Allow exact redefinition for types with recursive supertype reference (…

…JuliaLang#55380)

This PR allows redefining a type when the new type is exactly identical
to the previous one (like JuliaLang#17618, JuliaLang#20592 and JuliaLang#21024), even if the type
has a reference to itself in its supertype. That particular case used to
error (issue JuliaLang#54757), whereas with this PR:
```julia
julia> struct Rec <: AbstractVector{Rec} end

julia> struct Rec <: AbstractVector{Rec} end # this used to error

julia>
```


Fix JuliaLang#54757 by implementing the solution proposed there. Hence, this
should also fix downstream Revise bug
timholy/Revise.jl#813.

---------

Co-authored-by: N5N3 <[email protected]>

src/builtins.c

@@ -2197,6 +2197,9 @@ static int equiv_type(jl_value_t *ta, jl_value_t *tb)
     JL_GC_PUSH2(&a, &b);
     a = jl_rewrap_unionall((jl_value_t*)dta->super, dta->name->wrapper);
     b = jl_rewrap_unionall((jl_value_t*)dtb->super, dtb->name->wrapper);
+    // if tb recursively refers to itself in its supertype, assume that it refers to ta
+    // before checking whether the supertypes are equal
+    b = jl_substitute_datatype(b, dtb, dta);
     if (!jl_types_equal(a, b))
         goto no;
     JL_TRY {

src/jltypes.c

@@ -1607,6 +1607,118 @@ jl_value_t *jl_rewrap_unionall_(jl_value_t *t, jl_value_t *u)
     return t;
 }
 
+// Create a copy of type expression t where any occurrence of data type x is replaced by y.
+// If x does not occur in t, return t without any copy.
+// For example, jl_substitute_datatype(Foo{Bar}, Foo{T}, Qux{S}) is Qux{Bar}, with T and S
+// free type variables.
+// To substitute type variables, use jl_substitute_var instead.
+jl_value_t *jl_substitute_datatype(jl_value_t *t, jl_datatype_t * x, jl_datatype_t * y)
+{
+    if jl_is_datatype(t) {
+        jl_datatype_t *typ = (jl_datatype_t*)t;
+        // For datatypes call itself recursively on the parameters to form new parameters.
+        // Then, if typename(t) == typename(x), rewrap the wrapper of y around the new
+        // parameters. Otherwise, do the same around the wrapper of t.
+        // This ensures that the types and supertype are properly set.
+        // Start by check whether there is a parameter that needs replacing.
+        long i_firstnewparam = -1;
+        size_t nparams = jl_svec_len(typ->parameters);
+        jl_value_t *firstnewparam = NULL;
+        JL_GC_PUSH1(&firstnewparam);
+        for (size_t i = 0; i < nparams; i++) {
+            jl_value_t *param = NULL;
+            JL_GC_PUSH1(&param);
+            param = jl_svecref(typ->parameters, i);
+            firstnewparam = jl_substitute_datatype(param, x, y);
+            if (param != firstnewparam) {
+                i_firstnewparam = i;
+                JL_GC_POP();
+                break;
+            }
+            JL_GC_POP();
+        }
+        // If one of the parameters needs to be updated, or if the type name is that to
+        // substitute, create a new datataype
+        if (i_firstnewparam != -1 || typ->name == x->name) {
+            jl_datatype_t *uw = typ->name == x->name ? y : typ; // substitution occurs here
+            jl_value_t *wrapper = uw->name->wrapper;
+            jl_datatype_t *w = (jl_datatype_t*)jl_unwrap_unionall(wrapper);
+            jl_svec_t *sv = jl_alloc_svec_uninit(jl_svec_len(uw->parameters));
+            JL_GC_PUSH1(&sv);
+            jl_value_t **vals = jl_svec_data(sv);
+            // no JL_GC_PUSHARGS(vals, ...) since GC is already aware of sv
+            for (long i = 0; i < i_firstnewparam; i++) { // copy the identical parameters
+                vals[i] = jl_svecref(typ->parameters, i); // value
+            }
+            if (i_firstnewparam != -1) { // insert the first non-identical parameter
+                vals[i_firstnewparam] = firstnewparam;
+            }
+            for (size_t i = i_firstnewparam+1; i < nparams; i++) { // insert the remaining parameters
+                vals[i] = jl_substitute_datatype(jl_svecref(typ->parameters, i), x, y);
+            }
+            if (jl_is_tuple_type(wrapper)) {
+                // special case for tuples, since the wrapper (Tuple) does not have as
+                // many parameters as t (it only has a Vararg instead).
+                t = jl_apply_tuple_type(sv, 0);
+            } else {
+                t = jl_instantiate_type_in_env((jl_value_t*)w, (jl_unionall_t*)wrapper, vals);
+            }
+            JL_GC_POP();
+        }
+        JL_GC_POP();
+    }
+    else if jl_is_unionall(t) { // recursively call itself on body and var bounds
+        jl_unionall_t* ut = (jl_unionall_t*)t;
+        jl_value_t *lb = NULL;
+        jl_value_t *ub = NULL;
+        jl_value_t *body = NULL;
+        JL_GC_PUSH3(&lb, &ub, &body);
+        lb = jl_substitute_datatype(ut->var->lb, x, y);
+        ub = jl_substitute_datatype(ut->var->ub, x, y);
+        body = jl_substitute_datatype(ut->body, x, y);
+        if (lb != ut->var->lb || ub != ut->var->ub) {
+            jl_tvar_t *newtvar = jl_new_typevar(ut->var->name, lb, ub);
+            JL_GC_PUSH1(&newtvar);
+            body = jl_substitute_var(body, ut->var, (jl_value_t*)newtvar);
+            t = jl_new_struct(jl_unionall_type, newtvar, body);
+            JL_GC_POP();
+        }
+        else if (body != ut->body) {
+            t = jl_new_struct(jl_unionall_type, ut->var, body);
+        }
+        JL_GC_POP();
+    }
+    else if jl_is_uniontype(t) { // recursively call itself on a and b
+        jl_uniontype_t *u = (jl_uniontype_t*)t;
+        jl_value_t *a = NULL;
+        jl_value_t *b = NULL;
+        JL_GC_PUSH2(&a, &b);
+        a = jl_substitute_datatype(u->a, x, y);
+        b = jl_substitute_datatype(u->b, x, y);
+        if (a != u->a || b != u->b) {
+            t = jl_new_struct(jl_uniontype_type, a, b);
+        }
+        JL_GC_POP();
+    }
+    else if jl_is_vararg(t) { // recursively call itself on T
+        jl_vararg_t *vt = (jl_vararg_t*)t;
+        if (vt->T) { // vt->T could be NULL
+            jl_value_t *rT = NULL;
+            JL_GC_PUSH1(&rT);
+            rT = jl_substitute_datatype(vt->T, x, y);
+            if (rT != vt->T) {
+                jl_task_t *ct = jl_current_task;
+                t = jl_gc_alloc(ct->ptls, sizeof(jl_vararg_t), jl_vararg_type);
+                jl_set_typetagof((jl_vararg_t *)t, jl_vararg_tag, 0);
+                ((jl_vararg_t *)t)->T = rT;
+                ((jl_vararg_t *)t)->N = vt->N;
+            }
+            JL_GC_POP();
+        }
+    }
+    return t;
+}
+
 static jl_value_t *lookup_type_stack(jl_typestack_t *stack, jl_datatype_t *tt, size_t ntp,
                                      jl_value_t **iparams)
 {

src/julia_internal.h

@@ -769,6 +769,7 @@ jl_unionall_t *jl_rename_unionall(jl_unionall_t *u);
 JL_DLLEXPORT jl_value_t *jl_unwrap_unionall(jl_value_t *v JL_PROPAGATES_ROOT) JL_NOTSAFEPOINT;
 JL_DLLEXPORT jl_value_t *jl_rewrap_unionall(jl_value_t *t, jl_value_t *u);
 JL_DLLEXPORT jl_value_t *jl_rewrap_unionall_(jl_value_t *t, jl_value_t *u);
+jl_value_t* jl_substitute_datatype(jl_value_t *t, jl_datatype_t * x, jl_datatype_t * y);
 int jl_count_union_components(jl_value_t *v);
 JL_DLLEXPORT jl_value_t *jl_nth_union_component(jl_value_t *v JL_PROPAGATES_ROOT, int i) JL_NOTSAFEPOINT;
 int jl_find_union_component(jl_value_t *haystack, jl_value_t *needle, unsigned *nth) JL_NOTSAFEPOINT;

test/core.jl

@@ -5611,6 +5611,26 @@ end
     x::Array{T} where T<:Integer
 end
 
+# issue #54757, type redefinitions with recursive reference in supertype
+struct T54757{A>:Int,N} <: AbstractArray{Tuple{X,Tuple{Vararg},Union{T54757{Union{X,Integer}},T54757{A,N}},Vararg{Y,N}} where {X,Y<:T54757}, N}
+    x::A
+    y::Union{A,T54757{A,N}}
+    z::T54757{A}
+end
+
+struct T54757{A>:Int,N} <: AbstractArray{Tuple{X,Tuple{Vararg},Union{T54757{Union{X,Integer}},T54757{A,N}},Vararg{Y,N}} where {X,Y<:T54757}, N}
+    x::A
+    y::Union{A,T54757{A,N}}
+    z::T54757{A}
+end
+
+@test_throws ErrorException struct T54757{A>:Int,N} <: AbstractArray{Tuple{X,Tuple{Vararg},Union{T54757{Union{X,Integer}},T54757{A}},Vararg{Y,N}} where {X,Y<:T54757}, N}
+    x::A
+    y::Union{A,T54757{A,N}}
+    z::T54757{A}
+end
+
+
 let a = Vector{Core.TypeofBottom}(undef, 2)
     @test a[1] == Union{}
     @test a == [Union{}, Union{}]