Capture structured bindings by value

Capturing structured bindings by reference is not allowed
before C++20. Here we can instead capture by value as a work around,
because the shifted indices are thin wrappers.

dune/fufem/backends/istlmatrixbackend.hh

@@ -86,7 +86,9 @@ namespace Impl {
     {
       assert(jjj.size()>0);
       auto [j, jj] = splitIndex(jjj);
-      return visitMatrixEntry(matrix, _0, j, [&](auto&& matrix_0j) -> decltype(auto) {
+      // We have to capture jj explicitly by value, because capturing structured bindings
+      // by reference is not allowed before C++20
+      return visitMatrixEntry(matrix, _0, j, [&, jj=jj](auto&& matrix_0j) -> decltype(auto) {
         return visitMatrixEntryRecursive(matrix_0j, iii, jj, f);
       });
     }
@@ -94,7 +96,9 @@ namespace Impl {
     {
       assert(iii.size()>0);
       auto [i, ii] = splitIndex(iii);
-      return visitMatrixEntry(matrix, i, _0, [&](auto&& matrix_i0) -> decltype(auto) {
+      // We have to capture ii explicitly by value, because capturing structured bindings
+      // by reference is not allowed before C++20
+      return visitMatrixEntry(matrix, i, _0, [&, ii=ii](auto&& matrix_i0) -> decltype(auto) {
         return visitMatrixEntryRecursive(matrix_i0, ii, jjj, f);
       });
     }
@@ -104,7 +108,9 @@ namespace Impl {
       assert(jjj.size()>0);
       auto [i, ii] = splitIndex(iii);
       auto [j, jj] = splitIndex(jjj);
-      return visitMatrixEntry(matrix, i, j, [&](auto&& matrix_ij) -> decltype(auto) {
+      // We have to capture ii and jj explicitly by value, because capturing structured bindings
+      // by reference is not allowed before C++20
+      return visitMatrixEntry(matrix, i, j, [&, ii=ii, jj=jj](auto&& matrix_ij) -> decltype(auto) {
         return visitMatrixEntryRecursive(matrix_ij, ii, jj, f);
       });
     }