add big if -> True to syntax to remove in v2

Author: giacomocavalieri

docs/v2.md

@@ -12,20 +12,20 @@ Due to a bug in the parser we accept `[1, ..]` as a valid list value.
 This pattern doesn't make sense as one could write `a` instead. We don't want
 two ways of doing the same thing.
 
-- [X] Emits warning when used.
-- [X] Formatter rewrites it to desired syntax.
+- [x] Emits warning when used.
+- [x] Formatter rewrites it to desired syntax.
 
 ## Shadowing imported values
 
 Do not allow shadowing an imported value, the same way one can't define two
 top level functions with the same name.
 
-- [ ] Emits warning when used.
+- [x] Emits warning when used.
 
 ## Import one module multiple times
 
-Do not one module to be imported multiple times. This is currently accepted so
-long as each import uses a different alias.
+Do not allow one module to be imported multiple times. This is currently
+accepted so long as each import uses a different alias.
 
 - [x] Emits warning when used.
 
@@ -34,3 +34,11 @@ long as each import uses a different alias.
 On JavaScript there is a deprecated `fn` property. This was a mistake, it
 should have been `function`. It still exists today due to backwards
 compatibility.
+
+## Do not allow guard with no condition
+
+It doesn't make sense to have an `if` guard followed by no condition, but the
+compiler allows this: `case wibble { big if -> True }`
+
+- [ ] Emits warning when used.
+- [x] Formatter rewrites it to desired syntax.