Add tests for old solc detection

packages/hardhat-core/test/builtin-tasks/compile.ts

@@ -4,9 +4,11 @@ import * as path from "path";
 
 import { TASK_COMPILE_SOLIDITY_GET_COMPILATION_JOBS_FAILURE_REASONS } from "../../src/builtin-tasks/task-names";
 import { SOLIDITY_FILES_CACHE_FILENAME } from "../../src/internal/constants";
+import { ERRORS } from "../../src/internal/core/errors-list";
 import { globSync } from "../../src/internal/util/glob";
 import { CompilationJobCreationErrorReason } from "../../src/types/builtin-tasks";
 import { useEnvironment } from "../helpers/environment";
+import { expectHardhatErrorAsync } from "../helpers/errors";
 import { useFixtureProject } from "../helpers/project";
 import { mockFile } from "../utils/mock-file";
 
@@ -709,4 +711,38 @@ Read about compiler configuration at https://hardhat.org/config
       );
     });
   });
+
+  describe("old versions of solidity", function () {
+    useFixtureProject("old-solidity-versions");
+
+    describe("project with an old version of solidity", function () {
+      useEnvironment("old-solidity-version.js");
+
+      it("should throw an error", async function () {
+        await expectHardhatErrorAsync(async () => {
+          await this.env.run("compile");
+        }, ERRORS.BUILTIN_TASKS.COMPILE_TASK_UNSUPPORTED_SOLC_VERSION);
+      });
+    });
+
+    describe("project with an old version of solidity (multiple compilers)", function () {
+      useEnvironment("old-solidity-version-multiple-compilers.js");
+
+      it("should throw an error", async function () {
+        await expectHardhatErrorAsync(async () => {
+          await this.env.run("compile");
+        }, ERRORS.BUILTIN_TASKS.COMPILE_TASK_UNSUPPORTED_SOLC_VERSION);
+      });
+    });
+
+    describe("project with an old version of solidity in an override", function () {
+      useEnvironment("old-solidity-version-in-override.js");
+
+      it("should throw an error", async function () {
+        await expectHardhatErrorAsync(async () => {
+          await this.env.run("compile");
+        }, ERRORS.BUILTIN_TASKS.COMPILE_TASK_UNSUPPORTED_SOLC_VERSION);
+      });
+    });
+  });
 });

packages/hardhat-core/test/fixture-projects/old-solidity-versions/contracts/Foo.sol

@@ -0,0 +1,3 @@
+pragma solidity ^0.4.0;
+
+contract Foo {}

packages/hardhat-core/test/fixture-projects/old-solidity-versions/old-solidity-version-in-override.js

@@ -0,0 +1,12 @@
+module.exports = {
+  solidity: {
+    compilers: [
+      {
+        version: "0.7.0",
+      },
+    ],
+    overrides: {
+      "contracts/Foo.sol": { version: "0.4.8" },
+    },
+  },
+};

packages/hardhat-core/test/fixture-projects/old-solidity-versions/old-solidity-version-multiple-compilers.js

@@ -0,0 +1,12 @@
+module.exports = {
+  solidity: {
+    compilers: [
+      {
+        version: "0.8.6",
+      },
+      {
+        version: "0.4.9",
+      },
+    ],
+  },
+};

packages/hardhat-core/test/fixture-projects/old-solidity-versions/old-solidity-version.js

@@ -0,0 +1,3 @@
+module.exports = {
+  solidity: "0.4.10",
+};

packages/hardhat-core/test/helpers/environment.ts

@@ -7,12 +7,16 @@ declare module "mocha" {
   }
 }
 
-export function useEnvironment() {
+export function useEnvironment(configPath?: string) {
   beforeEach("Load environment", function () {
+    if (configPath !== undefined) {
+      process.env.HARDHAT_CONFIG = configPath;
+    }
     this.env = require("../../src/internal/lib/hardhat-lib");
   });
 
   afterEach("reset hardhat context", function () {
+    delete process.env.HARDHAT_CONFIG;
     resetHardhatContext();
   });
 }