[CSDiagnostics] Tailor diagnostic for Copyable conformance mismatch in key path context

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -683,6 +683,15 @@ bool MissingConformanceFailure::diagnoseAsError() {
     }
   }
 
+  if (isExpr<KeyPathExpr>(anchor)) {
+    if (auto *P = dyn_cast<ProtocolDecl>(protocolType->getAnyNominal())) {
+      if (P->isSpecificProtocol(KnownProtocolKind::Copyable)) {
+        emitDiagnostic(diag::expr_keypath_noncopyable_type, nonConformingType);
+        return true;
+      }
+    }
+  }
+
   if (diagnoseAsAmbiguousOperatorRef())
     return true;