[ConstraintSystem] Propagate holes up to result of optional chaining

If underlying type of an optional chain has been marked as a hole,
e.g. due to a reference to an invalid or missing member, let's
propagate that information to object type of an optional that
represents a result of an optional chain.

Resolves: rdar://80941497

Author: xedin

lib/Sema/CSBindings.cpp

@@ -1929,6 +1929,24 @@ bool TypeVariableBinding::attempt(ConstraintSystem &cs) const {
     type = type->reconstituteSugar(/*recursive=*/false);
   }
 
+  // If type variable has been marked as a possible hole due to
+  // e.g. reference to a missing member. Let's propagate that
+  // information to the object type of the optional type it's
+  // about to be bound to.
+  //
+  // In some situations like pattern bindings e.g. `if let x = base?.member`
+  // - if `member` doesn't exist, `x` cannot be determined either, which
+  // leaves `OptionalEvaluationExpr` representing outer type of `base?.member`
+  // without any contextual information, so even though `x` would get
+  // bound to result type of the chain, underlying type variable wouldn't
+  // be resolved, so we need to propagate holes up the conversion chain.
+  if (TypeVar->getImpl().canBindToHole()) {
+    if (auto objectTy = type->getOptionalObjectType()) {
+      if (auto *typeVar = objectTy->getAs<TypeVariableType>())
+        cs.recordPotentialHole(typeVar);
+    }
+  }
+
   ConstraintSystem::TypeMatchOptions options;
 
   options |= ConstraintSystem::TMF_GenerateConstraints;

test/Constraints/result_builder_diags.swift

@@ -797,3 +797,21 @@ func test_weak_with_nonoptional_type() {
     42
   }
 }
+
+// rdar://80941497 - compiler fails to produce diagnostic when referencing missing member in optional context
+func test_missing_member_in_optional_context() {
+  struct Test {
+  }
+
+  var test: Test? = nil
+
+  tuplify(true) { c in
+    if let prop = test?.prop { // expected-error {{value of type 'Test' has no member 'prop'}}
+      0
+    }
+
+    if let method = test?.method() { // expected-error {{value of type 'Test' has no member 'method'}}
+      1
+    }
+  }
+}

test/expr/unary/keypath/keypath.swift

@@ -1089,8 +1089,7 @@ func rdar74711236() {
     var s = S()
 
     s.arr = {
-      // FIXME: Missing member reference is pattern needs a better diagnostic
-      if let type = context?.store { // expected-error {{type of expression is ambiguous without more context}}
+        if let type = context?.store { // expected-error {{value of type 'Context' has no member 'store'}}
         // `isSupported` should be an invalid declaration to trigger a crash in `map(\.option)`
         let isSupported = context!.supported().contains(type)
         return (isSupported ? [type] : []).map(\.option)