Require transitive gating, and add examples

Author: lukewagner

design/mvp/WIT.md

@@ -905,8 +905,28 @@ unstable-gate ::= '@unstable' '(' feature-field ')'
 feature-field ::= 'feature' '=' id
 since-gate ::= '@since' '(' 'version' '=' '"' <valid semver> '"' ( ',' feature-field )? ')'
 ```
+
 As part of WIT validation, any item that refers to another gated item must also
-be gated.
+be compatibly gated. For example, this is an error:
+```wit
+interface i {
+  @since("1.0.1")
+  type t1 = u32;
+
+  type t2 = t1; // error
+}
+```
+Additionally, if an item is *contained* by a gated item, it must also be
+compatibly gated. For example, this is an error:
+```wit
+@since("1.0.2")
+interface i {
+  foo: func();  // error: no gate
+
+  @since("1.0.1")
+  bar: func();  // also error: weaker gate
+}
+```
 
 ## Package declaration