From 8bf99cbbde9cb01681f980b527d06abc3fe22d95 Mon Sep 17 00:00:00 2001
From: "Steven G. Johnson" <stevenj@mit.edu>
Date: Thu, 21 Jul 2016 19:27:44 -0400
Subject: [PATCH] make x[...] .= ... assign in-place (fixes bug in #17510)

---
 doc/manual/functions.rst |  4 ++++
 src/julia-syntax.scm     | 16 +++++++++++++---
 test/broadcast.jl        |  8 ++++++++
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/doc/manual/functions.rst b/doc/manual/functions.rst
index e898f663201ef..1a97ca273e989 100644
--- a/doc/manual/functions.rst
+++ b/doc/manual/functions.rst
@@ -660,6 +660,10 @@ calls do not allocate new arrays over and over again for the results
 except that, as above, the ``broadcast!`` loop is fused with any nested
 "dot" calls.  For example, ``X .= sin.(Y)`` is equivalent to
 ``broadcast!(sin, X, Y)``, overwriting ``X`` with ``sin.(Y)`` in-place.
+If the left-hand side is a ``getindex`` expression, e.g.
+``X[2:end] .= sin(Y)``, then it translates to ``broadcast!`` on a ``view``,
+e.g. ``broadcast!(sin, view(X, 2:length(X)), Y)``, so that the left-hand
+side is updated in-place.
 
 (In future versions of Julia, operators like ``.*`` will also be handled with
 the same mechanism: they will be equivalent to ``broadcast`` calls and
diff --git a/src/julia-syntax.scm b/src/julia-syntax.scm
index c08d48843e963..d1b7da8301c7b 100644
--- a/src/julia-syntax.scm
+++ b/src/julia-syntax.scm
@@ -1542,6 +1542,15 @@
         (cadr expr)  ;; eta reduce `x->f(x)` => `f`
         `(-> ,argname (block ,@splat ,expr)))))
 
+(define (ref-to-view expr)
+  (if (and (pair? expr) (eq? (car expr) 'ref))
+      (let* ((ex (partially-expand-ref expr))
+             (stmts (butlast (cdr ex)))
+             (refex (last    (cdr ex)))
+             (nuref `(call view ,(caddr refex) ,@(cdddr refex))))
+        `(block ,@stmts ,nuref))
+      expr))
+
 ; fuse nested calls to expr == f.(args...) into a single broadcast call,
 ; or a broadcast! call if lhs is non-null.
 (define (expand-fuse-broadcast lhs rhs)
@@ -1657,14 +1666,15 @@
                         (cons farg new-fargs) (cons arg new-args) renames varfarg vararg))))))
           (cf (cdadr f) args '() '() '() '() '()))
         e)) ; (not (fuse? e))
-  (let ((e (compress-fuse (dot-to-fuse rhs)))) ; an expression '(fuse func args) if expr is a dot call
+  (let ((e (compress-fuse (dot-to-fuse rhs))) ; an expression '(fuse func args) if expr is a dot call
+        (lhs_ (ref-to-view lhs))) ; x[...] expressions on lhs turn in to view(x, ...) to update x in-place
     (if (fuse? e)
         (if (null? lhs)
             (expand-forms `(call broadcast ,(from-lambda (cadr e)) ,@(caddr e)))
-            (expand-forms `(call broadcast! ,(from-lambda (cadr e)) ,lhs ,@(caddr e))))
+            (expand-forms `(call broadcast! ,(from-lambda (cadr e)) ,lhs_ ,@(caddr e))))
         (if (null? lhs)
             (expand-forms e)
-            (expand-forms `(call broadcast! identity ,lhs ,e))))))
+            (expand-forms `(call broadcast! identity ,lhs_ ,e))))))
 
 ;; table mapping expression head to a function expanding that form
 (define expand-table
diff --git a/test/broadcast.jl b/test/broadcast.jl
index 31a2166a4ee14..52aeacc859041 100644
--- a/test/broadcast.jl
+++ b/test/broadcast.jl
@@ -265,6 +265,14 @@ let x = [1:4;], y = x
     @test y === x == [8,8,8,8]
     y .-= 1:4
     @test y === x == [7,6,5,4]
+    x[1:2] .= 1
+    @test y === x == [1,1,5,4]
+    x[1:2] .+= [2,3]
+    @test y === x == [3,4,5,4]
+    x[:] .= 0
+    @test y === x == [0,0,0,0]
+    x[2:end] .= 1:3
+    @test y === x == [0,1,2,3]
 end
 
 # PR 16988