Give an example in the non-exhaustive diagnostic (#44702)

* When reporting a switch is not exhaustive, give an example why.
Fixes #43943
* Skip a flaky test
Related to #45234

src/Compilers/CSharp/Portable/Binder/SwitchExpressionBinder.cs

@@ -93,11 +93,15 @@ private bool CheckSwitchExpressionExhaustive(
             // nulls is the job of the nullable walker.
             if (!hasErrors)
             {
-                foreach (var n in TopologicalSort.IterativeSort<BoundDecisionDagNode>(new[] { decisionDag.RootNode }, nonNullSuccessors))
+                var nodes = TopologicalSort.IterativeSort<BoundDecisionDagNode>(new[] { decisionDag.RootNode }, nonNullSuccessors);
+                foreach (var n in nodes)
                 {
                     if (n is BoundLeafDecisionDagNode leaf && leaf.Label == defaultLabel)
                     {
-                        diagnostics.Add(ErrorCode.WRN_SwitchExpressionNotExhaustive, node.SwitchKeyword.GetLocation());
+                        diagnostics.Add(
+                            ErrorCode.WRN_SwitchExpressionNotExhaustive,
+                            node.SwitchKeyword.GetLocation(),
+                            PatternExplainer.SamplePatternForPathToDagNode(BoundDagTemp.ForOriginalInput(boundInputExpression), nodes, n, nullPaths: false));
                         return true;
                     }
                 }

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5750,7 +5750,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>The syntax 'var' for a pattern is not permitted to refer to a type, but '{0}' is in scope here.</value>
   </data>
   <data name="WRN_SwitchExpressionNotExhaustive" xml:space="preserve">
-    <value>The switch expression does not handle all possible values of its input type (it is not exhaustive).</value>
+    <value>The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern '{0}' is not covered.</value>
   </data>
   <data name="WRN_SwitchExpressionNotExhaustive_Title" xml:space="preserve">
     <value>The switch expression does not handle all possible values of its input type (it is not exhaustive).</value>
@@ -5981,7 +5981,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>Type '{0}' cannot be embedded because it has a non-abstract member. Consider setting the 'Embed Interop Types' property to false.</value>
   </data>
   <data name="WRN_SwitchExpressionNotExhaustiveForNull" xml:space="preserve">
-    <value>The switch expression does not handle some null inputs (it is not exhaustive).</value>
+    <value>The switch expression does not handle some null inputs (it is not exhaustive). For example, the pattern '{0}' is not covered.</value>
   </data>
   <data name="WRN_SwitchExpressionNotExhaustiveForNull_Title" xml:space="preserve">
     <value>The switch expression does not handle some null inputs.</value>

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker_Patterns.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Linq;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.PooledObjects;
@@ -590,7 +591,13 @@ private void VisitSwitchExpressionCore(BoundSwitchExpression node, bool inferTyp
                 labelStateMap.TryGetValue(node.DefaultLabel, out var defaultLabelState) && defaultLabelState.believedReachable)
             {
                 SetState(defaultLabelState.state);
-                ReportDiagnostic(ErrorCode.WRN_SwitchExpressionNotExhaustiveForNull, ((SwitchExpressionSyntax)node.Syntax).SwitchKeyword.GetLocation());
+                var nodes = node.DecisionDag.TopologicallySortedNodes;
+                var leaf = nodes.Where(n => n is BoundLeafDecisionDagNode leaf && leaf.Label == node.DefaultLabel).First();
+                ReportDiagnostic(
+                    ErrorCode.WRN_SwitchExpressionNotExhaustiveForNull,
+                    ((SwitchExpressionSyntax)node.Syntax).SwitchKeyword.GetLocation(),
+                    PatternExplainer.SamplePatternForPathToDagNode(BoundDagTemp.ForOriginalInput(node.Expression), nodes, leaf, nullPaths: true)
+                    );
             }
 
             // collect expressions, conversions and result types

src/Compilers/CSharp/Portable/Utilities/IValueSet.cs

@@ -4,6 +4,8 @@
 
 #nullable enable
 
+using System;
+
 namespace Microsoft.CodeAnalysis.CSharp
 {
     /// <summary>
@@ -51,6 +53,12 @@ internal interface IValueSet
         /// Does this value set contain no values?
         /// </summary>
         bool IsEmpty { get; }
+
+        /// <summary>
+        /// Produce a sample value contained in the set. Throws <see cref="ArgumentException"/> if the set is empty. If the set
+        /// contains values but we cannot produce a particular value (e.g. for the set `nint > int.MaxValue`), returns null.
+        /// </summary>
+        ConstantValue? Sample { get; }
     }
 
     /// <summary>

src/Compilers/CSharp/Portable/Utilities/IValueSetFactory.cs

@@ -33,6 +33,16 @@ internal interface IValueSetFactory
         /// Produce a random value for testing.
         /// </summary>
         ConstantValue RandomValue(Random random);
+
+        /// <summary>
+        /// The set containing all values of the type.
+        /// </summary>
+        IValueSet AllValues { get; }
+
+        /// <summary>
+        /// The empty set of values.
+        /// </summary>
+        IValueSet NoValues { get; }
     }
 
     /// <summary>

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.BoolValueSet.cs

@@ -40,6 +40,9 @@ public static BoolValueSet Create(bool hasFalse, bool hasTrue)
 
             bool IValueSet.IsEmpty => !_hasFalse && !_hasTrue;
 
+            ConstantValue IValueSet.Sample => ConstantValue.Create(_hasTrue ? true : _hasFalse ? false : throw new ArgumentException());
+
+
             public bool Any(BinaryOperatorKind relation, bool value)
             {
                 switch (relation, value)

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.BoolValueSetFactory.cs

@@ -23,6 +23,10 @@ private sealed class BoolValueSetFactory : IValueSetFactory<bool>
 
             private BoolValueSetFactory() { }
 
+            IValueSet IValueSetFactory.AllValues => BoolValueSet.AllValues;
+
+            IValueSet IValueSetFactory.NoValues => BoolValueSet.None;
+
             public IValueSet<bool> Related(BinaryOperatorKind relation, bool value)
             {
                 switch (relation, value)

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.ByteTC.cs

@@ -19,6 +19,8 @@ private struct ByteTC : INumericTC<byte>
 
             byte INumericTC<byte>.MaxValue => byte.MaxValue;
 
+            byte INumericTC<byte>.Zero => 0;
+
             bool INumericTC<byte>.Related(BinaryOperatorKind relation, byte left, byte right)
             {
                 switch (relation)

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.CharTC.cs

@@ -20,6 +20,8 @@ private struct CharTC : INumericTC<char>
 
             char INumericTC<char>.MaxValue => char.MaxValue;
 
+            char INumericTC<char>.Zero => (char)0;
+
             bool INumericTC<char>.Related(BinaryOperatorKind relation, char left, char right)
             {
                 switch (relation)
@@ -49,11 +51,7 @@ char INumericTC<char>.Next(char value)
 
             string INumericTC<char>.ToString(char c)
             {
-                var isPrintable = char.IsWhiteSpace(c) ||
-                    // exclude the Unicode character categories containing non-rendering,
-                    // unknown, or incomplete characters.
-                    char.GetUnicodeCategory(c) switch { UnicodeCategory.Control => false, UnicodeCategory.OtherNotAssigned => false, UnicodeCategory.Surrogate => false, _ => true };
-                return isPrintable ? $"'{c}'" : $"\\u{(int)c:X4}";
+                return ObjectDisplay.FormatPrimitive(c, ObjectDisplayOptions.EscapeNonPrintableCharacters | ObjectDisplayOptions.UseQuotes);
             }
 
             char INumericTC<char>.Prev(char value)

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.DecimalTC.cs

@@ -30,6 +30,8 @@ private struct DecimalTC : INumericTC<decimal>
 
             decimal INumericTC<decimal>.MaxValue => decimal.MaxValue;
 
+            decimal INumericTC<decimal>.Zero => 0M;
+
             public decimal FromConstantValue(ConstantValue constantValue) => constantValue.IsBad ? 0m : constantValue.DecimalValue;
 
             public ConstantValue ToConstantValue(decimal value) => ConstantValue.Create(value);

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.DecimalValueSetFactory.cs

@@ -16,6 +16,10 @@ private sealed class DecimalValueSetFactory : IValueSetFactory<decimal>, IValueS
 
             private readonly IValueSetFactory<decimal> _underlying = NumericValueSetFactory<decimal, DecimalTC>.Instance;
 
+            IValueSet IValueSetFactory.AllValues => NumericValueSet<decimal, DecimalTC>.AllValues;
+
+            IValueSet IValueSetFactory.NoValues => NumericValueSet<decimal, DecimalTC>.NoValues;
+
             public IValueSet<decimal> Related(BinaryOperatorKind relation, decimal value) => _underlying.Related(relation, DecimalTC.Normalize(value));
 
             IValueSet IValueSetFactory.Random(int expectedSize, Random random) => _underlying.Random(expectedSize, random);

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.DoubleTC.cs

@@ -21,6 +21,8 @@ private struct DoubleTC : FloatingTC<double>, INumericTC<double>
 
             double FloatingTC<double>.NaN => double.NaN;
 
+            double INumericTC<double>.Zero => 0.0;
+
             /// <summary>
             /// The implementation of Next depends critically on the internal representation of an IEEE floating-point
             /// number.  Every bit sequence between the representation of 0 and MaxValue represents a distinct

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.EnumeratedValueSet.cs

@@ -4,6 +4,7 @@
 
 #nullable enable
 
+using System;
 using System.Collections.Immutable;
 using System.Linq;
 using Roslyn.Utilities;
@@ -24,18 +25,50 @@ private sealed class EnumeratedValueSet<T, TTC> : IValueSet<T> where TTC : struc
             /// In <see cref="_included"/>, then members are listed by inclusion.  Otherwise all members
             /// are assumed to be contained in the set unless excluded.
             /// </summary>
-            private bool _included;
+            private readonly bool _included;
 
-            private ImmutableHashSet<T> _membersIncludedOrExcluded;
+            private readonly ImmutableHashSet<T> _membersIncludedOrExcluded;
 
             private EnumeratedValueSet(bool included, ImmutableHashSet<T> membersIncludedOrExcluded) =>
                 (this._included, this._membersIncludedOrExcluded) = (included, membersIncludedOrExcluded);
 
-            public static EnumeratedValueSet<T, TTC> AllValues = new EnumeratedValueSet<T, TTC>(included: false, ImmutableHashSet<T>.Empty);
+            public static readonly EnumeratedValueSet<T, TTC> AllValues = new EnumeratedValueSet<T, TTC>(included: false, ImmutableHashSet<T>.Empty);
+
+            public static readonly EnumeratedValueSet<T, TTC> NoValues = new EnumeratedValueSet<T, TTC>(included: true, ImmutableHashSet<T>.Empty);
 
             internal static EnumeratedValueSet<T, TTC> Including(T value) => new EnumeratedValueSet<T, TTC>(included: true, ImmutableHashSet<T>.Empty.Add(value));
 
-            bool IValueSet.IsEmpty => _included && _membersIncludedOrExcluded.IsEmpty;
+            public bool IsEmpty => _included && _membersIncludedOrExcluded.IsEmpty;
+
+            ConstantValue IValueSet.Sample
+            {
+                get
+                {
+                    if (IsEmpty) throw new ArgumentException();
+                    var tc = default(TTC);
+                    if (_included)
+                        return tc.ToConstantValue(_membersIncludedOrExcluded.OrderBy(k => k).First());
+                    if (typeof(T) == typeof(string))
+                    {
+                        // try some simple strings.
+                        if (this.Any(BinaryOperatorKind.Equal, (T)(object)""))
+                            return tc.ToConstantValue((T)(object)"");
+                        for (char c = 'A'; c <= 'z'; c++)
+                            if (this.Any(BinaryOperatorKind.Equal, (T)(object)c.ToString()))
+                                return tc.ToConstantValue((T)(object)c.ToString());
+                    }
+                    // If that doesn't work, choose from a sufficiently large random selection of values.
+                    // Since this is an excluded set, they cannot all be excluded
+                    var candidates = tc.RandomValues(_membersIncludedOrExcluded.Count + 1, new Random(0), _membersIncludedOrExcluded.Count + 1);
+                    foreach (var value in candidates)
+                    {
+                        if (this.Any(BinaryOperatorKind.Equal, value))
+                            return tc.ToConstantValue(value);
+                    }
+
+                    throw ExceptionUtilities.Unreachable;
+                }
+            }
 
             public bool Any(BinaryOperatorKind relation, T value)
             {

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.EnumeratedValueSetFactory.cs

@@ -19,7 +19,11 @@ internal static partial class ValueSetFactory
         /// </summary>
         private sealed class EnumeratedValueSetFactory<T, TTC> : IValueSetFactory<T> where TTC : struct, IEquatableValueTC<T> where T : notnull
         {
-            public static EnumeratedValueSetFactory<T, TTC> Instance = new EnumeratedValueSetFactory<T, TTC>();
+            public static readonly EnumeratedValueSetFactory<T, TTC> Instance = new EnumeratedValueSetFactory<T, TTC>();
+
+            IValueSet IValueSetFactory.AllValues => EnumeratedValueSet<T, TTC>.AllValues;
+
+            IValueSet IValueSetFactory.NoValues => EnumeratedValueSet<T, TTC>.NoValues;
 
             private EnumeratedValueSetFactory() { }
 
@@ -44,11 +48,11 @@ bool IValueSetFactory.Related(BinaryOperatorKind relation, ConstantValue left, C
                 return tc.FromConstantValue(left).Equals(tc.FromConstantValue(right));
             }
 
-            IValueSet IValueSetFactory.Random(int expectedSize, Random random)
+            public IValueSet Random(int expectedSize, Random random)
             {
                 TTC tc = default;
                 T[] values = tc.RandomValues(expectedSize, random, expectedSize * 2);
-                IValueSet<T> result = EnumeratedValueSet<T, TTC>.AllValues.Complement();
+                IValueSet<T> result = EnumeratedValueSet<T, TTC>.NoValues;
                 Debug.Assert(result.IsEmpty);
                 foreach (T value in values)
                     result = result.Union(Related(Equal, value));

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.FloatingValueSet.cs

@@ -5,6 +5,7 @@
 #nullable enable
 
 using System;
+using System.Diagnostics;
 using System.Text;
 using Roslyn.Utilities;
 
@@ -47,7 +48,27 @@ internal static IValueSet<TFloating> Random(int expectedSize, Random random)
                     numbers: (IValueSet<TFloating>)NumericValueSetFactory<TFloating, TFloatingTC>.Instance.Random(expectedSize, random), hasNaN: hasNan);
             }
 
-            bool IValueSet.IsEmpty => !_hasNaN && _numbers.IsEmpty;
+            public bool IsEmpty => !_hasNaN && _numbers.IsEmpty;
+
+            ConstantValue IValueSet.Sample
+            {
+                get
+                {
+                    if (IsEmpty)
+                        throw new ArgumentException();
+
+                    if (!_numbers.IsEmpty)
+                    {
+                        var sample = _numbers.Sample;
+                        Debug.Assert(sample is { });
+                        return sample;
+                    }
+
+                    Debug.Assert(_hasNaN);
+                    var tc = default(TFloatingTC);
+                    return tc.ToConstantValue(tc.NaN);
+                }
+            }
 
             public static IValueSet<TFloating> Related(BinaryOperatorKind relation, TFloating value)
             {

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.FloatingValueSetFactory.cs

@@ -16,6 +16,10 @@ private sealed class FloatingValueSetFactory<TFloating, TFloatingTC> : IValueSet
 
             private FloatingValueSetFactory() { }
 
+            IValueSet IValueSetFactory.AllValues => FloatingValueSet<TFloating, TFloatingTC>.AllValues;
+
+            IValueSet IValueSetFactory.NoValues => FloatingValueSet<TFloating, TFloatingTC>.NoValues;
+
             public IValueSet<TFloating> Related(BinaryOperatorKind relation, TFloating value) =>
                 FloatingValueSet<TFloating, TFloatingTC>.Related(relation, value);
 

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.INumericTC.cs

@@ -59,6 +59,11 @@ private interface INumericTC<T>
             /// </summary>
             T Random(Random random);
 
+            /// <summary>
+            /// Produce the zero value for the type.
+            /// </summary>
+            T Zero { get; }
+
             /// <summary>
             /// A formatter for values of type <typeparamref name="T"/>.  This is needed for testing because
             /// the default ToString output for float and double changed between desktop and .net core,

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.IntTC.cs

@@ -19,6 +19,8 @@ private struct IntTC : INumericTC<int>
 
             int INumericTC<int>.MaxValue => int.MaxValue;
 
+            int INumericTC<int>.Zero => 0;
+
             public bool Related(BinaryOperatorKind relation, int left, int right)
             {
                 switch (relation)

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.LongTC.cs

@@ -19,6 +19,8 @@ private struct LongTC : INumericTC<long>
 
             long INumericTC<long>.MaxValue => long.MaxValue;
 
+            long INumericTC<long>.Zero => 0;
+
             bool INumericTC<long>.Related(BinaryOperatorKind relation, long left, long right)
             {
                 switch (relation)

src/Compilers/CSharp/Portable/Utilities/ValueSetFactory.NintValueSet.cs

@@ -4,6 +4,7 @@
 
 #nullable enable
 
+using System;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Roslyn.Utilities;
 
@@ -17,6 +18,8 @@ private sealed class NintValueSet : IValueSet<int>, IValueSet
         {
             public readonly static NintValueSet AllValues = new NintValueSet(hasSmall: true, values: NumericValueSet<int, IntTC>.AllValues, hasLarge: true);
 
+            public readonly static NintValueSet NoValues = new NintValueSet(hasSmall: false, values: NumericValueSet<int, IntTC>.NoValues, hasLarge: false);
+
             private readonly IValueSet<int> _values;
 
             /// <summary>
@@ -42,7 +45,24 @@ internal NintValueSet(bool hasSmall, IValueSet<int> values, bool hasLarge)
                 _hasLarge = hasLarge;
             }
 
-            bool IValueSet.IsEmpty => !_hasSmall && !_hasLarge && _values.IsEmpty;
+            public bool IsEmpty => !_hasSmall && !_hasLarge && _values.IsEmpty;
+
+            ConstantValue? IValueSet.Sample
+            {
+                get
+                {
+                    if (IsEmpty)
+                        throw new ArgumentException();
+
+                    if (!_values.IsEmpty)
+                        return _values.Sample;
+
+                    // We do not support sampling from a nint value set without a specific value. The caller
+                    // must arrange another way to get a sample, since we can return no specific value.  This
+                    // occurs when the value set was constructed from a pattern like `> (nint)int.MaxValue`.
+                    return null;
+                }
+            }
 
             public bool All(BinaryOperatorKind relation, int value)
             {