[CodeCompletion] Add test case that we correctly compute type relations for constrained existentials

Author: ahoppen

test/IDE/complete_typerelation.swift

@@ -0,0 +1,30 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-ide-test -batch-code-completion -source-filename %s -filecheck %raw-FileCheck -completion-output-dir %t -code-complete-call-pattern-heuristics -disable-objc-attr-requires-foundation-module
+
+// SE-0353
+struct ConstraintedExistentialTest {
+  protocol Producer<Event> {
+    associatedtype Event
+  }
+
+  struct StringProducer: Producer {
+    typealias Event = String
+  }
+
+  struct IntProducer: Producer {
+    typealias Event = Int
+  }
+
+  func takeStringProducer(producer: any Producer<String>) {}
+
+  let intProducer: IntProducer
+  let stringProducer: StringProducer
+
+  func test() {
+    takeStringProducer(self.#^IN_CONSTRAINTED_EXISTENTIAL_CONTEXT^#)
+  }
+// IN_CONSTRAINTED_EXISTENTIAL_CONTEXT: Begin completions
+// IN_CONSTRAINTED_EXISTENTIAL_CONTEXT-DAG: Decl[InstanceVar]/CurrNominal:      intProducer[#IntProducer#];
+// IN_CONSTRAINTED_EXISTENTIAL_CONTEXT-DAG: Decl[InstanceVar]/CurrNominal/TypeRelation[Convertible]: stringProducer[#StringProducer#];
+// IN_CONSTRAINTED_EXISTENTIAL_CONTEXT: End completions
+}