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@@ -837,7 +837,7 @@ If the interfaces discussed above are the building blocks for type-of-types,
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may be composed together. Unlike interfaces which are nominal, the name of a
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named constraint is not a part of its value. Two different named constraints
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with the same definition are equivalent even if they have different names. This
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-is because types don't explicitly specify which named constraints they
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+is because types don't have to explicitly specify which named constraints they
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implement, types automatically implement any named constraints they can satisfy.
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A named constraint definition can contain interface requirements using `impl`
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@@ -856,6 +856,35 @@ constraint VectorLegoFish {
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}
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```
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+An `impl` requirement may alternatively be on a named constraint, instead of an
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+interface, to add all the requirements of another named constraint without
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+adding any of the names:
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+
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+```
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+constraint DrawVectorLegoFish {
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+ // The same as requiring both `Vector` and `LegoFish`.
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+ impl as VectorLegoFish;
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+ // A regular interface requirement. No syntactic difference.
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+ impl as Drawable;
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+}
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+```
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+
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+In general, Carbon makes no syntactic distinction between the uses of named
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+constraints and interfaces, so one may be replaced with the other without
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+affecting users. To accomplish this, Carbon allows a named constraint to be used
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+whenever an interface may be. This includes all of these
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+[uses of interfaces](#interfaces-recap):
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+
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+- A type may `impl` a named constraint to say that it implements all of the
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+ requirements of the named constraint, as
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+ [described below](#extends-and-impl-with-named-constraints).
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+- A named constraint may be used as a namespace name in
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+ [a qualified name](#qualified-member-names-and-compound-member-access). For
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+ example, `VectorLegoFish.VAdd` refers to the same name as `Vector.Add`.
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+- A named constraint may be used as a
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+ [type-of-type](terminology.md#type-of-type) for
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+ [a generic type parameter](#generics).
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+
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We don't expect developers to directly define many named constraints, but other
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constructs we do expect them to use will be defined in terms of them. For
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example, we can define the Carbon builtin `Type` as:
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