template<typename T> int globalVar = 123;
                                          ^^^---- apparent type int
^^^^^^^^^^^^^^^^^^^^^^^^^^^^--- effective type (int)<T> [ with T resovled at instantiation site]

(int)<T> : is not a correct C++, it is invented for the purpose of this talk.

So

auto var = globalVar<double>;

Apparent type of var is int.

But effective type is (int)<double>, in words it say, a variable template of type int instantiated with type double.

>>>Thank you Sébastien for the perfect short summary. 

Oops, Sebastian, not Sébastien,  sorry :)


Sent from my Galaxy


-------- Original message --------
From: Sebastian Wittmeier via Std-Proposals <std-proposals@lists.isocpp.org>
Date: 7/30/24 6:05 PM (GMT+04:00)
To: std-proposals@lists.isocpp.org
Cc: Sebastian Wittmeier <wittmeier@projectalpha.org>
Subject: Re: [std-proposals] Function overload set type information loss

template<typename T>
int globalVar = 123;
 
This is a variable template;
 
 
globalVar<double> var;
 

You have kind of 3 types here, not 2: globalVar<double>, double and int

 

 

As far as I understand, you want to extract the template parameter double from globalVar<double>, before it is resolved.

 

But you also want to get it from wherever it is used with auto:

auto var = globalVar<double>;

 

 

In this case, you want to get double from var.