[CSSimplify] Open key path type before assignment to its record requirements

`Root` and `Value` generic parameters now have conformance requirements
which we need to account for post-binding inference.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -12267,6 +12267,21 @@ bool ConstraintSystem::resolveKeyPath(TypeVariableType *typeVar,
                                       Type contextualType,
                                       TypeMatchOptions flags,
                                       ConstraintLocatorBuilder locator) {
+  // Key path types recently gained Copyable, Escapable requirements.
+  // The solver cannot account for that during inference because Root
+  // and Value types are required to be only resolved enough to infer
+  // a capability of a key path itself.
+  if (auto *BGT = contextualType->getAs<BoundGenericType>()) {
+    auto keyPathTy = openUnboundGenericType(
+        BGT->getDecl(), BGT->getParent(), locator, /*isTypeResolution=*/false);
+
+    assignFixedType(
+        typeVar, keyPathTy, /*updateState=*/true,
+        /*notifyInference=*/!flags.contains(TMF_BindingTypeVariable));
+    addConstraint(ConstraintKind::Equal, keyPathTy, contextualType, locator);
+    return true;
+  }
+
   assignFixedType(typeVar, contextualType, /*updateState=*/true,
                   /*notifyInference=*/!flags.contains(TMF_BindingTypeVariable));
   return true;