[docs] clarify broadcasting in constraint documentation (#594)

Author: odow

docs/src/manual/types.md

@@ -100,6 +100,11 @@ constraint = x == 0
 constraint = x >= 1
 ```
 
+Note that constraints apply elementwise automatically; that is, `x >= 1` means
+that `x[i, j] >= 1` for `i in 1:5` and `j in 1:5`. Consequently, broadcasting
+should not be used to constrain arrays, that is, use `x >= y` instead of
+`x .>= y`.
+
 Matrices can also be constrained to be positive semidefinite.
 
 ```julia

test/test_utilities.jl

@@ -1282,6 +1282,15 @@ function test_variable_primal_start()
     return
 end
 
+function test_broadcasting()
+    A = [1 2; 3 4]
+    x = Variable(2)
+    y = [1.1, 2.2]
+    @test_throws MethodError (A * x) .<= y
+    @test sprint(show, x .== y) == sprint(show, [x[i] == y[i] for i in 1:2])
+    return
+end
+
 end  # TestUtilities
 
 TestUtilities.runtests()