fix #21923, regressions in circular type definitions

src/jltypes.c

@@ -1343,10 +1343,7 @@ static jl_value_t *inst_type_w_(jl_value_t *t, jl_typeenv_t *env, jl_typestack_t
         }
         jl_typeenv_t newenv = { ua->var, (jl_value_t*)((jl_unionall_t*)res)->var, env };
         jl_value_t *newbody = inst_type_w_(ua->body, &newenv, &top, check);
-        if (newbody == ua->body) {
-            res = t;
-        }
-        else if (newbody == (jl_value_t*)jl_emptytuple_type) {
+        if (newbody == (jl_value_t*)jl_emptytuple_type) {
             // NTuple{0} => Tuple{} can make a typevar disappear
             res = (jl_value_t*)jl_emptytuple_type;
         }

src/julia-syntax.scm

@@ -3315,7 +3315,7 @@ f(x) = yt(x)
         (set-car! (lam:vinfo lam) (append (car (lam:vinfo lam)) `((,g Any 2))))
         g))
     ;; evaluate the arguments of a call, creating temporary locations as needed
-    (define (compile-args lst break-labels)
+    (define (compile-args lst break-labels (linearize #t))
       (if (null? lst) '()
           (let ((temps? (or *very-linear-mode*
                             (any (lambda (e)
@@ -3332,9 +3332,9 @@ f(x) = yt(x)
               (if (null? lst)
                   (reverse! vals)
                   (let* ((arg (car lst))
-                         (aval (compile arg break-labels #t #f)))
+                         (aval (compile arg break-labels #t #f linearize)))
                     (loop (cdr lst)
-                          (cons (if (and temps? (not simple?)
+                          (cons (if (and temps? linearize (not simple?)
                                          (not (simple-atom? arg))  (not (ssavalue? arg))
                                          (not (simple-atom? aval)) (not (ssavalue? aval))
                                          (not (and (pair? arg)
@@ -3361,7 +3361,7 @@ f(x) = yt(x)
     ;; value must be returned.
     ;; `tail` means we are in tail position, where a value needs to be `return`ed
     ;; from the current function.
-    (define (compile e break-labels value tail)
+    (define (compile e break-labels value tail (linearize-args #t))
       (if (or (not (pair? e)) (memq (car e) '(null ssavalue quote inert top core copyast the_exception $
                                                    globalref outerref cdecl stdcall fastcall thiscall llvmcall)))
           (let ((e1 (if (and arg-map (symbol? e))
@@ -3386,11 +3386,11 @@ f(x) = yt(x)
                               ;; NOTE: 2nd and 3rd arguments of ccall must be left in place
                               ;;       the 1st should be compiled if an atom.
                               (append (list)
-                                      (cond (atom? (cadr e) (compile-args (list (cadr e)) break-labels))
+                                      (cond (atom? (cadr e) (compile-args (list (cadr e)) break-labels linearize-args))
                                             (else (cadr e)))
                                       (list-head (cddr e) 2)
-                                      (compile-args (list-tail e 4) break-labels))
-                              (compile-args (cdr e) break-labels)))
+                                      (compile-args (list-tail e 4) break-labels linearize-args))
+                              (compile-args (cdr e) break-labels linearize-args)))
                     (callex (cons (car e) args)))
                (cond (tail (emit-return callex))
                      (value callex)
@@ -3602,7 +3602,11 @@ f(x) = yt(x)
                ((composite_type)
                 (let* ((para (compile (caddr e) break-labels #t #f))
                        (supe (compile (list-ref e 4) break-labels #t #f))
-                       (ftys (compile (list-ref e 5) break-labels #t #f)))
+                       ;; composite_type has an unconventional evaluation rule that
+                       ;; needs to do work around the evaluation of the field types,
+                       ;; so the field type expressions need to be kept in place as
+                       ;; much as possible. (part of issue #21923)
+                       (ftys (compile (list-ref e 5) break-labels #t #f #f)))
                   (emit `(composite_type ,(cadr e) ,para ,(cadddr e) ,supe ,ftys ,@(list-tail e 6)))))
                (else
                 (emit e)))

test/core.jl

@@ -245,6 +245,16 @@ abstract type Sup2a_ end
 abstract type Sup2b_{A <: Sup2a_, B} <: Sup2a_ end
 @test_throws ErrorException @eval abstract type Qux2_{T} <: Sup2b_{Qux2_{Int}, T} end # wrapped in eval to avoid #16793
 
+# issue #21923
+struct A21923{T,N}; v::Vector{A21923{T}}; end
+@test fieldtype(A21923,1) == Vector{A21923{T}} where T
+struct B21923{T,N}; v::Vector{B21923{T,M} where M}; end
+@test fieldtype(B21923, 1) == Vector{B21923{T,M} where M} where T
+struct C21923{T,N}; v::C21923{T,M} where M; end
+@test fieldtype(C21923, 1) == C21923
+struct D21923{T,N}; v::D21923{T}; end
+@test fieldtype(D21923, 1) == D21923
+
 # issue #3890
 mutable struct A3890{T1}
     x::Matrix{Complex{T1}}