prevent allocation of Memory (layout and object) when not concrete (#58064)

Fix #54969

Author: vtjnash

src/datatype.c

@@ -500,7 +500,13 @@ void jl_get_genericmemory_layout(jl_datatype_t *st)
     jl_value_t *kind = jl_tparam0(st);
     jl_value_t *eltype = jl_tparam1(st);
     jl_value_t *addrspace = jl_tparam2(st);
-    if (!jl_is_typevar(eltype) && !jl_is_type(eltype)) {
+    if (!st->isconcretetype) {
+        // Since parent dt has an opaque layout, we may end up here being asked to copy that layout to subtypes,
+        // but we don't actually want to do that unless this object is constructable (or at least has a layout).
+        // The real layout is stored only on the wrapper.
+        return;
+    }
+    if (!jl_is_type(eltype)) {
         // this is expected to have a layout, but since it is not constructable, we don't care too much what it is
         static const jl_datatype_layout_t opaque_ptr_layout = {0, 0, 1, -1, sizeof(void*), {0}};
         st->layout = &opaque_ptr_layout;

src/julia.h

@@ -1491,7 +1491,7 @@ JL_DLLEXPORT jl_value_t *jl_unwrap_unionall(jl_value_t *v JL_PROPAGATES_ROOT) JL
 #define jl_inlinedatatype_layout(t) (((jl_datatype_t*)t)->layout)
 STATIC_INLINE const jl_datatype_layout_t *jl_datatype_layout(jl_datatype_t *t) JL_NOTSAFEPOINT
 {
-    if (jl_is_layout_opaque(t->layout)) // e.g. GenericMemory
+    if (t->layout == NULL || jl_is_layout_opaque(t->layout)) // e.g. GenericMemory
         t = (jl_datatype_t*)jl_unwrap_unionall(t->name->wrapper);
     return t->layout;
 }

test/core.jl

@@ -4951,6 +4951,9 @@ let ft = Base.datatype_fieldtypes
     @test !isdefined(ft(B12238.body.body)[1], :instance)  # has free type vars
 end
 
+# issue #54969
+@test !isdefined(Memory.body, :instance)
+
 # `where` syntax in constructor definitions
 (A12238{T} where T<:Real)(x) = 0
 @test A12238{<:Real}(0) == 0