Signature constraints on templated enums and aliases?

[H. S. Teoh via Digitalmars-d]

Recently, very nice syntactic sugar was added for eponymous templates
that expand to enums or aliases:

	// Original verbose form
	template myEnum(T) {
		enum myEnum = T.someStaticValue;
	}

	// New concise form:
	enum myEnum(T) = T.someStaticValue;

	// Original verbose form
	template MyType(T) {
		alias MyType = typeof(T.someField);
	}

	// New concise form:
	alias MyType(T) = T.someField;

This is wonderful, since it lets us shorten common templated enums and
aliases the same way templated functions are. However, there's one
missing component: signature contraints.

	// sig constraints work for templated functions:
	auto myFunc(T)(T t) if (isInputRange!T) { ... }

	// But not for enums or aliases:
	//enum myEnum(T) = T.someValue if (is(typeof(T.someValue))); //NG

	// Instead you're forced to write it in long-form:
	template myEnum(T)
		if (is(typeof(T.someValue)))
	{
		enum myEnum = T.someValue;
	}

Is there any chance of extending the syntax of enums and aliases to also
allow sig constraints, the same way they are allowed in function
declarations?


T

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