@@ -0,0 +1,120 @@
+//
+// Copyright (c) 2019-2022 Angouri.
+// AngouriMath is licensed under MIT.
+// Details: https://github.com/asc-community/AngouriMath/blob/master/LICENSE.md.
+// Website: https://am.angouri.org.
+//
+
+using AngouriMath;
+using AngouriMath.Core;
+using AngouriMath.Extensions;
+using Xunit;
+
+namespace AngouriMath.Tests.Common
+{
+    /// <summary>
+    /// Issues that are open in the tracker but no longer reproduce. Nothing here is a
+    /// fix: each test pins behaviour that already works, so that its issue can be closed
+    /// without leaving the fix that closed it unprotected. Every case is the reporter's
+    /// own, taken from the issue.
+    /// </summary>
+    public sealed class AlreadyFixedIssuesTest
+    {
+        private static readonly System.TimeSpan Budget = System.TimeSpan.FromSeconds(30);
+
+        /// <summary>
+        /// Runs work that used to hang or overflow the stack, and fails if it does not
+        /// finish. Asserted as termination rather than as a duration, so the test does not
+        /// turn flaky on a slow machine, and so a regression fails the run instead of
+        /// hanging it.
+        /// </summary>
+        private static T Terminates<T>(System.Func<T> work, string what)
+        {
+            var task = System.Threading.Tasks.Task.Run(work);
+            Assert.True(task.Wait(Budget), $"{what} did not finish within {Budget.TotalSeconds}s");
+            return task.Result;
+        }
+
+        // https://github.com/asc-community/AngouriMath/issues/263
+        // InnerSimplify never returned on the reporter's quotient, and neither did the
+        // limit it was reached from.
+        private const string Issue263Quotient =
+            "(x ^ 15 - 3 ^ 15 - 15 * 3 ^ 14 * (x - 3)) / (x - 3) ^ 2";
+
+        [Fact]
+        public void Issue263_InnerSimplifyTerminates() =>
+            Terminates(() => Issue263Quotient.ToEntity().InnerSimplified,
+                "InnerSimplify of the #263 quotient");
+
+        [Fact]
+        public void Issue263_LimitTerminates() =>
+            Terminates(() => Issue263Quotient.ToEntity().Limit("x", 3, ApproachFrom.Left),
+                "the #263 limit");
+
+        // https://github.com/asc-community/AngouriMath/issues/347
+        // Taking a limit of a boolean expression overflowed the stack. It is still not
+        // evaluated -- there is nothing to evaluate -- but it comes back rather than
+        // taking the process down.
+        [Fact]
+        public void Issue347_LimitOfABooleanDoesNotCrash() =>
+            Terminates(() => "a and x".ToEntity().Limit("x", "+oo"), "lim (a and x)");
+
+        // https://github.com/asc-community/AngouriMath/issues/362
+        // Simplified to (-3/2 * d - d / 2) * t, which is the same number written worse.
+        [Fact]
+        public void Issue362_CoefficientsAreCollected() =>
+            Assert.Equal("(-2) * d * t", "(-1/2d-3/2d)t".ToEntity().Simplify().Stringize());
+
+        // https://github.com/asc-community/AngouriMath/issues/424
+        // Piecewise was to become compileable.
+        [Theory]
+        [InlineData("5 provided x = 3", 3.0, 5.0)]
+        [InlineData("x provided true", 7.0, 7.0)]
+        [InlineData("2 * x provided x > 0", 4.0, 8.0)]
+        public void Issue424_PiecewiseCompiles(string expression, double argument, double expected) =>
+            Assert.Equal(expected, expression.ToEntity().Compile<double, double>("x")(argument), 9);
+
+        // https://github.com/asc-community/AngouriMath/issues/415
+        // Intersections and unions of intervals were to be simplified.
+        [Fact]
+        public void Issue415_IntervalsIntersect() =>
+            Assert.Equal(@"[3; 5]".ToEntity(), @"[1; 5] /\ [3; 8]".ToEntity().Simplify());
+
+        [Fact]
+        public void Issue415_IntervalsUnite() =>
+            Assert.Equal(@"[1; 8]".ToEntity(), @"[1; 5] \/ [3; 8]".ToEntity().Simplify());
+
+        // https://github.com/asc-community/AngouriMath/issues/550
+        // The reporter's small system did not solve. The 25x26 one from the same report
+        // is a question of scale and is tracked separately at #608.
+        [Fact]
+        public void Issue550_SmallLinearSystemSolves()
+        {
+            var equations = new[]
+            {
+                "x - y + 2 * z - 6",
+                "2 * x + 3 * y + 2 * z - 11",
+                "x + 2 * y + z - 8"
+            };
+            var solution = MathS.Equations(equations[0], equations[1], equations[2]).Solve("x", "y", "z");
+            Assert.NotNull(solution);
+            var (x, y, z) = (solution![0, 0], solution[0, 1], solution[0, 2]);
+            // Checked by substitution rather than against printed values, so the test does
+            // not depend on which form the solver hands the roots back in.
+            foreach (var equation in equations)
+            {
+                var residual = equation.ToEntity()
+                    .Substitute("x", x).Substitute("y", y).Substitute("z", z)
+                    .EvalNumerical();
+                Assert.True(residual.Abs().EDecimal.ToDouble() < 1e-9,
+                    $"{equation} left {residual.Stringize()} at the reported solution");
+            }
+        }
+
+        // https://github.com/asc-community/AngouriMath/issues/164
+        // Reported as leaving sixteen uncollected terms.
+        [Fact]
+        public void Issue164_RepeatedFactorCollects() =>
+            Assert.Equal("(1 + x) ^ 4".ToEntity(), "(x+1)^2*(x+2-1)^2".ToEntity().Simplify());
+    }
+}