Why Does Template Deduction Fail with Implicit Type Conversion for User-Defined Types?

Implicit Type Conversion and Template Deduction

When attempting to implicitly convert an int to a Scalar object within a template function call, the template argument deduction may fail. This occurs because template argument deduction does not automatically take into account user-defined conversions, such as the one between int and Scalar.

In the provided code:

<code class="cpp">func(a, 2);</code>

the compiler attempts to implicitly convert the int 2 to a Scalar object, but the template argument deduction fails as it does not consider user-defined conversions. To resolve this, the argument must be explicitly converted at the caller site:

<code class="cpp">func(a, Scalar<int>{2}); // C++14</code>

Alternatively, if C 17 is used, a deduction guide can be defined for Scalar, allowing the following syntax:

<code class="cpp">func(a, Scalar{2});</code>

Finally, explicit instantiation of the template function with the specified type argument can also bypass the need for implicit conversion:

<code class="cpp">func<int>(a, 2); // Assuming Scalar<T>::Scalar(T) is not explicit</code>

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