[ConstraintSystem] Look through l-value while checking whether dynamic key path is recursive

Fix a crash in dynamic member lookup attempting to recursively
lookup a member on the same base type.

The problem is related to `isSelfRecursiveKeyPathDynamicMemberLookup`
which failed to look through l-value wrapping base type on each side of
the comparison, that's important because it's possible to have l-value
mismatch due to the fact that implicit call always gets l-value base
type but actual subscript which triggered dynamic lookup might be r-value.

Resolves: [SR-11743](https://bugs.swift.org/browse/SR-11743)
Resolves: rdar://problem/57091169

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -5546,7 +5546,15 @@ static bool isSelfRecursiveKeyPathDynamicMemberLookup(
       return baseDecl == keyPathRootDecl;
     }
 
-    if (baseTy->isEqual(keyPathRootTy))
+    // Previous base type could be r-value because that could be
+    // a base type of subscript "as written" for which we attempt
+    // a dynamic member lookup.
+    auto baseTy1 = baseTy->getRValueType();
+    // Root type of key path is always wrapped in an l-value
+    // before lookup is performed, so we need to unwrap that.
+    auto baseTy2 = keyPathRootTy->getRValueType();
+
+    if (baseTy1->isEqual(baseTy2))
       return true;
   }
 
@@ -5848,6 +5856,10 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
       using KPDynamicMemberElt = LocatorPathElt::KeyPathDynamicMember;
       if (auto kpElt = memberLocator->getLastElementAs<KPDynamicMemberElt>()) {
         auto *keyPath = kpElt->getKeyPathDecl();
+        if (isSelfRecursiveKeyPathDynamicMemberLookup(*this, baseTy,
+                                                      memberLocator))
+          return;
+
         if (auto *storage = dyn_cast<AbstractStorageDecl>(decl)) {
           // If this is an attempt to access read-only member via
           // writable key path, let's fail this choice early.

test/Constraints/keypath_dynamic_member_lookup.swift

@@ -483,3 +483,16 @@ func testDynamicMemberWithDefault(_ x: SR_11933) {
   // CHECK: [[OUTER_KP:%[0-9]+]] = keypath $KeyPath<SR_11933, Int>, (root $SR_11933; gettable_property $Int,  id @$s29keypath_dynamic_member_lookup8SR_11933V0B6MemberSis7KeyPathCyAA21HasDefaultedSubscriptVSiG_tcig : $@convention(method) (@guaranteed KeyPath<HasDefaultedSubscript, Int>, SR_11933) -> Int, getter @$s29keypath_dynamic_member_lookup8SR_11933V0B6MemberSis7KeyPathCyAA21HasDefaultedSubscriptVSiG_tcipACTK : $@convention(thin) (@in_guaranteed SR_11933, UnsafeRawPointer) -> @out Int, indices [%$0 : $KeyPath<HasDefaultedSubscript, Int> : $KeyPath<HasDefaultedSubscript, Int>], indices_equals @$ss7KeyPathCy29keypath_dynamic_member_lookup21HasDefaultedSubscriptVSiGTH : $@convention(thin) (UnsafeRawPointer, UnsafeRawPointer) -> Bool, indices_hash @$ss7KeyPathCy29keypath_dynamic_member_lookup21HasDefaultedSubscriptVSiGTh : $@convention(thin) (UnsafeRawPointer) -> Int) ([[INNER_KP]])
   _ = \SR_11933.[]
 }
+
+@dynamicMemberLookup
+protocol SR_11743 {
+  subscript(dynamicMember member: KeyPath<Self, Any>) -> Any { get }
+}
+
+extension SR_11743 {
+  subscript(dynamicMember member: KeyPath<Self, Any>) -> Any {
+    self[keyPath: member] // Ok
+    // CHECK: function_ref @swift_getAtKeyPath
+    // CHECK-NEXT: apply %{{.*}}<Self, Any>({{.*}})
+  }
+}