Why Can't the Compiler Deduce `T` in `test(TMap::Type)`?

Unable to Deduce T from Template::Type

In the provided code, an error occurs when trying to deduce the template argument T for the test function. To understand why, it's crucial to delve into the concept of "non-deducible context."

In the template definition of TMap, you define an alias called Type to be a std::map. When attempting to deduce T from the function call test(tmap), the compiler encounters a non-deducible context.

Consider the following scenario: if TMap had a specialization for SomeType where Type is defined as std::map, how would the compiler determine the type SomeType based solely on the knowledge that TMap::Type is std::map? This deduction is not possible.

Moreover, if another specialization of TMap exists for OtherType where the Type is again std::map, the situation becomes even more ambiguous. Given the known equivalence of TMap::Type and TMap::Type, the compiler cannot discern between SomeType and OtherType.

It is important to remember that the compiler cannot make assumptions about the relationship between the type arguments used in the template definition. Hence, in such non-deducible contexts, the compiler cannot automatically deduce the template argument T.

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