Merge pull request #14373 from OmarTawfik/fix-14287-const-tuple-deconstruction

Forbid modifiers on a deconstuction declaration

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

@@ -608,7 +608,19 @@ private static void FlattenDeconstructVariables(ArrayBuilder<DeconstructionVaria
 
         internal BoundLocalDeconstructionDeclaration BindDeconstructionDeclarationStatement(DeconstructionDeclarationStatementSyntax node, DiagnosticBag diagnostics)
         {
-            return new BoundLocalDeconstructionDeclaration(node, BindDeconstructionDeclaration(node, node.Assignment.VariableComponent, node.Assignment.Value, diagnostics));
+            bool modifierErrors;
+
+            // No modifiers are allowed in deconstruction declarations
+            ModifierUtils.MakeAndCheckNontypeMemberModifiers(
+                modifiers: node.Modifiers,
+                defaultAccess: DeclarationModifiers.None,
+                allowedModifiers: DeclarationModifiers.None,
+                errorLocation: node.Assignment.VariableComponent.Location,
+                diagnostics: diagnostics,
+                modifierErrors: out modifierErrors);
+
+            var assignment = BindDeconstructionDeclaration(node, node.Assignment.VariableComponent, node.Assignment.Value, diagnostics);
+            return new BoundLocalDeconstructionDeclaration(node, assignment, hasErrors: modifierErrors);
         }
 
         internal BoundDeconstructionAssignmentOperator BindDeconstructionDeclaration(CSharpSyntaxNode node, VariableComponentSyntax declaration, ExpressionSyntax right,

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

@@ -2546,5 +2546,46 @@ static void M((int, int) t)
             }
         }
 
+        [Fact, WorkItem(14287, "https://github.com/dotnet/roslyn/issues/14287")]
+        public void TupleDeconstructionStatementWithTypesCannotBeConst()
+        {
+            string source = @"
+class C
+{
+    static void Main()
+    {
+        const (int x, int y) = (1, 2);
+    }
+}
+";
+
+            var comp = CreateCompilationWithMscorlib(source, references: new[] { ValueTupleRef, SystemRuntimeFacadeRef });
+            comp.VerifyDiagnostics(
+                // (6,15): error CS0106: The modifier 'const' is not valid for this item
+                //         const (int x, int y) = (1, 2);
+                Diagnostic(ErrorCode.ERR_BadMemberFlag, "(int x, int y)").WithArguments("const").WithLocation(6, 15)
+                );
+        }
+
+        [Fact, WorkItem(14287, "https://github.com/dotnet/roslyn/issues/14287")]
+        public void TupleDeconstructionStatementWithoutTypesCannotBeConst()
+        {
+            string source = @"
+class C
+{
+    static void Main()
+    {
+        const var (x, y) = (1, 2);
+    }
+}
+";
+
+            var comp = CreateCompilationWithMscorlib(source, references: new[] { ValueTupleRef, SystemRuntimeFacadeRef });
+            comp.VerifyDiagnostics(
+                // (6,15): error CS0106: The modifier 'const' is not valid for this item
+                //         const var (x, y) = (1, 2);
+                Diagnostic(ErrorCode.ERR_BadMemberFlag, "var (x, y)").WithArguments("const").WithLocation(6, 15)
+                );
+        }
     }
 }