Nate Chandler 0143ce25c2 Implicitly convert single exprs from uninhabited. ...

When type-checking a return statement's result, pass a new
ContextualTypePurpose when that return statement appears in a function
with a single expression.  When solving the corresponding constraint
system, the conversion constraint will have a new ConstraintKind.  When
matching types, check whether the constraint kind is this new kind,
meaning that the constraint is between a function's single expression
and the function's result.  If it is, allow a conversion from
an uninhabited type (the expression's type) to anything (the function's
result type) by adding an uninhabited upcast restriction to the vector
of conversions.  Finally, at coercion time, upon encountering this
restriction, call coerceUninhabited, replacing the original expression
with an UninhabitedUpcastExpr.  Finally, at SILGen time, treat this
UninhabitedUpcastExpr as a ForcedCheckedCastExpr.

Eliminates the bare ConstraintSystem usage from
typeCheckFunctionBodyUntil, ensuring that the same code path is followed
for all function bodies.

2019-04-24 10:04:17 -07:00

261 KiB

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