add @compat for .=

README.md

@@ -62,6 +62,11 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * `@compat Nullable(value, hasvalue)` to handle the switch from the `Nullable` `:isnull` field to `:hasvalue` field. [#18510](https://github.com/JuliaLang/julia/pull/18510)
 
+* `@compat x .= y` converts to an in-place assignment to `x` (via `broadcast!`) [#17510](https://github.com/JuliaLang/julia/pull/17510).
+  However, beware that `.=` in Julia 0.4 has the precedence of `==`, not of assignment `=`, so if the right-hand-side `y`
+  includes expressions with lower precedence than `==` you should enclose it in parentheses `x .= (y)` to ensure the
+  correct order of evaluation.
+
 ## Type Aliases
 
 * `String` has undergone multiple changes: in Julia 0.3 it was an abstract type and then got renamed to `AbstractString` in 0.4; in 0.5, `ASCIIString` and `ByteString` were deprecated, and `UTF8String` was renamed to the (now concrete) type `String`.

src/Compat.jl

@@ -708,6 +708,12 @@ function _compat(ex::Expr)
                 import Base.writemime
             end
         end
+    elseif VERSION < v"0.5.0-dev+5575" && isexpr(ex, :comparison) #17510
+        if length(ex.args) == 3 && ex.args[2] == :.=
+            return :(Base.broadcast!(Base.identity, $(_compat(ex.args[1])), $(_compat(ex.args[3]))))
+        elseif length(ex.args) > 3 && ex.args[2] == :.=
+            return :(Base.broadcast!(Base.identity, $(_compat(ex.args[1])), $(_compat(Expr(:comparison, ex.args[3:end]...)))))
+        end
     end
     return Expr(ex.head, map(_compat, ex.args)...)
 end

test/runtests.jl

@@ -1525,6 +1525,16 @@ let io = IOBuffer()
     @test String(take!(io)) == "bbb"
 end
 
+# julia#17510
+let x = [1,2,3]
+    @compat x .= [3,4,5]
+    @test x == [3,4,5]
+    @compat x .= x .== 4
+    @test x == [0,1,0]
+    @compat x .= 7
+    @test x == [7,7,7]
+end
+
 let s = "Koala test: 🐨"
     @test transcode(UInt16, s) == UInt16[75,111,97,108,97,32,116,101,115,116,58,32,55357,56360]
     @test transcode(UInt32, s) == UInt32[75,111,97,108,97,32,116,101,115,116,58,32,128040]