[clang-format] Brace breaking for C# lambdas

[jbcoe]

Workaround for broken parsing of C# lambdas.

[jbcoe]

Amend comment - behaviour with addUnwrappedLine is expected and applied fix is fine.

[krasimir]

Thank you!

clang/lib/Format/UnwrappedLineParser.cpp

case tok::identifier: {
       break;
     }
     case tok::equal:
       // Fat arrows (=>) have tok::TokenKind tok::equal but TokenType
       // TT_JsFatArrow. The always start an expression or a child block if
       // followed by a curly.
       if (FormatTok->is(TT_JsFatArrow)) {
         nextToken();
-        if (FormatTok->is(tok::l_brace))
+        if (FormatTok->is(tok::l_brace)) {
+          // C# may break after => if the next character is a newline.
+          if (Style.isCSharp() && Style.BraceWrapping.AfterFunction == true) {
+            // calling `addUnwrappedLine()` here causes odd parsing errors.
+            FormatTok->MustBreakBefore = true;
+          }
           parseChildBlock();
+        }
         break;
       }
 
       nextToken();
       if (FormatTok->Tok.is(tok::l_brace)) {
         // Block kind should probably be set to BK_BracedInit for any language.
         // C# needs this change to ensure that array initialisers and object
         // initialisers are indented the same way.

clang/unittests/Format/FormatTestCSharp.cpp

var x = foo(className, $@"some code:
 
   // Interpolated string with embedded multiline string.
   Code = R"(Console.WriteLine($"{string.Join(@",
     ", values)}");)";
 
   EXPECT_EQ(Code, format(Code, Style));
 }
 
+TEST_F(FormatTestCSharp, CSharpLambdas) {
+  FormatStyle GoogleStyle = getGoogleStyle(FormatStyle::LK_CSharp);
+  FormatStyle MicrosoftStyle = getMicrosoftStyle(FormatStyle::LK_CSharp);
+
+  verifyFormat(R"(//
+class MyClass {
+  Action<string> greet = name => {
+    string greeting = $"Hello {name}!";
+    Console.WriteLine(greeting);
+  };
+})",
+               GoogleStyle);
+
+  // Microsoft Style:
+  // https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/lambda-expressions#statement-lambdas
+  verifyFormat(R"(//
+class MyClass
+{
+    Action<string> greet = name =>
+    {
+        string greeting = $"Hello {name}!";
+        Console.WriteLine(greeting);
+    };
+})",
+               MicrosoftStyle);
+}
+
 TEST_F(FormatTestCSharp, CSharpObjectInitializers) {
   FormatStyle Style = getGoogleStyle(FormatStyle::LK_CSharp);
 
   // Start code fragments with a comment line so that C++ raw string literals
   // as seen are identical to expected formatted code.
 
   verifyFormat(R"(//
 Shape[] shapes = new[] {