fix missing field type initialization vars (#44797)

We were accidentally passing the start of the list instead of the end of
the list, resulting in some values passing through uninitialized.

Fix #42297 regression

src/jltypes.c

@@ -1934,7 +1934,7 @@ void jl_reinstantiate_inner_types(jl_datatype_t *t) // can throw!
  for (i = 0; i < n; i++)
  env[i].val = jl_svecref(ndt->parameters, i);
 
- ndt->super = (jl_datatype_t*)inst_type_w_((jl_value_t*)t->super, env, &top, 1);
+ ndt->super = (jl_datatype_t*)inst_type_w_((jl_value_t*)t->super, &env[n - 1], &top, 1);
  jl_gc_wb(ndt, ndt->super);
  }
 
@@ -1944,7 +1944,7 @@ void jl_reinstantiate_inner_types(jl_datatype_t *t) // can throw!
  for (i = 0; i < n; i++)
  env[i].val = jl_svecref(ndt->parameters, i);
  assert(ndt->types == NULL);
- ndt->types = inst_ftypes(t->types, env, &top);
+ ndt->types = inst_ftypes(t->types, &env[n - 1], &top);
  jl_gc_wb(ndt, ndt->types);
  if (ndt->isconcretetype) { // cacheable
  jl_compute_field_offsets(ndt);

test/core.jl

@@ -337,6 +337,15 @@ end
 #struct S22624{A,B,C} <: Ref{S22624{Int64,A}}; end
 @test_broken @isdefined S22624
 
+# issue #42297
+mutable struct Node42297{T, V}
+ value::V
+ next::Union{Node42297{T, T}, Node42297{T, Val{T}}, Nothing}
+ Node42297{T}(value) where {T} = new{T, typeof(value)}(value, nothing)
+end
+@test fieldtype(Node42297{Int,Val{Int}}, 1) === Val{Int}
+@test fieldtype(Node42297{Int,Int}, 1) === Int
+
 # issue #3890
 mutable struct A3890{T1}
  x::Matrix{Complex{T1}}