jltypes: ensure revising structs is safe (#51303)

(cherry picked from commit 1142759)

Author: vtjnash

src/jltypes.c

@@ -2491,6 +2491,8 @@ void jl_reinstantiate_inner_types(jl_datatype_t *t) // can throw!
 
     for (j = 0; j < jl_array_len(partial); j++) {
         jl_datatype_t *ndt = (jl_datatype_t*)jl_array_ptr_ref(partial, j);
+        if (ndt == NULL)
+            continue;
         assert(jl_unwrap_unionall(ndt->name->wrapper) == (jl_value_t*)t);
         for (i = 0; i < n; i++)
             env[i].val = jl_svecref(ndt->parameters, i);
@@ -2502,6 +2504,8 @@ void jl_reinstantiate_inner_types(jl_datatype_t *t) // can throw!
     if (t->types != jl_emptysvec) {
         for (j = 0; j < jl_array_len(partial); j++) {
             jl_datatype_t *ndt = (jl_datatype_t*)jl_array_ptr_ref(partial, j);
+            if (ndt == NULL)
+                continue;
             for (i = 0; i < n; i++)
                 env[i].val = jl_svecref(ndt->parameters, i);
             assert(ndt->types == NULL);
@@ -2510,7 +2514,9 @@ void jl_reinstantiate_inner_types(jl_datatype_t *t) // can throw!
             if (ndt->isconcretetype) { // cacheable
                 jl_compute_field_offsets(ndt);
             }
+            jl_array_ptr_set(partial, j, NULL);
         }
+        t->name->partial = NULL;
     }
     else {
         assert(jl_field_names(t) == jl_emptysvec);

test/core.jl

@@ -7544,6 +7544,19 @@ end
 struct T36104   # check that redefining it works, issue #21816
     v::Vector{T36104}
 end
+struct S36104{K,V}
+    v::S36104{K,V}
+    S36104{K,V}() where {K,V} = new()
+    S36104{K,V}(x::S36104) where {K,V} = new(x)
+end
+@test !isdefined(Base.unwrap_unionall(Base.ImmutableDict).name, :partial)
+@test !isdefined(S36104.body.body.name, :partial)
+@test hasfield(typeof(S36104.body.body.name), :partial)
+struct S36104{K,V}   # check that redefining it works
+    v::S36104{K,V}
+    S36104{K,V}() where {K,V} = new()
+    S36104{K,V}(x::S36104) where {K,V} = new(x)
+end
 # with a gensymmed unionall
 struct Symmetric{T,S<:AbstractMatrix{<:T}} <: AbstractMatrix{T}
     data::S