Fix handling of ;init kwarg and test it is actually rewritten

src/rewrite_generic.jl

@@ -39,18 +39,17 @@ This most commonly happens in situations like `x^2`.
 """
 _rewrite_generic(::Expr, x::Number) = x, true
 
-function _is_generator(expr)
-    return Meta.isexpr(expr, :call, 2) && Meta.isexpr(expr.args[2], :generator)
-end
-
-function _is_flatten(expr)
-    return Meta.isexpr(expr, :call, 2) && Meta.isexpr(expr.args[2], :flatten)
+function _is_generic_generator_call(expr)
+    if Meta.isexpr(expr, :call, 2)
+        return _is_generator_or_flatten(expr.args[2])
+    end
+    return Meta.isexpr(expr, :call, 3) &&
+           Meta.isexpr(expr.args[2], :parameters) &&
+           _is_generator_or_flatten(expr.args[3])
 end
 
-_is_generator_or_flatten(expr) = _is_generator(expr) || _is_flatten(expr)
-
-function _is_parameters(expr)
-    return Meta.isexpr(expr, :call, 3) && Meta.isexpr(expr.args[2], :parameters)
+function _is_generator_or_flatten(expr)
+    return Meta.isexpr(expr, :generator) || Meta.isexpr(expr, :flatten)
 end
 
 function _is_kwarg(expr, kwarg::Symbol)
@@ -113,7 +112,7 @@ function _rewrite_generic(stack::Expr, expr::Expr)
     elseif Meta.isexpr(expr.args[2], :(...))
         # If the first argument is a splat.
         return esc(expr), false
-    elseif _is_generator_or_flatten(expr) || _is_parameters(expr)
+    elseif _is_generic_generator_call(expr)
         if !(expr.args[1] in (:sum, :Σ, :∑))
             # We don't know what this is. Return the expression and don't let
             # future callers mutate.

test/test_rewrite_generic.jl

@@ -675,6 +675,22 @@ function test_issue_76_trailing_dimensions()
     return
 end
 
+function test_issue_359()
+    for (expr, has_zero) in Pair{Expr,Bool}[
+        :(sum(i for i in I))=>true,
+        :(sum(i for i in I; init = 0))=>false,
+        :(sum(i for i in I, init in 0))=>false,
+    ]
+        stack = quote end
+        root, is_mutable = MA._rewrite_generic(stack, expr)
+        @test root isa Symbol
+        @test is_mutable
+        @test length(stack.args) == 3
+        @test occursin(".Zero()", string(stack.args[2])) == has_zero
+    end
+    return
+end
+
 end  # module
 
 TestRewriteGeneric.runtests()