[ConstraintSystem] Implicitly open existential type of for-in sequence

This allows to use `for-in` statement to iterate over i.e. `any Collection`
and other existentials that conform to `Sequence` protocol.

Author: xedin

lib/Sema/CSApply.cpp

@@ -8556,6 +8556,7 @@ static Optional<SolutionApplicationTarget> applySolutionToForEachStmt(
   auto resultTarget = target;
   auto &forEachStmtInfo = resultTarget.getForEachStmtInfo();
   auto *stmt = target.getAsForEachStmt();
+  auto *parsedSequence = stmt->getSequence();
   bool isAsync = stmt->getAwaitLoc().isValid();
 
   // Simplify the various types.
@@ -8673,9 +8674,14 @@ static Optional<SolutionApplicationTarget> applySolutionToForEachStmt(
         stmt->getAwaitLoc().isValid() ? KnownProtocolKind::AsyncSequence
                                       : KnownProtocolKind::Sequence);
 
+    auto type = forEachStmtInfo.sequenceType->getRValueType();
+    if (type->isExistentialType()) {
+      auto *contextualLoc = solution.getConstraintLocator(
+          parsedSequence, LocatorPathElt::ContextualType(CTP_ForEachSequence));
+      type = Type(solution.OpenedExistentialTypes[contextualLoc]);
+    }
     auto sequenceConformance = TypeChecker::conformsToProtocol(
-        forEachStmtInfo.sequenceType->getRValueType(), sequenceProto,
-        dc->getParentModule());
+        type, sequenceProto, dc->getParentModule());
     assert(!sequenceConformance.isInvalid() &&
            "Couldn't find sequence conformance");
     stmt->setSequenceConformance(sequenceConformance);

lib/Sema/CSDiagnostics.cpp

@@ -1275,7 +1275,7 @@ bool MemberAccessOnOptionalBaseFailure::diagnoseAsError() {
         auto &solution = getSolution();
         auto *baseLoc = solution.getConstraintLocator(
             UDE->getBase(),
-            LocatorPathElt::ContextualType(CTP_ForEachStmt));
+            LocatorPathElt::ContextualType(CTP_ForEachSequence));
 
         if (hasFixFor(solution, baseLoc))
           return false;

lib/Sema/CSGen.cpp

@@ -3889,7 +3889,7 @@ generateForEachStmtConstraints(
   auto *sequenceExpr = stmt->getSequence();
   auto *dc = target.getDeclContext();
   auto contextualLocator = cs.getConstraintLocator(
-      sequenceExpr, LocatorPathElt::ContextualType(CTP_ForEachStmt));
+      sequenceExpr, LocatorPathElt::ContextualType(CTP_ForEachSequence));
 
   // The expression type must conform to the Sequence protocol.
   auto sequenceProto = TypeChecker::getProtocol(

lib/Sema/CSSimplify.cpp

@@ -7153,6 +7153,16 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyConformsToConstraint(
   } break;
   case ConstraintKind::ConformsTo:
   case ConstraintKind::LiteralConformsTo: {
+    // If existential type is used as a for-in sequence, let's open it
+    // and check whether underlying type conforms to `Sequence`.
+    if (type->isExistentialType()) {
+      if (auto elt = loc->getLastElementAs<LocatorPathElt::ContextualType>()) {
+        if (elt->getPurpose() == CTP_ForEachSequence) {
+          type = openExistentialType(type, loc).first;
+        }
+      }
+    }
+
     // Check whether this type conforms to the protocol.
     auto conformance = DC->getParentModule()->lookupConformance(
         type, protocol, /*allowMissing=*/true);

test/Interpreter/statements.swift

@@ -51,3 +51,13 @@ print(guardvalue)
 print("done");
 // CHECK-LABEL: done
 
+func testAnyCollection(_ c: any Collection) {
+  for v in c {
+    print(v)
+  }
+}
+
+testAnyCollection([1, 2.0, "Hello"])
+// CHECK-NEXT: 1
+// CHECK-NEXT: 2.0
+// CHECK-NEXT: Hello

test/stmt/foreach.swift

@@ -175,13 +175,19 @@ func testOptionalSequence() {
   }
 }
 
-// FIXME: Should this be allowed?
 func testExistentialSequence(s: any Sequence) {
-  for x in s { // expected-error {{type 'any Sequence' cannot conform to 'Sequence'}} expected-note {{only concrete types such as structs, enums and classes can conform to protocols}}
+  for x in s {
     _ = x
   }
 }
 
+// rdar://92177656 - implicit existential opening should work with for-in loops
+func testForEachWithAnyCollection(c: any Collection) {
+  for v in c {
+    print(v)
+  }
+}
+
 // Conditional conformance to Sequence and IteratorProtocol.
 protocol P { }
 
@@ -238,3 +244,4 @@ func testForEachWhereWithClosure(_ x: [Int]) {
   for i in x where foo({ i.byteSwapped == 5 }) {}
   for i in x where x.contains(where: { $0.byteSwapped == i }) {}
 }
+