Sema: Fix existential-metatype-to-Any conversion

We were incorrectly unwrapping too many levels of metatype, and
our ErasureExpr would end up with conformances for the instance
type and not the metatype itself.

In the old universe, this was not a problem, but now Any has two
protocol conformance members, so suddently this violated invariants.

This was a regression, introduced in 6027bf4.

Fixes rdar://125460667.

Author: slavapestov

lib/Sema/CSApply.cpp

@@ -6688,13 +6688,24 @@ Expr *ExprRewriter::coerceExistential(Expr *expr, Type toType,
   Type fromInstanceType = fromType;
   Type toInstanceType = toType;
 
-  // Look through metatypes
+  // For existential-to-existential coercions, open the source existential.
+  Type openedFromType;
+  if (fromType->isAnyExistentialType()) {
+    openedFromType = OpenedArchetypeType::getAny(fromType->getCanonicalType(),
+                                                 dc->getGenericSignatureOfContext());
+  }
+
+  Type openedFromInstanceType = openedFromType;
+
+  // Look through metatypes.
   while ((fromInstanceType->is<UnresolvedType>() ||
           fromInstanceType->is<AnyMetatypeType>()) &&
          toInstanceType->is<ExistentialMetatypeType>()) {
     if (!fromInstanceType->is<UnresolvedType>())
-      fromInstanceType = fromInstanceType->castTo<AnyMetatypeType>()->getInstanceType();
-    toInstanceType = toInstanceType->castTo<ExistentialMetatypeType>()->getExistentialInstanceType();
+      fromInstanceType = fromInstanceType->getMetatypeInstanceType();
+    if (openedFromInstanceType && !openedFromInstanceType->is<UnresolvedType>())
+      openedFromInstanceType = openedFromInstanceType->getMetatypeInstanceType();
+    toInstanceType = toInstanceType->getMetatypeInstanceType();
   }
 
   ASTContext &ctx = cs.getASTContext();
@@ -6758,20 +6769,13 @@ Expr *ExprRewriter::coerceExistential(Expr *expr, Type toType,
   }
 
   // For existential-to-existential coercions, open the source existential.
-  if (fromType->isAnyExistentialType()) {
-    fromType = OpenedArchetypeType::getAny(fromType->getCanonicalType(),
-                                           dc->getGenericSignatureOfContext());
-
+  if (openedFromType) {
     auto *archetypeVal = cs.cacheType(
-        new (ctx) OpaqueValueExpr(expr->getSourceRange(), fromType));
-
-    fromInstanceType = fromType;
-    while (auto *metatypeType = fromInstanceType->getAs<MetatypeType>())
-      fromInstanceType = metatypeType->getInstanceType();
+        new (ctx) OpaqueValueExpr(expr->getSourceRange(), openedFromType));
 
     auto conformances =
         dc->getParentModule()
-          ->collectExistentialConformances(fromInstanceType->getCanonicalType(),
+          ->collectExistentialConformances(openedFromInstanceType->getCanonicalType(),
                                            toInstanceType->getCanonicalType(),
                                            /*allowMissing=*/true);
 

test/SILOptimizer/rdar125460667.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-frontend -emit-sil %s
+
+@_transparent func f(a: Any.Type) -> Any {
+  return a
+}
+
+func g(a: Any.Type) {
+  f(a: a)
+}