[CSDiagnostics] Diagnose requirement failures in closure parameter positions

Fix "affected" declaration computation to handle situations when
a generic type happens to be in a closure parameter position.

Resolves: rdar://150068895

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -398,6 +398,14 @@ ValueDecl *RequirementFailure::getDeclRef() const {
     }
   }
 
+  if (auto *closure = getAsExpr<ClosureExpr>(getRawAnchor())) {
+    if (auto argElt =
+            getLocator()->getFirstElementAs<LocatorPathElt::TupleElement>()) {
+      auto *param = (*closure->getParameters())[argElt->getIndex()];
+      return getAffectedDeclFromType(getType(param));
+    }
+  }
+
   return getAffectedDeclFromType(getOwnerType());
 }
 

test/Constraints/closures.swift

@@ -1366,3 +1366,29 @@ do {
     try $0.missing // expected-error {{value of type 'Int' has no member 'missing'}}
   }
 }
+
+// Generic requirement failures associated with closure parameters should be diagnosed.
+protocol Input {
+  associatedtype Value
+  var value: Value { get }
+}
+
+protocol Idable {
+  associatedtype ID
+}
+
+struct TestInput<ItemID: Hashable, Value: Collection<ItemID>> : Input { // expected-note {{where 'Value' = 'Item.ID'}}
+  var value: Value
+}
+
+struct Container<I: Input> {
+  var data: (I) -> Void
+}
+
+func test_generic_closure_parameter_requirement_failure<Item: Idable>(
+  for itemType: Item.Type = Item.self,
+  _ payload: @escaping (_ itemID: Item.ID) -> Void
+) where Item.ID : Sendable {
+  Container(data: { (input: TestInput) in payload(input.value) })
+  // expected-error@-1 {{generic struct 'TestInput' requires that 'Item.ID' conform to 'Collection'}}
+}