specify that types can be omitted from augmenting declarations, and will be inferred from the original

working/augmentation-libraries/feature-specification.md

@@ -318,6 +318,19 @@ It is a compile-time error if:
     augmentation can both declare a new declaration and augment it in the same
     file.*
 
+### Type inheritance
+
+When a type is omitted in an augmenting declaration, if the first declaration
+that introduced the member has a type specified in the ommitted types position,
+then that type is inherited.
+
+This applies to return types, parameter types, variable types, and type
+parameter bounds (but not type parameter names or regular parameter names, those
+need to be re-declared).
+
+It is a compile time error for a manually written type is not the same as a type
+that would be inherited if that manually writted type had been omitted.
+
 ### Augmented Expression
 
 The exact result of an `augmented` expression depends on what is being
@@ -474,7 +487,7 @@ The augmenting function does not have to pass the same arguments to
 `augmented()` as were passed to it. It may call it once, more than once, or not
 at all.
 
-It is a compile-time error if:
+It is a compile-time error if, after type inheritance:
 
 *   The signature of the function augmentation does not exactly match the
     original function. This means the return types must be the same; there must
@@ -706,7 +719,7 @@ original redirecting initializer if there is one.
 In the augmenting constructor's body, an `augmented()` call invokes the
 original constructor's body.
 
-It is a compile-time error if:
+It is a compile-time error if, after type inheritance:
 
 *   The signature of the constructor augmentation does not match the original
     constructor. This means the return types must be the same; there must be the