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

(cherry picked from commit 5f2abf6)

src/jltypes.c

@@ -1912,7 +1912,7 @@ 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);
     }
 
@@ -1922,7 +1922,7 @@ 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

@@ -239,6 +239,15 @@ 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}}