Emit warning when using the Yul optimiser

test/cmdlineTests/gas_test_abiv2_optimize_yul/err

@@ -1,3 +1,4 @@
+Warning: The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.
 gas_test_abiv2_optimize_yul/input.sol:2:1: Warning: Experimental features are turned on. Do not use experimental features on live deployments.
 pragma experimental ABIEncoderV2;
 ^-------------------------------^

test/cmdlineTests/standard_optimizer_yul/input.json

@@ -0,0 +1,17 @@
+{
+	"language": "Solidity",
+	"sources":
+	{
+		"A":
+		{
+			"content": "pragma solidity >=0.0; contract C { function f() public pure {} }"
+		}
+	},
+	"settings":
+	{
+		"optimizer": {
+			"enabled": true,
+			"details": { "yul": true }
+		}
+	}
+}

test/cmdlineTests/standard_optimizer_yul/output.json

@@ -0,0 +1 @@
+{"errors":[{"component":"general","formattedMessage":"Warning: The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.\n","message":"The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.","severity":"warning","type":"Warning"}],"sources":{"A":{"id":0}}}

test/cmdlineTests/standard_optimizer_yulDetails/input.json

@@ -0,0 +1,16 @@
+{
+	"language": "Solidity",
+	"sources":
+	{
+		"A":
+		{
+			"content": "pragma solidity >=0.0; contract C { function f() public pure {} }"
+		}
+	},
+	"settings":
+	{
+		"optimizer": {
+			"details": { "yul": true, "yulDetails": {} }
+		}
+	}
+}

test/cmdlineTests/standard_optimizer_yulDetails/output.json

@@ -0,0 +1 @@
+{"errors":[{"component":"general","formattedMessage":"Warning: The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.\n","message":"The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.","severity":"warning","type":"Warning"}],"sources":{"A":{"id":0}}}

test/libsolidity/StandardCompiler.cpp

@@ -996,6 +996,12 @@ BOOST_AUTO_TEST_CASE(optimizer_settings_details_different)
 	)";
 	Json::Value result = compile(input);
 	BOOST_CHECK(containsAtMostWarnings(result));
+	BOOST_CHECK(containsError(
+		result,
+		"Warning",
+		"The Yul optimiser is still experimental. "
+		"Do not use it in production unless correctness of generated code is verified with extensive tests."
+	));
 	Json::Value contract = getContractResult(result, "fileA", "A");
 	BOOST_CHECK(contract.isObject());
 	BOOST_CHECK(contract["metadata"].isString());