Allow existential opening for parameters of optional type.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -1402,8 +1402,10 @@ shouldOpenExistentialCallArgument(
   if (param->isVariadic() && !param->getVarargBaseTy()->hasTypeSequence())
     return None;
 
-  // Look through an inout type on the formal type of the parameter.
-  auto formalParamTy = param->getInterfaceType()->getInOutObjectType();
+  // Look through an inout and optional types on the formal type of the
+  // parameter.
+  auto formalParamTy = param->getInterfaceType()->getInOutObjectType()
+      ->lookThroughSingleOptionalType();
 
   // If the argument is of an existential metatype, look through the
   // metatype on the parameter.
@@ -1412,8 +1414,8 @@ shouldOpenExistentialCallArgument(
     paramTy = paramTy->getMetatypeInstanceType();
   }
 
-  // Look through an inout type on the parameter.
-  paramTy = paramTy->getInOutObjectType();
+  // Look through an inout and optional types on the parameter.
+  paramTy = paramTy->getInOutObjectType()->lookThroughSingleOptionalType();
 
   // The parameter type must be a type variable.
   auto paramTypeVar = paramTy->getAs<TypeVariableType>();

test/Constraints/opened_existentials.swift

@@ -159,6 +159,16 @@ func passesInOut(i: Int) {
   takesInOut(&p)
 }
 
+func takesOptional<T: P>(_ value: T?) { }
+// expected-note@-1{{required by global function 'takesOptional' where 'T' = 'P'}}
+
+func passesToOptional(p: any P, pOpt: (any P)?) {
+  takesOptional(p) // okay
+  takesOptional(pOpt) // expected-error{{protocol 'P' as a type cannot conform to the protocol itself}}
+  // expected-note@-1{{only concrete types such as structs, enums and classes can conform to protocols}}
+}
+
+
 @available(SwiftStdlib 5.1, *)
 func testReturningOpaqueTypes(p: any P) {
   let q = p.getQ()