If ty1 and ty2 are basic types, return the TY that both can be implicitly converted to.
If ty1 and ty2 are basic types, return the TY that ty1 can be implicitly converted to to bring them to a common ty. It's symmetric, i.e. the operands can be swapped.
Provides an implicit conversion table for basic types.
Used to determine integer promotions and common types.
Specification: Integer Promotions, Usual Arithmetic Conversions.