Improve error message when = is used instead of == (#3892)

src/Containers/macro.jl

@@ -54,6 +54,11 @@ function parse_macro_arguments(
                 )
             elseif valid_kwargs !== nothing && !(arg.args[1] in valid_kwargs)
                 error_fn("unsupported keyword argument `$(arg.args[1])`.")
+            elseif !(arg.args[1] isa Symbol)
+                error_fn(
+                    "Invalid keyword argument detected. If you are trying to " *
+                    "construct an equality constraint, use `==` instead of `=`.",
+                )
             end
             kwargs[arg.args[1]] = arg.args[2]
         else

test/test_macros.jl

@@ -2514,4 +2514,16 @@ function test_do_not_mutate_expression_in_set()
     return
 end
 
+function test_malformed_kwarg()
+    model = Model()
+    @variable(model, x[1:1])
+    @test_throws_parsetime(
+        ErrorException(
+            "In `@constraint(model, x[1] = 1)`: Invalid keyword argument detected. If you are trying to construct an equality constraint, use `==` instead of `=`.",
+        ),
+        @constraint(model, x[1] = 1),
+    )
+    return
+end
+
 end  # module