Fix issues with `dynamic` in pattern-matching (#17202)

* Add breaking changes doc for post-VS2017
Fixes #17089 

docs/compilers/CSharp/Compiler Breaking Changes - post VS2017.md

+Changes since VS2017 (C# 7)
+===========================
+
+- https://github.com/dotnet/roslyn/issues/17089
+In C# 7, the compiler accepted a pattern of the form `dynamic identifier`, e.g. `if (e is dynamic x)`. This was accepted only if the expression `e` was statically of type `dynamic`. The compiler now rejects the use of the type `dynamic` for a pattern variable declaration, as no object's runtime type is `dynamic`.

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

@@ -128,6 +128,9 @@ internal BoundExpression ConvertPatternExpression(TypeSymbol inputType, CSharpSy
             return convertedExpression;
         }
 
+        /// <summary>
+        /// Check that the pattern type is valid for the operand. Return true if an error was reported.
+        /// </summary>
         private bool CheckValidPatternType(
             CSharpSyntaxNode typeSyntax,
             BoundExpression operand,
@@ -139,6 +142,11 @@ internal BoundExpression ConvertPatternExpression(TypeSymbol inputType, CSharpSy
         {
             Debug.Assert((object)operandType != null);
             Debug.Assert((object)patternType != null);
+
+            // Because we do not support recursive patterns, we always have an operand
+            Debug.Assert((object)operand != null);
+            // Once we support recursive patterns that will be relaxed.
+
             if (operandType.IsErrorType() || patternType.IsErrorType())
             {
                 return false;
@@ -156,6 +164,12 @@ internal BoundExpression ConvertPatternExpression(TypeSymbol inputType, CSharpSy
             }
             else if (!isVar)
             {
+                if (patternType.IsDynamic())
+                {
+                    Error(diagnostics, ErrorCode.ERR_PatternDynamicType, typeSyntax);
+                    return true;
+                }
+
                 HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                 Conversion conversion =
                     operand != null
@@ -164,6 +178,10 @@ internal BoundExpression ConvertPatternExpression(TypeSymbol inputType, CSharpSy
                 diagnostics.Add(typeSyntax, useSiteDiagnostics);
                 switch (conversion.Kind)
                 {
+                    case ConversionKind.ExplicitDynamic:
+                    case ConversionKind.ImplicitDynamic:
+                        // Since the input was `dynamic`, which is equivalent to `object`, there must also
+                        // exist some unboxing, identity, or reference conversion as well, making the conversion legal.
                     case ConversionKind.Boxing:
                     case ConversionKind.ExplicitNullable:
                     case ConversionKind.ExplicitReference:

src/Compilers/CSharp/Portable/Binder/Semantics/Conversions/ConversionsBase.cs

@@ -528,11 +528,6 @@ private Conversion ClassifyImplicitBuiltInConversionSlow(TypeSymbol source, Type
                 return conversion;
             }
 
-            //if (HasImplicitDynamicConversion(source, destination))
-            //{
-            //    return Conversion.ImplicitDynamic;
-            //}
-
             return Conversion.NoConversion;
         }
 
@@ -1861,14 +1856,13 @@ private static bool HasImplicitDynamicConversionFromExpression(TypeSymbol expres
             // An implicit dynamic conversion exists from an expression of type dynamic to any type T.
 
             Debug.Assert((object)destination != null);
-            return (object)expressionType != null && expressionType.Kind == SymbolKind.DynamicType && !destination.IsPointerType();
+            return expressionType?.Kind == SymbolKind.DynamicType && !destination.IsPointerType();
         }
 
         private static bool HasExplicitDynamicConversion(TypeSymbol source, TypeSymbol destination)
         {
-            // SPEC: An explicit dynamic conversion exists from an expression of type dynamic to any type T. 
-            // ISSUE: Note that this is exactly the same as an implicit dynamic conversion. Conversion of
-            // ISSUE: dynamic to int is both an explicit dynamic conversion and an implicit dynamic conversion.
+            // SPEC: An explicit dynamic conversion exists from an expression of [sic] type dynamic to any type T.
+            // ISSUE: The "an expression of" part of the spec is probably an error; see https://github.com/dotnet/csharplang/issues/132
 
             Debug.Assert((object)source != null);
             Debug.Assert((object)destination != null);

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -7495,6 +7495,15 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to It is not legal to use the type &apos;dynamic&apos; in a pattern..
+        /// </summary>
+        internal static string ERR_PatternDynamicType {
+            get {
+                return ResourceManager.GetString("ERR_PatternDynamicType", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to The switch case has already been handled by a previous case..
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5017,4 +5017,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_TypeForwardedToMultipleAssemblies" xml:space="preserve">
     <value>Module '{0}' in assembly '{1}' is forwarding the type '{2}' to multiple assemblies: '{3}' and '{4}'.</value>
   </data>
+  <data name="ERR_PatternDynamicType" xml:space="preserve">
+    <value>It is not legal to use the type 'dynamic' in a pattern.</value>
+  </data>
 </root>
\ No newline at end of file

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1457,6 +1457,7 @@ internal enum ErrorCode
         ERR_AttributesInLocalFuncDecl = 8205,
         ERR_TypeForwardedToMultipleAssemblies = 8206,
         ERR_ExpressionTreeContainsDiscard = 8207,
+        ERR_PatternDynamicType = 8208,
         #endregion more stragglers for C# 7
 
         ERR_Merge_conflict_marker_encountered = 8300,

src/Compilers/CSharp/Test/Semantic/Semantics/PatternMatchingTests.cs

@@ -5619,5 +5619,60 @@ public static void Main()
             var expectedOutput = @"eval";
             var comp = CompileAndVerify(compilation, expectedOutput: expectedOutput);
         }
+
+        [Fact]
+        [WorkItem(17089, "https://github.com/dotnet/roslyn/issues/17089")]
+        public void Dynamic_01()
+        {
+            var source =
+@"using System;
+public class X
+{
+    static void M1(dynamic d)
+    {
+        if (d is 1)
+        {
+            Console.Write('r');
+        }
+        else if (d is int i)
+        {
+            Console.Write('o');
+        }
+        else if (d is var z)
+        {
+            long l = z;
+            Console.Write('s');
+        }
+    }
+    static void M2(dynamic d)
+    {
+        switch (d)
+        {
+            case 1:
+                Console.Write('l');
+                break;
+            case int i:
+                Console.Write('y');
+                break;
+            case var z:
+                long l = z;
+                Console.Write('n');
+                break;
+        }
+    }
+    public static void Main(string[] args)
+    {
+        M1(1);
+        M1(2);
+        M1(3L);
+        M2(1);
+        M2(2);
+        M2(3L);
+    }
+}
+";
+            var compilation = CreateCompilationWithMscorlib45(source, references: new MetadataReference[] { CSharpRef, SystemCoreRef }, options: TestOptions.ReleaseExe);
+            var comp = CompileAndVerify(compilation, expectedOutput: "roslyn");
+        }
     }
 }

src/Compilers/CSharp/Test/Semantic/Semantics/PatternSwitchTests.cs

@@ -2756,7 +2756,7 @@ static void Main()
             var comp = CompileAndVerify(compilation, expectedOutput: @"pass");
         }
 
-        [Fact(Skip = "https://github.com/dotnet/roslyn/issues/17089")]
+        [Fact]
         [WorkItem(17089, "https://github.com/dotnet/roslyn/issues/17089")]
         public void Dynamic_01()
         {
@@ -2769,12 +2769,79 @@ static void Main()
         dynamic d = 1;
         switch (d)
         {
-            case dynamic x: // should be an error
+            case dynamic x: // error 1
                 break;
         }
+        if (d is dynamic y) {} // error 2
+        if (d is var z) // ok
+        {
+            long l = z;
+            string s = z;
+        }
+    }
+}
+";
+            var compilation = CreateCompilationWithMscorlib45(source, options: TestOptions.ReleaseExe);
+            compilation.VerifyDiagnostics(
+                // (9,18): error CS8207: It is not legal to use the type 'dynamic' in a pattern.
+                //             case dynamic x: // error 1
+                Diagnostic(ErrorCode.ERR_PatternDynamicType, "dynamic"),
+                // (12,18): error CS8207: It is not legal to use the type 'dynamic' in a pattern.
+                //         if (d is dynamic y) {} // error 2
+                Diagnostic(ErrorCode.ERR_PatternDynamicType, "dynamic").WithLocation(12, 18)
+                );
+        }
+
+        [Fact]
+        [WorkItem(17089, "https://github.com/dotnet/roslyn/issues/17089")]
+        public void Dynamic_02()
+        {
+            var source =
+@"
+class Program
+{
+    static void Main()
+    {
+        dynamic d = 1;
         switch (d)
         {
-            case int i: // should not be an error
+            case object x:
+            case var y: // OK, catches null
+            case var z: // error: subsumed
+                break;
+        }
+        switch (d)
+        {
+            case object x:
+            case null:
+            case var y: // error: subsumed
+                break;
+        }
+        switch (d)
+        {
+            case object x:
+            case 1: // error: subsumed
+                break;
+        }
+        switch (d)
+        {
+            case object x:
+            case int y: // error: subsumed
+                break;
+        }
+        switch (d)
+        {
+            case object x:
+            case (dynamic)null:
+            case (string)null: // error: subsumed
+                break;
+        }
+        switch (d)
+        {
+            case int i:
+            case long l:
+            case object o:
+            case var v:
                 break;
         }
     }
@@ -2782,7 +2849,21 @@ static void Main()
 ";
             var compilation = CreateCompilationWithMscorlib45(source, options: TestOptions.ReleaseExe);
             compilation.VerifyDiagnostics(
-                // Per https://github.com/dotnet/roslyn/issues/17089, the compiler does not produce correct diagnostics for this.
+                // (11,18): error CS8120: The switch case has already been handled by a previous case.
+                //             case var z: // error: subsumed
+                Diagnostic(ErrorCode.ERR_PatternIsSubsumed, "var z").WithLocation(11, 18),
+                // (18,18): error CS8120: The switch case has already been handled by a previous case.
+                //             case var y: // error: subsumed
+                Diagnostic(ErrorCode.ERR_PatternIsSubsumed, "var y").WithLocation(18, 18),
+                // (24,13): error CS8120: The switch case has already been handled by a previous case.
+                //             case 1: // error: subsumed
+                Diagnostic(ErrorCode.ERR_PatternIsSubsumed, "case 1:"),
+                // (30,18): error CS8120: The switch case has already been handled by a previous case.
+                //             case int y: // error: subsumed
+                Diagnostic(ErrorCode.ERR_PatternIsSubsumed, "int y").WithLocation(30, 18),
+                // (37,13): error CS0152: The switch statement contains multiple cases with the label value 'null'
+                //             case (string)null: // error: subsumed
+                Diagnostic(ErrorCode.ERR_DuplicateCaseLabel, "case (string)null:").WithArguments("null").WithLocation(37, 13)
                 );
         }
     }