[CS] Support IUOs for compound function references

This was previously an artificial limitation of the
FunctionRefKind representation, there's no reason
we shouldn't support IUOs for functions referenced
using a compound name.

Author: hamishknight

lib/Sema/ConstraintSystem.cpp

@@ -1955,11 +1955,6 @@ OverloadChoice::getIUOReferenceKind(ConstraintSystem &cs,
   auto refKind = getFunctionRefInfo();
   assert(!forSecondApplication || refKind.isDoubleApply());
 
-  // Compound references currently never produce IUOs.
-  // FIXME(FunctionRefInfo): They should.
-  if (refKind.isCompoundName())
-    return std::nullopt;
-
   switch (refKind.getApplyLevel()) {
   case FunctionRefInfo::ApplyLevel::Unapplied:
     // Such references never produce IUOs.

test/Constraints/iuo.swift

@@ -260,3 +260,26 @@ let _: Int? = returnsIUOFn()()
 let _: Int = returnsIUOFn()() // expected-error {{value of optional type 'Int?' must be unwrapped to a value of type 'Int'}}
 // expected-note@-1 {{coalesce using '??' to provide a default when the optional value contains 'nil'}}
 // expected-note@-2 {{force-unwrap using '!' to abort execution if the optional value contains 'nil'}}
+
+// Make sure it works for compound function references.
+func testCompoundRefs() {
+  func hasArgLabel(x: Int) -> Int! { x }
+  struct S {
+    func hasArgLabel(x: Int) -> Int! { x }
+  }
+  let _ = hasArgLabel(x:)(0)
+  let _: Int? = hasArgLabel(x:)(0)
+  let _: Int = hasArgLabel(x:)(0)
+
+  let _ = S.hasArgLabel(x:)(S())(0)
+  let _: Int? = S.hasArgLabel(x:)(S())(0)
+  let _: Int = S.hasArgLabel(x:)(S())(0)
+
+  let _ = S().hasArgLabel(x:)(0)
+  let _: Int? = S().hasArgLabel(x:)(0)
+  let _: Int = S().hasArgLabel(x:)(0)
+
+  // We still don't allow IUOs for the function itself.
+  let _: (Int) -> Int = hasArgLabel(x:) // expected-error {{cannot convert value of type '(Int) -> Int?' to specified type '(Int) -> Int}}
+  let _: (Int) -> Int? = hasArgLabel(x:)
+}