[flang][Evaluate] Restrict ConstantBase constructor overload (#138456)
ConstantBase has a constructor that takes a value of any type as an
input: template <typename T> ConstantBase(const T &). A derived type
Constant<T> is a member of many Expr<T> classes (as an alternative in
the member variant).
When trying (erroneously) to create Expr<T> from a wrong input, if the
specific instance of Expr<T> contains Constant<T>, it's that constructor
that will be instantiated, leading to cryptic and confusing errors.
Eliminate the constructor from overload for invalid input values to help
produce more meaningful diagnostics.
llvm-project
304c7a87 - [flang][Evaluate] Restrict ConstantBase constructor overload (#138456)
