Requires type arguments T and U to have a common type. If they have a common type, that type is bound to CommonType symbol that can be used for example in concepts that refine this concept.
Types T and U have a common type if the types T and U are exactly the same type, type T is implicitly convertible to type U, or type U is implicitly convertible to type U. In the first case the common type will be T, in the second case it will be U and in the third case it will be T.
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