Merge pull request #28115 from wachulski/fix/27866/else-without-if-error-message

Fix 'else without if' parsing - better error message

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

@@ -3930,6 +3930,15 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to &apos;else&apos; cannot start a statement..
+        /// </summary>
+        internal static string ERR_ElseCannotStartStatement {
+            get {
+                return ResourceManager.GetString("ERR_ElseCannotStartStatement", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Empty character literal.
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5638,4 +5638,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ExpressionTreeCantContainRefStruct" xml:space="preserve">
     <value>Expression tree cannot contain value of ref struct or restricted type '{0}'.</value>
   </data>
-</root>
+  <data name="ERR_ElseCannotStartStatement" xml:space="preserve">
+    <value>'else' cannot start a statement.</value>
+  </data>
+</root>
\ No newline at end of file

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

@@ -1635,7 +1635,8 @@ internal enum ErrorCode
         ERR_NullableDirectiveQualifierExpected = 8637,
         WRN_CantInferNullabilityOfMethodTypeArgs = 8638,
         WRN_NoBestNullabilityArrayElements = 8639,
-        ERR_ExpressionTreeCantContainRefStruct = 8640
+        ERR_ExpressionTreeCantContainRefStruct = 8640,
+        ERR_ElseCannotStartStatement = 8641
 
         #endregion diagnostics introduced for C# 8.0
 

src/Compilers/CSharp/Portable/Parser/LanguageParser.cs

@@ -6678,6 +6678,7 @@ private StatementSyntax ParseStatementNoDeclaration(bool allowAnyExpression)
                 case SyntaxKind.GotoKeyword:
                     return this.ParseGotoStatement();
                 case SyntaxKind.IfKeyword:
+                case SyntaxKind.ElseKeyword: // Including 'else' keyword to handle 'else without if' error cases 
                     return this.ParseIfStatement();
                 case SyntaxKind.LockKeyword:
                     return this.ParseLockStatement();
@@ -7338,6 +7339,7 @@ private bool IsPossibleStatement(bool acceptAccessibilityMods)
                 case SyntaxKind.ForEachKeyword:
                 case SyntaxKind.GotoKeyword:
                 case SyntaxKind.IfKeyword:
+                case SyntaxKind.ElseKeyword:
                 case SyntaxKind.LockKeyword:
                 case SyntaxKind.ReturnKeyword:
                 case SyntaxKind.SwitchKeyword:
@@ -7933,14 +7935,18 @@ private GotoStatementSyntax ParseGotoStatement()
 
         private IfStatementSyntax ParseIfStatement()
         {
-            Debug.Assert(this.CurrentToken.Kind == SyntaxKind.IfKeyword);
-            var @if = this.EatToken(SyntaxKind.IfKeyword);
+            Debug.Assert(this.CurrentToken.Kind == SyntaxKind.IfKeyword || this.CurrentToken.Kind == SyntaxKind.ElseKeyword);
+
+            bool firstTokenIsElse = this.CurrentToken.Kind == SyntaxKind.ElseKeyword;
+            var @if = firstTokenIsElse
+                ? this.EatToken(SyntaxKind.IfKeyword, ErrorCode.ERR_ElseCannotStartStatement)
+                : this.EatToken(SyntaxKind.IfKeyword);
             var openParen = this.EatToken(SyntaxKind.OpenParenToken);
             var condition = this.ParseExpressionCore();
             var closeParen = this.EatToken(SyntaxKind.CloseParenToken);
-            var statement = this.ParseEmbeddedStatement();
-            var elseClause = ParseElseClauseOpt();
-
+            var statement = firstTokenIsElse ? this.ParseExpressionStatement() : this.ParseEmbeddedStatement();
+            var elseClause = this.ParseElseClauseOpt();
+            
             return _syntaxFactory.IfStatement(@if, openParen, condition, closeParen, statement, elseClause);
         }
 
@@ -8775,6 +8781,7 @@ private static bool IsInvalidSubExpression(SyntaxKind kind)
                 case SyntaxKind.ForEachKeyword:
                 case SyntaxKind.GotoKeyword:
                 case SyntaxKind.IfKeyword:
+                case SyntaxKind.ElseKeyword:
                 case SyntaxKind.LockKeyword:
                 case SyntaxKind.ReturnKeyword:
                 case SyntaxKind.SwitchKeyword:

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

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf

@@ -47,6 +47,11 @@
         <target state="new">Type parameter '{1}' has the 'unmanaged' constraint so '{1}' cannot be used as a constraint for '{0}'</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_ElseCannotStartStatement">
+        <source>'else' cannot start a statement.</source>
+        <target state="new">'else' cannot start a statement.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_ExplicitNonNullTypesAttribute">
         <source>Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</source>
         <target state="new">Explicit application of 'System.Runtime.CompilerServices.NonNullTypesAttribute' is not allowed.</target>

src/Compilers/CSharp/Test/Syntax/Parsing/StatementParsingTests.cs

@@ -2054,6 +2054,40 @@ public void TestIfElse()
             Assert.NotNull(ss.Else.Statement);
         }
 
+        [Fact]
+        public void TestIfElseIf()
+        {
+            var text = "if (a) { } else if (b) { }";
+            var statement = this.ParseStatement(text);
+
+            Assert.NotNull(statement);
+            Assert.Equal(SyntaxKind.IfStatement, statement.Kind());
+            Assert.Equal(text, statement.ToString());
+            Assert.Equal(0, statement.Errors().Length);
+
+            var ss = (IfStatementSyntax)statement;
+            Assert.NotNull(ss.IfKeyword);
+            Assert.Equal(SyntaxKind.IfKeyword, ss.IfKeyword.Kind());
+            Assert.NotNull(ss.OpenParenToken);
+            Assert.NotNull(ss.Condition);
+            Assert.Equal("a", ss.Condition.ToString());
+            Assert.NotNull(ss.CloseParenToken);
+            Assert.NotNull(ss.Statement);
+
+            Assert.NotNull(ss.Else);
+            Assert.NotNull(ss.Else.ElseKeyword);
+            Assert.Equal(SyntaxKind.ElseKeyword, ss.Else.ElseKeyword.Kind());
+            Assert.NotNull(ss.Else.Statement);
+
+            var subIf = (IfStatementSyntax) ss.Else.Statement;
+            Assert.NotNull(subIf.IfKeyword);
+            Assert.Equal(SyntaxKind.IfKeyword, subIf.IfKeyword.Kind());
+            Assert.NotNull(subIf.Condition);
+            Assert.Equal("b", subIf.Condition.ToString());
+            Assert.NotNull(subIf.CloseParenToken);
+            Assert.NotNull(subIf.Statement);
+        }
+
         [Fact]
         public void TestLock()
         {
@@ -2741,6 +2775,311 @@ public void NullExceptionInLabeledStatement()
                 );
         }
 
+        [WorkItem(27866, "https://github.com/dotnet/roslyn/issues/27866")]
+        [Fact]
+        public void ParseElseWithoutPrecedingIfStatement()
+        {
+            UsingStatement("else {}",
+                // (1,1): error CS8641: 'else' cannot start a statement.
+                // else {}
+                Diagnostic(ErrorCode.ERR_ElseCannotStartStatement, "else").WithLocation(1, 1),
+                // (1,1): error CS1003: Syntax error, '(' expected
+                // else {}
+                Diagnostic(ErrorCode.ERR_SyntaxError, "else").WithArguments("(", "else").WithLocation(1, 1),
+                // (1,1): error CS1525: Invalid expression term 'else'
+                // else {}
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 1),
+                // (1,1): error CS1026: ) expected
+                // else {}
+                Diagnostic(ErrorCode.ERR_CloseParenExpected, "else").WithLocation(1, 1),
+                // (1,1): error CS1525: Invalid expression term 'else'
+                // else {}
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 1),
+                // (1,1): error CS1002: ; expected
+                // else {}
+                Diagnostic(ErrorCode.ERR_SemicolonExpected, "else").WithLocation(1, 1)
+                );
+            N(SyntaxKind.IfStatement);
+            {
+                M(SyntaxKind.IfKeyword);
+                M(SyntaxKind.OpenParenToken);
+                M(SyntaxKind.IdentifierName);
+                {
+                    M(SyntaxKind.IdentifierToken);
+                }
+                M(SyntaxKind.CloseParenToken);
+                M(SyntaxKind.ExpressionStatement);
+                {
+                    M(SyntaxKind.IdentifierName);
+                    {
+                        M(SyntaxKind.IdentifierToken);
+                    }
+                    M(SyntaxKind.SemicolonToken);
+                }
+                N(SyntaxKind.ElseClause);
+                {
+                    N(SyntaxKind.ElseKeyword);
+                    N(SyntaxKind.Block);
+                    {
+                        N(SyntaxKind.OpenBraceToken);
+                        N(SyntaxKind.CloseBraceToken);
+                    }
+                }
+            }
+            EOF();
+        }
+
+        [WorkItem(27866, "https://github.com/dotnet/roslyn/issues/27866")]
+        [Fact]
+        public void ParseElseAndElseWithoutPrecedingIfStatement()
+        {
+            UsingStatement("{ else {} else {} }",
+                // (1,3): error CS8641: 'else' cannot start a statement.
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_ElseCannotStartStatement, "else").WithLocation(1, 3),
+                // (1,3): error CS1003: Syntax error, '(' expected
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_SyntaxError, "else").WithArguments("(", "else").WithLocation(1, 3),
+                // (1,3): error CS1525: Invalid expression term 'else'
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 3),
+                // (1,3): error CS1026: ) expected
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_CloseParenExpected, "else").WithLocation(1, 3),
+                // (1,3): error CS1525: Invalid expression term 'else'
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 3),
+                // (1,3): error CS1002: ; expected
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_SemicolonExpected, "else").WithLocation(1, 3),
+                // (1,11): error CS8641: 'else' cannot start a statement.
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_ElseCannotStartStatement, "else").WithLocation(1, 11),
+                // (1,11): error CS1003: Syntax error, '(' expected
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_SyntaxError, "else").WithArguments("(", "else").WithLocation(1, 11),
+                // (1,11): error CS1525: Invalid expression term 'else'
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 11),
+                // (1,11): error CS1026: ) expected
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_CloseParenExpected, "else").WithLocation(1, 11),
+                // (1,11): error CS1525: Invalid expression term 'else'
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 11),
+                // (1,11): error CS1002: ; expected
+                // { else {} else {} }
+                Diagnostic(ErrorCode.ERR_SemicolonExpected, "else").WithLocation(1, 11)
+                );
+            N(SyntaxKind.Block);
+            {
+                N(SyntaxKind.OpenBraceToken);
+                N(SyntaxKind.IfStatement);
+                {
+                    M(SyntaxKind.IfKeyword);
+                    M(SyntaxKind.OpenParenToken);
+                    M(SyntaxKind.IdentifierName);
+                    {
+                        M(SyntaxKind.IdentifierToken);
+                    }
+                    M(SyntaxKind.CloseParenToken);
+                    M(SyntaxKind.ExpressionStatement);
+                    {
+                        M(SyntaxKind.IdentifierName);
+                        {
+                            M(SyntaxKind.IdentifierToken);
+                        }
+                        M(SyntaxKind.SemicolonToken);
+                    }
+                    N(SyntaxKind.ElseClause);
+                    {
+                        N(SyntaxKind.ElseKeyword);
+                        N(SyntaxKind.Block);
+                        {
+                            N(SyntaxKind.OpenBraceToken);
+                            N(SyntaxKind.CloseBraceToken);
+                        }
+                    }
+                }
+                N(SyntaxKind.IfStatement);
+                {
+                    M(SyntaxKind.IfKeyword);
+                    M(SyntaxKind.OpenParenToken);
+                    M(SyntaxKind.IdentifierName);
+                    {
+                        M(SyntaxKind.IdentifierToken);
+                    }
+                    M(SyntaxKind.CloseParenToken);
+                    M(SyntaxKind.ExpressionStatement);
+                    {
+                        M(SyntaxKind.IdentifierName);
+                        {
+                            M(SyntaxKind.IdentifierToken);
+                        }
+                        M(SyntaxKind.SemicolonToken);
+                    }
+                    N(SyntaxKind.ElseClause);
+                    {
+                        N(SyntaxKind.ElseKeyword);
+                        N(SyntaxKind.Block);
+                        {
+                            N(SyntaxKind.OpenBraceToken);
+                            N(SyntaxKind.CloseBraceToken);
+                        }
+                    }
+                }
+                N(SyntaxKind.CloseBraceToken);
+            }
+            EOF();
+        }
+
+        [WorkItem(27866, "https://github.com/dotnet/roslyn/issues/27866")]
+        [Fact]
+        public void ParseSubsequentElseWithoutPrecedingIfStatement()
+        {
+            UsingStatement("{ if (a) { } else { } else { } }",
+                // (1,23): error CS8641: 'else' cannot start a statement.
+                // { if (a) { } else { } else { } }
+                Diagnostic(ErrorCode.ERR_ElseCannotStartStatement, "else").WithLocation(1, 23),
+                // (1,23): error CS1003: Syntax error, '(' expected
+                // { if (a) { } else { } else { } }
+                Diagnostic(ErrorCode.ERR_SyntaxError, "else").WithArguments("(", "else").WithLocation(1, 23),
+                // (1,23): error CS1525: Invalid expression term 'else'
+                // { if (a) { } else { } else { } }
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 23),
+                // (1,23): error CS1026: ) expected
+                // { if (a) { } else { } else { } }
+                Diagnostic(ErrorCode.ERR_CloseParenExpected, "else").WithLocation(1, 23),
+                // (1,23): error CS1525: Invalid expression term 'else'
+                // { if (a) { } else { } else { } }
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 23),
+                // (1,23): error CS1002: ; expected
+                // { if (a) { } else { } else { } }
+                Diagnostic(ErrorCode.ERR_SemicolonExpected, "else").WithLocation(1, 23)
+                );
+            N(SyntaxKind.Block);
+            {
+                N(SyntaxKind.OpenBraceToken);
+                N(SyntaxKind.IfStatement);
+                {
+                    N(SyntaxKind.IfKeyword);
+                    N(SyntaxKind.OpenParenToken);
+                    N(SyntaxKind.IdentifierName);
+                    {
+                        N(SyntaxKind.IdentifierToken, "a");
+                    }
+                    N(SyntaxKind.CloseParenToken);
+                    N(SyntaxKind.Block);
+                    {
+                        N(SyntaxKind.OpenBraceToken);
+                        N(SyntaxKind.CloseBraceToken);
+                    }
+                    N(SyntaxKind.ElseClause);
+                    {
+                        N(SyntaxKind.ElseKeyword);
+                        N(SyntaxKind.Block);
+                        {
+                            N(SyntaxKind.OpenBraceToken);
+                            N(SyntaxKind.CloseBraceToken);
+                        }
+                    }
+                }
+                N(SyntaxKind.IfStatement);
+                {
+                    M(SyntaxKind.IfKeyword);
+                    M(SyntaxKind.OpenParenToken);
+                    M(SyntaxKind.IdentifierName);
+                    {
+                        M(SyntaxKind.IdentifierToken);
+                    }
+                    M(SyntaxKind.CloseParenToken);
+                    M(SyntaxKind.ExpressionStatement);
+                    {
+                        M(SyntaxKind.IdentifierName);
+                        {
+                            M(SyntaxKind.IdentifierToken);
+                        }
+                        M(SyntaxKind.SemicolonToken);
+                    }
+                    N(SyntaxKind.ElseClause);
+                    {
+                        N(SyntaxKind.ElseKeyword);
+                        N(SyntaxKind.Block);
+                        {
+                            N(SyntaxKind.OpenBraceToken);
+                            N(SyntaxKind.CloseBraceToken);
+                        }
+                    }
+                }
+                N(SyntaxKind.CloseBraceToken);
+            }
+            EOF();
+        }
+
+        [WorkItem(27866, "https://github.com/dotnet/roslyn/issues/27866")]
+        [Fact]
+        public void ParseElseKeywordPlacedAsIfEmbeddedStatement()
+        {
+            UsingStatement("if (a) else {}",
+                // (1,8): error CS8641: 'else' cannot start a statement.
+                // if (a) else {}
+                Diagnostic(ErrorCode.ERR_ElseCannotStartStatement, "else").WithLocation(1, 8),
+                // (1,8): error CS1003: Syntax error, '(' expected
+                // if (a) else {}
+                Diagnostic(ErrorCode.ERR_SyntaxError, "else").WithArguments("(", "else").WithLocation(1, 8),
+                // (1,8): error CS1525: Invalid expression term 'else'
+                // if (a) else {}
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 8),
+                // (1,8): error CS1026: ) expected
+                // if (a) else {}
+                Diagnostic(ErrorCode.ERR_CloseParenExpected, "else").WithLocation(1, 8),
+                // (1,8): error CS1525: Invalid expression term 'else'
+                // if (a) else {}
+                Diagnostic(ErrorCode.ERR_InvalidExprTerm, "else").WithArguments("else").WithLocation(1, 8),
+                // (1,8): error CS1002: ; expected
+                // if (a) else {}
+                Diagnostic(ErrorCode.ERR_SemicolonExpected, "else").WithLocation(1, 8)
+                );
+            N(SyntaxKind.IfStatement);
+            {
+                N(SyntaxKind.IfKeyword);
+                N(SyntaxKind.OpenParenToken);
+                N(SyntaxKind.IdentifierName);
+                {
+                    N(SyntaxKind.IdentifierToken, "a");
+                }
+                N(SyntaxKind.CloseParenToken);
+                N(SyntaxKind.IfStatement);
+                {
+                    M(SyntaxKind.IfKeyword);
+                    M(SyntaxKind.OpenParenToken);
+                    M(SyntaxKind.IdentifierName);
+                    {
+                        M(SyntaxKind.IdentifierToken);
+                    }
+                    M(SyntaxKind.CloseParenToken);
+                    M(SyntaxKind.ExpressionStatement);
+                    {
+                        M(SyntaxKind.IdentifierName);
+                        {
+                            M(SyntaxKind.IdentifierToken);
+                        }
+                        M(SyntaxKind.SemicolonToken);
+                    }
+                    N(SyntaxKind.ElseClause);
+                    {
+                        N(SyntaxKind.ElseKeyword);
+                        N(SyntaxKind.Block);
+                        {
+                            N(SyntaxKind.OpenBraceToken);
+                            N(SyntaxKind.CloseBraceToken);
+                        }
+                    }
+                }
+            }
+            EOF();
+        }
+
         private sealed class TokenAndTriviaWalker : CSharpSyntaxWalker
         {
             public int Tokens;