swift234

[Constraint solver] Track whether type variables can bind to lvalues.

The type checker has a poor habit of inferring lvalue types where it
shouldn't, or (more rarely) dropping lvalue types when it should keep
them. Only allow a type variable to bind to an lvalue type in specific
situations where it makes sense (e.g., the type variable represents an
overload set that may contain a variable). Fixes <rdar://problem/13827562>.



Swift SVN r7179