mjhjmtu

This code compiles fine in g++ (coliru), but not MSVC (godbolt and my VS2017).

#include <type_traits>
#include <iostream>
template<class T> void f()
 constexpr bool b=std::is_same_v<T,int>; //#1
 auto func_x=[&]()
 if constexpr(b) //#error
 else
 
 ;
 func_x();

int main()
 f<int>();

(6): error C2131: expression did not evaluate to a constant

(6): note: failure was caused by a read of a variable outside its lifetime

(6): note: see usage of 'this'

Which one (g++ or MSVC) is wrong?

What is this in "see usage of 'this'"??

How to work around it while keep the compile-time guarantee?

In my real case, b (#1) is a complex statement depends on a few other constexpr variables.

Gcc is right. b (as constexpr variable) doesn't need to be captured in fact.

A lambda expression can read the value of a variable without capturing
it if the variable

• is constexpr and has no mutable members.

GCC LIVE

It seems if making b static then MSVC could access b without capturing.

template<class T> void f()
 constexpr static bool b=std::is_same_v<T,int>;
 auto func_x=()
 if constexpr(b)
 else
 
 ;
 func_x();

MSVC LIVE

And

How to work around it while keep the compile-time guarantee?

We can't keep the constexpr-ness for the captured variables. They become non-static data members of the lambda closure type and non-static data members can't be constexpr.

How to work around it while keep the compile-time guarantee?

Marking the constexpr bool as static serves as a work around.

See Demo

Alternately, you can use the condition in the if constexpr instead of assigning it to a bool. Like below:

if constexpr(std::is_same_v<T,int>)

See Demo

Note that there have been bugs raised for MSVC regarding constexpr with respect to lambda expressions.

One such is: problems with capturing constexpr in lambda

and another is: if constexpr in lambda