

Fixes #6504

Uncurried functions with 1 unit argument are emitted without the parameter. This leaves a possible undefined id in the emitted code if the parameter is a variable.
This PR limits the optimization to cases where the parameter is explicitly `()`, as opposed to e.g. a variable whose type is inferred to be unit.

Author: cristianoc

CHANGELOG.md

@@ -19,6 +19,7 @@
 
 #### :bug: Bug Fix
 - Fix issue where an inline record with attributes did not parse. https://github.com/rescript-lang/rescript-compiler/pull/6499
+- Fix issue with uncurried function with 1 arg being a variable where an undefined variable could be emitted. https://github.com/rescript-lang/rescript-compiler/pull/6507
 
 # 11.0.0-rc.6
 

jscomp/core/lam_pass_alpha_conversion.ml

@@ -71,7 +71,7 @@ let alpha_conversion (meta : Lam_stats.t) (lam : Lam.t) : Lam.t =
         | None -> Lam.prim ~primitive ~args:[ simpl arg ] loc)
     | Lprim { primitive = Pjs_fn_make_unit; args = [ arg ]; loc } ->
       let arg = match arg with
-      | Lfunction ({arity=1; params=[x]; attr; body}) ->
+      | Lfunction ({arity=1; params=[x]; attr; body}) when Ident.name x = "param" (* "()" *) ->
         Lam.function_ ~params:[x] ~attr:{attr with oneUnitArg=true} ~body ~arity:1
       | _ -> arg in
       simpl arg

jscomp/test/UncurriedAlways.js

@@ -65,3 +65,13 @@ module OptMixed = {
   let c3 = ptl(~x="x", ~z="z", ~d2="d2<-200", ~d4="d4<-400")
   Js.log2("c3:", c3)
 }
+
+let fn = cb => {
+  cb()
+}
+
+fn(s => Js.log(#foo(s)))
+
+let fn1 = (a, b, ()) => a() + b 
+
+let a = fn1(() => 1, 2, _)