Fix off-by-one in generator nospecialize

I had an off-by-one in #50556, since the argument slots actually start
at 2 and `iota` starts at `0`. This was breaking StaticArrays precompiles,
which attempts to precompile a generator with its abstract signature and
without the nospecialize, those signatures are not compileable.

Author: Keno

src/julia-syntax.scm

@@ -296,7 +296,8 @@
                                   (if (eq? n '|#self#|) (gensy) n))
                                 arg-names))))
     (let ((body (insert-after-meta body  ;; don't specialize on generator arguments
-                                   `((meta nospecialize ,@(map (lambda (idx) `(slot ,(+ idx 1))) (iota (length arg-names))))))))
+                                   ;; arg-names slots start at 2 (after name)
+                                   `((meta nospecialize ,@(map (lambda (idx) `(slot ,(+ idx 2))) (iota (length arg-names))))))))
       `(block
         (global ,name)
         (function (call ,name ,@arg-names) ,body)))))

test/precompile.jl

@@ -1771,6 +1771,20 @@ precompile_test_harness("Issue #48391") do load_path
     @test_throws ErrorException isless(x, x)
 end
 
+precompile_test_harness("Generator nospecialize") do load_path
+    write(joinpath(load_path, "GenNoSpec.jl"),
+        """
+        module GenNoSpec
+        @generated function f(x...)
+            :((\$(Base.Meta.quot(x)),))
+        end
+        @assert precompile(Tuple{typeof(which(f, (Any,Any)).generator.gen), Any, Any})
+        end
+        """)
+    ji, ofile = Base.compilecache(Base.PkgId("GenNoSpec"))
+    @eval using GenNoSpec
+end
+
 empty!(Base.DEPOT_PATH)
 append!(Base.DEPOT_PATH, original_depot_path)
 empty!(Base.LOAD_PATH)