fix #43960, evaluation order of splat inside ref (#44024)

(cherry picked from commit 546a774)

src/julia-syntax.scm

@@ -130,31 +130,29 @@
 ;; returns (values index-list stmts) where stmts are statements that need
 ;; to execute first.
 (define (process-indices a i)
-  (let loop ((lst i)
-             (n   1)
-             (stmts '())
-             (tuples '())
-             (ret '()))
-    (if (null? lst)
-        (values (reverse ret) (reverse stmts))
-        (let ((idx  (car lst))
-              (last (null? (cdr lst))))
-          (if (and (pair? idx) (eq? (car idx) '...))
-              (if (symbol-like? (cadr idx))
-                  (loop (cdr lst) (+ n 1)
-                        stmts
-                        (cons (cadr idx) tuples)
-                        (cons `(... ,(replace-beginend (cadr idx) a n tuples last))
-                              ret))
-                  (let ((g (make-ssavalue)))
-                    (loop (cdr lst) (+ n 1)
-                          (cons `(= ,g ,(replace-beginend (cadr idx) a n tuples last))
-                                stmts)
-                          (cons g tuples)
-                          (cons `(... ,g) ret))))
-              (loop (cdr lst) (+ n 1)
-                    stmts tuples
-                    (cons (replace-beginend idx a n tuples last) ret)))))))
+  (let ((has-va? (any vararg? i)))
+    (let loop ((lst i)
+               (n   1)
+               (stmts '())
+               (tuples '())
+               (ret '()))
+      (if (null? lst)
+          (values (reverse ret) (reverse stmts))
+          (let* ((idx0 (car lst))
+                 (idx  (if (vararg? idx0) (cadr idx0) idx0))
+                 (last (null? (cdr lst)))
+                 (replaced (replace-beginend idx a n tuples last))
+                 (idx      (if (or (not has-va?) (simple-atom? replaced)) replaced (make-ssavalue))))
+            (loop (cdr lst) (+ n 1)
+                  (if (eq? idx replaced)
+                      stmts
+                      (cons `(= ,idx ,replaced)
+                            stmts))
+                  (if (vararg? idx0) (cons idx tuples) tuples)
+                  (cons (if (vararg? idx0)
+                            `(... ,idx)
+                            idx)
+                        ret)))))))
 
 ;; GF method does not need to keep decl expressions on lambda args
 ;; except for rest arg

test/syntax.jl

@@ -1193,6 +1193,14 @@ end
 @test [(0,0)... 1] == [0 0 1]
 @test Float32[(0,0)... 1] == Float32[0 0 1]
 
+# issue #43960, evaluation order of splatting in `ref`
+let a = [], b = [4,3,2,1]
+    f() = (push!(a, 1); 2)
+    g() = (push!(a, 2); ())
+    @test b[f(), g()...] == 3
+    @test a == [1,2]
+end
+
 @testset "raw_str macro" begin
     @test raw"$" == "\$"
     @test raw"\n" == "\\n"