[Diagnostics] Ignore result type failures if one side is a hole

If one of the sides in a application result conversion is a hole
it could only mean that the fix has been recorded earlier (at the
point where hole was introduced) and failure at function result
position could be safely ignored.

Resolves: rdar://79757320

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -4621,6 +4621,12 @@ bool ConstraintSystem::repairFailures(
   }
 
   case ConstraintLocator::FunctionResult: {
+    if (lhs->isPlaceholder() || rhs->isPlaceholder()) {
+      recordAnyTypeVarAsPotentialHole(lhs);
+      recordAnyTypeVarAsPotentialHole(rhs);
+      return true;
+    }
+
     auto *loc = getConstraintLocator(anchor, {path.begin(), path.end() - 1});
     // If this is a mismatch between contextual type and (trailing)
     // closure with explicitly specified result type let's record it
@@ -4641,6 +4647,7 @@ bool ConstraintSystem::repairFailures(
         break;
       }
     }
+
     // Handle function result coerce expression wrong type conversion.
     if (isExpr<CoerceExpr>(anchor)) {
       auto *fix =

test/Constraints/generics.swift

@@ -905,3 +905,28 @@ func rdar78781552() {
     // expected-error@-4 {{missing argument for parameter 'filter' in call}}
   }
 }
+
+// rdar://79757320 - failured to produce a diagnostic when unresolved dependent member is used in function result position
+
+protocol R_79757320 {
+  associatedtype In
+  associatedtype Out
+}
+
+func rdar79757320() {
+  struct Value<W> {
+    init(_: W) {}
+
+    func formatted() -> String { "" }
+    func formatted<T : R_79757320>(_: T) -> T.Out where T.In == Value<W> { fatalError() }
+  }
+
+  struct Container {
+    var value: String
+  }
+
+  // FIXME: There has to be a way to propagate holes that makes it easy to suppress failures caused by missing members.
+  _ = Container(value: Value(42).formatted(.S(a: .a, b: .b(0)))) // expected-error {{type 'R_79757320' has no member 'S'}}
+  // expected-error@-1 {{cannot infer contextual base in reference to member 'a'}}
+  // expected-error@-2 {{cannot infer contextual base in reference to member 'b'}}
+}