Sema: Fix crash with circular type witness edge case

Fixes #75371

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -4683,7 +4683,10 @@ void ConformanceChecker::resolveSingleWitness(ValueDecl *requirement) {
                                       ReferencedAssociatedTypesRequest{requirement},
                                       TinyPtrVector<AssociatedTypeDecl *>());
   for (auto assocType : referenced) {
-    if (Conformance->getTypeWitness(assocType)->hasError()) {
+    auto typeWitness = Conformance->getTypeWitness(assocType);
+    if (!typeWitness)
+      return;
+    if (typeWitness->hasError()) {
       Conformance->setInvalid();
       return;
     }

test/AssociatedTypeInference/issue-75371.swift

@@ -0,0 +1,18 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+// https://github.com/swiftlang/swift/issues/75371
+
+public protocol MyProtocol {
+    associatedtype Value = Self
+
+    static var originalValue: Value { get }
+    static var copyValue: Value { get }
+}
+
+public struct MyStruct: MyProtocol {}
+
+extension MyStruct {
+    public static let originalValue = Self()
+    public static let copyValue = originalValue
+}
+