*Created by: mhoemmen* @trilinos/stratimikos @trilinos/thyra @rrdrake Stratimikos [uses the macro](https://github.com/trilinos/Trilinos/blob/6f4bf237f689a547430c4183c41459d11946a60a/packages/stratimikos/adapters/amesos2/src/Thyra_Amesos2LinearOpWithSolve.cpp#L53) `TEUCHOS_CLASS_TEMPLATE_INSTANT_REAL_SCALAR_TYPES` to do explicit template instantiation (ETI) for its Amesos2 adapter, `Thyra::Amesos2LinearOpWithSolveFactory`. [This macro depends on the](https://github.com/trilinos/Trilinos/blob/6f4bf237f689a547430c4183c41459d11946a60a/packages/teuchos/core/src/Teuchos_ExplicitInstantiationHelpers.hpp#L66) `HAVE_TEUCHOS_FLOAT` macro, which in turn depends on the `Teuchos_ENABLE_FLOAT` CMake option. However, whether it's valid to use Tpetra objects with `Scalar=float` depends on a different CMake option, `Tpetra_INST_FLOAT`. `Tpetra_INST_FLOAT` and `Teuchos_ENABLE_FLOAT` both take their default values from the common CMake option `Trilinos_ENABLE_FLOAT`. However, #4059 shows that it's possible to override those defaults and set `Teuchos_ENABLE_FLOAT=ON` but `Tpetra_INST_FLOAT=OFF`. This will result in link errors in packages that use `TEUCHOS_CLASS_TEMPLATE_INSTANT_REAL_SCALAR_TYPES` to instantiate code that uses Tpetra objects. ## Possible Solution At configure time, forbid setting `Teuchos_ENABLE_FLOAT=ON` but `Tpetra_INST_FLOAT=OFF`. This is most easily done in Tpetra, since Tpetra has a required dependence on Teuchos. One could fix this in other ways, but I prefer solutions that avoid combinatorial explosion of build options. ## Related Issues * Follows #4059 * Related to #4059, #2009