SFINAE working in return type but not as template parameter

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You should take a look at 14.5.6.1 Function template overloading (C++11 standard) where function templates equivalency is defined. In short, default template arguments are not considered, so in the 1st case you have the same function template defined twice. In the 2nd case you have expression referring template parameters used in the return type (again see 14.5.6.1/4). Since this expression is part of signature you get two different function template declarations and thus SFINAE get a chance to work.

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