Fix validation to always check the configured type for an arg (#80)

Also update the readme to always refer to the boolean type correctly.

README.rst

@@ -653,9 +653,10 @@ Named arguments support the following configuration options:
 - **type** : str
    The type that the provided value will be cast to. The set of acceptable options is
    {"string", "float", "integer", "boolean"}. If not provided then the default behaviour
-   is to keep values as strings. Setting the type to "bool" makes the resulting argument
-   a flag that if provided will set the value to the boolean opposite of the default
-   value – i.e. *true* if no default value is given, or false if :toml:`default = true`.
+   is to keep values as strings. Setting the type to "boolean" makes the resulting
+   argument a flag that if provided will set the value to the boolean opposite of the
+   default value – i.e. *true* if no default value is given, or false if
+   :toml:`default = true`.
 
 
 Arguments for cmd and shell tasks

poethepoet/task/args.py

@@ -123,13 +123,14 @@ def validate_def(cls, task_name: str, args_def: ArgsDef) -> Optional[str]:
                         f"Unexpected 'name' option for arg {arg_name!r} of task "
                         f"{task_name!r}"
                     )
-                error = cls._validate_type(params, arg_name, task_name)
-                if error:
-                    return error
                 arg_params.append((params, arg_name, task_name))
 
         positional_multiple = None
         for params, arg_name, task_name in arg_params:
+            error = cls._validate_type(params, arg_name, task_name)
+            if error:
+                return error
+
             error = cls._validate_params(params, arg_name, task_name)
             if error:
                 return error