Remove broken conversion of @fastmath x[i] += 1 (#50347)

Fixes #47241

base/fastmath.jl

@@ -112,19 +112,10 @@ function make_fastmath(expr::Expr)
  if isa(var, Symbol)
  # simple assignment
  expr = :($var = $op($var, $rhs))
- elseif isa(var, Expr) && var.head === :ref
- var = var::Expr
- # array reference
- arr = var.args[1]
- inds = var.args[2:end]
- arrvar = gensym()
- indvars = Any[gensym() for _ in inds]
- expr = quote
- $(Expr(:(=), arrvar, arr))
- $(Expr(:(=), Base.exprarray(:tuple, indvars), Base.exprarray(:tuple, inds)))
- $arrvar[$(indvars...)] = $op($arrvar[$(indvars...)], $rhs)
- end
  end
+ # It is hard to optimize array[i += 1] += 1
+ # and array[end] += 1 without bugs. (#47241)
+ # We settle for not optimizing the op= call.
  end
  Base.exprarray(make_fastmath(expr.head), Base.mapany(make_fastmath, expr.args))
 end

test/fastmath.jl

@@ -284,3 +284,12 @@ end
  end
  end
 end
+
+@testset "+= with indexing (#47241)" begin
+ i = 0
+ x = zeros(2)
+ @fastmath x[i += 1] += 1
+ @fastmath x[end] += 1
+ @test x == [1, 1]
+ @test i == 1
+end