[SOLVED] Right bit shift in body of lambda used as a template argument doesn't compile on GCC

Issue

GCC doesn't compile this code, while other compilers (clang, msvc) do

template<auto T>
struct S { };

int main() {
  S<[]{int x,y; x<<y;}> s1; // compiles fine
  S<[]{int x,y; x>>y;}> s2; // error
}

error:

error: expected ';' before '>>' token
      |     S<[]{int x,y; x>>y;}> s2;
      |                    ^~
      |                    ;

However when I explicitly call operator>>, GCC accepts it

struct S2 {
  void operator>>(S2 arg) { }
};
S<[]{S2 x,y; x.operator>>(y);}> s2; // compiles

It also does compile when I move the lambda definition outside of template parameter list

auto lam = []{int x,y; x>>y;}; 
S<lam> s; // compiles

Is it a compiler bug?

Solution

g++ is actually correct here. From [temp.names]/3 (C++20 Draft N4860):

When a name is considered to be a template-name, and it is followed by a <, the < is always taken as the delimiter of a template-argument-list and never as the less-than operator. When parsing a template-argument-list, the first non-nested > is taken as the ending delimiter rather than a greater-than operator. Similarly, the first non-nested >> is treated as two consecutive but distinct > tokens, the first of which is taken as the end of the template-argument-list and completes the template-id. [Note: The second > token produced by this replacement rule may terminate an enclosing template-id construct or it may be part of a different construct (e.g., a cast). — end note] [Example:

template<int i> class X { /* ... */ };
X< 1>2 > x1;                            // syntax error
X<(1>2)> x2;                            // OK

template<class T> class Y { /* ... */ };
Y<X<1>> x3;                             // OK, same as Y<X<1> > x3;
Y<X<6>>1>> x4;                          // syntax error
Y<X<(6>>1)>> x5;                        // OK

— end example]

The >> is treated as two > tokens. You can wrap it in brackets to enforce what you are trying to do if you want.

template<auto T>
struct S { };

int main() {
  S<[]{int x,y; (x>>y);}> s2; // now compiles fine
}

Note: it may be that both accepting or rejecting is correct, since it's a bit vague what is meant by nested. See comments.

Answered By - ChrisMM

Answer Checked By - Marilyn (WPSolving Volunteer)