Penalize type mismatches that lead to @dynamicMemberLookup types

Author: Greg Titus

lib/Sema/CSSimplify.cpp

@@ -15269,12 +15269,19 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
     // Passing a closure to a parameter that doesn't expect one should
     // be scored lower because there might be an overload that expects
     // a closure but has other issues e.g. wrong number of parameters.
-    if (!type2->lookThroughAllOptionalTypes()->is<FunctionType>()) {
+    auto nonOptional2 = type2->lookThroughAllOptionalTypes();
+    if (!nonOptional2->is<FunctionType>()) {
       auto argument = simplifyLocatorToAnchor(fix->getLocator());
       if (isExpr<ClosureExpr>(argument)) {
         impact += 2;
       }
     }
+    // A parameter expecting dynamic member lookup may mask other
+    // missing member errors. Discourage choosing this overload if
+    // there are other options.
+    if (nonOptional2->hasDynamicMemberLookupAttribute()) {
+      impact += 6;
+    }
 
     // De-prioritize `Builtin.RawPointer` and `OpaquePointer` parameters
     // because they usually clash with generic parameter mismatches e.g.

test/Constraints/issue-74700.swift

@@ -0,0 +1,29 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol Idable<ID> {
+  associatedtype ID
+  var id: ID { get }
+}
+func test74700() {
+  @dynamicMemberLookup struct Binding<C> {
+    subscript(dynamicMember member: String) -> String { "" }
+  }
+  struct ForEach<Data, ID, Content> where Data : RandomAccessCollection, ID: Hashable {
+    init(_ data: Data, content: (Data.Element) -> Content) where ID == Data.Element.ID, Data.Element : Idable {}
+    // expected-note@-1 {{where 'Data.Element' = 'User'}}
+    init<C>(_ data: Binding<C>, content: (Binding<C.Element>) -> Content) where Data == Array<C>, ID == C.Element.ID, C : MutableCollection, C : RandomAccessCollection, C.Element : Idable, C.Index : Hashable {}
+  }
+
+  struct User {
+    let name: String // expected-note {{'name' declared here}}
+  }
+
+  struct ContentView {
+    let users: [User] = []
+    func body() -> Void {
+      ForEach(users) { user in // expected-error {{initializer 'init(_:content:)' requires that 'User' conform to 'Idable'}}
+        return user.nam // expected-error {{value of type 'User' has no member 'nam'; did you mean 'name'?}}
+      }
+    }
+  }
+}