What is decltype with two arguments?

Solution 1:

It's an comma-separated list of expressions, the type is identical to the type of the last expression in the list. It's usually used to verify that the first expression is valid (compilable, think SFINAE), the second is used to specify that decltype should return in case the first expression is valid.

Solution 2:

decltype does not take two arguments. Simply, it can can have an expression as its argument, and the comma operator is one way of creating expressions. Per Paragraph 5.18/1:

[...] A pair of expressions separated by a comma is evaluated left-to-right; the left expression is a discarded-value expression (Clause 5). Every value computation and side effect associated with the left expression is sequenced before every value computation and side effect associated with the right expression. The type and value of the result are the type and value of the right operand; the result is of the same value category as its right operand, and is a bit-field if its right operand is a glvalue and a bit-field. If the value of the right operand is a temporary (12.2), the result is that temporary.

Therefore:

static_assert(std::is_same<decltype(42, 3.14), double>::value, "Will not fire");