Support opening of arguments of existential metatype type.

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -1364,12 +1364,19 @@ shouldOpenExistentialCallArgument(
   if (param->isVariadic() && !param->getVarargBaseTy()->hasTypeSequence())
     return None;
 
+  // If the argument is of an existential metatype, look through the
+  // metatype on the parameter.
+  auto formalParamTy = param->getInterfaceType();
+  if (argTy->is<AnyMetatypeType>()) {
+    formalParamTy = formalParamTy->getMetatypeInstanceType();
+    paramTy = paramTy->getMetatypeInstanceType();
+  }
+
   // The parameter type must be a type variable.
   auto paramTypeVar = paramTy->getAs<TypeVariableType>();
   if (!paramTypeVar)
     return None;
 
-  auto formalParamTy = param->getInterfaceType();
   auto genericParam = formalParamTy->getAs<GenericTypeParamType>();
   if (!genericParam)
     return None;

test/Constraints/opened_existentials.swift

@@ -61,6 +61,16 @@ func testMultipleOpened(a: any P, b: any P & Q) {
   let _: Int = r2  // expected-error{{cannot convert value of type '(Q, Q)?' to specified type 'Int'}}  
 }
 
+// --- Opening existential metatypes
+func conjureValue<T: P>(of type: T.Type) -> T? {
+  nil
+}
+
+func testMagic(pt: any P.Type) {
+  let pOpt = conjureValue(of: pt)
+  let _: Int = pOpt // expected-error{{cannot convert value of type 'P?' to specified type 'Int'}}
+}
+
 // --- With primary associated types and opaque parameter types
 protocol CollectionOf: Collection {
   @_primaryAssociatedType associatedtype Element