[ConstraintSystem] Don't include self-recursive dynamic member results as "inaccessible"

Solver has to keep track of excluded dynamic member results while
performing lookup because otherwise, in diagnostic modem it might
include such results as inaccessible.

Resolves: rdar://problem/61084565

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -5812,6 +5812,15 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
     }
   }
 
+  // Exclude some of the dynamic member choices from results
+  // because using such choices would result in a self-recursive reference.
+  //
+  // This is required because if there are no viable/unviable choices
+  // `performMemberLookup` is going to attempt to lookup inaccessible
+  // members and results would include dynamic member subscripts which
+  // have already been excluded.
+  llvm::SmallPtrSet<ValueDecl *, 2> excludedDynamicMembers;
+
   // Local function that adds the given declaration if it is a
   // reasonable choice.
   auto addChoice = [&](OverloadChoice candidate) {
@@ -5988,8 +5997,10 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
       if (auto kpElt = memberLocator->getLastElementAs<KPDynamicMemberElt>()) {
         auto *keyPath = kpElt->getKeyPathDecl();
         if (isSelfRecursiveKeyPathDynamicMemberLookup(*this, baseTy,
-                                                      memberLocator))
+                                                      memberLocator)) {
+          excludedDynamicMembers.insert(candidate.getDecl());
           return;
+        }
 
         if (auto *storage = dyn_cast<AbstractStorageDecl>(decl)) {
           // If this is an attempt to access read-only member via
@@ -6225,6 +6236,9 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
         return result;
       }
 
+      if (excludedDynamicMembers.count(cand))
+        continue;
+
       result.addUnviable(getOverloadChoice(cand, /*isBridged=*/false,
                                            /*isUnwrappedOptional=*/false),
                          MemberLookupResult::UR_Inaccessible);

test/Constraints/subscript.swift

@@ -203,3 +203,16 @@ func test_generic_subscript_requirements_mismatch_diagnostics() {
 
   s[number: ["hello"]] // expected-error {{subscript 'subscript(number:)' requires that 'String' conform to 'BinaryInteger'}}
 }
+
+// rdar://61084565 - infinite recursion in dynamic member lookup
+func rdar61084565() {
+  @dynamicMemberLookup
+  struct Foo {
+    subscript(dynamicMember _: KeyPath<Foo, Int>) -> Int {
+      return 42
+    }
+  }
+
+  let a = Foo()
+  a[] // expected-error {{value of type 'Foo' has no subscripts}}
+}