[Diagnostics] Assign missing member eagerly when there is no context

Without contextual information it won't be possible to bind a missing
member to a concrete type later, so let's bind them eagerly and propagate
placeholders outward.

Resolves: rdar://152021264
Resolves: swiftlang#81770

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -3484,6 +3484,15 @@ class ConstraintSystem {
     return nullptr;
   }
 
+  Expr *getSemanticsProvidingParentExpr(Expr *expr) {
+    while (auto *parent = getParentExpr(expr)) {
+      if (parent->getSemanticsProvidingExpr() == parent)
+        return parent;
+      expr = parent;
+    }
+    return nullptr;
+  }
+
   /// Retrieve the depth of the given expression.
   std::optional<unsigned> getExprDepth(Expr *expr) {
     if (auto result = getExprDepthAndParent(expr))

lib/Sema/CSSimplify.cpp

@@ -11438,6 +11438,14 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
             // `key path` constraint can't be retired until all components
             // are simplified.
             addTypeVariableConstraintsToWorkList(memberTypeVar);
+          } else if (locator->getAnchor().is<Expr *>() &&
+                     !getSemanticsProvidingParentExpr(
+                         getAsExpr(locator->getAnchor()))) {
+            // If there are no contextual expressions that could provide
+            // a type for the member type variable, let's default it to
+            // a placeholder eagerly so it could be propagated to the
+            // pattern if necessary.
+            recordTypeVariablesAsHoles(memberTypeVar);
           } else if (locator->isLastElement<LocatorPathElt::PatternMatch>()) {
             // Let's handle member patterns specifically because they use
             // equality instead of argument application constraint, so allowing

test/Constraints/closures.swift

@@ -803,7 +803,6 @@ overloaded { print("hi"); print("bye") } // multiple expression closure without
 // expected-error@-1 {{ambiguous use of 'overloaded'}}
 
 func not_overloaded(_ handler: () -> Int) {}
-// expected-note@-1 {{'not_overloaded' declared here}}
 
 not_overloaded { } // empty body
 // expected-error@-1 {{cannot convert value of type '()' to closure result type 'Int'}}
@@ -1347,3 +1346,23 @@ do {
   let _ = V(value: { [Int]() }) // expected-error {{add () to forward '@autoclosure' parameter}} {{31-31=()}}
   let _ = V(other: { [Int]() }) // expected-error {{cannot convert value of type '[Int]' to closure result type 'String'}}
 }
+
+// https://github.com/swiftlang/swift/issues/81770
+do {
+  func test(_: Int) {}
+  func test(_: Int = 42, _: (Int) -> Void) {}
+
+  test {
+    if let _ = $0.missing { // expected-error {{value of type 'Int' has no member 'missing'}}
+    }
+  }
+
+  test {
+    if let _ = (($0.missing)) { // expected-error {{value of type 'Int' has no member 'missing'}}
+    }
+  }
+
+  test { // expected-error {{invalid conversion from throwing function of type '(Int) throws -> Void' to non-throwing function type '(Int) -> Void'}}
+    try $0.missing // expected-error {{value of type 'Int' has no member 'missing'}}
+  }
+}