Error when '=' syntax is used to define UDT (#2721)

Today we allow something like `T = Type(Number)` which is incorrect and
should not be allowed . this PR adds error to that scenario

---------

src/libraries/Microsoft.PowerFx.Core/Localization/Strings.cs

@@ -853,5 +853,6 @@ namespace Microsoft.PowerFx.Core.Localization
         public static ErrorResourceKey ErrReachedMaxJsonLength = new ErrorResourceKey("ErrReachedMaxJsonLength");
 
         public static ErrorResourceKey ErrUserDefinedTypesDisabled = new ErrorResourceKey("ErrUserDefinedTypesDisabled");
+        public static ErrorResourceKey ErrUserDefinedTypeIncorrectSyntax = new ErrorResourceKey("ErrUserDefinedTypeIncorrectSyntax");
     }
 }

src/libraries/Microsoft.PowerFx.Core/Parser/TexlParser.cs

@@ -362,6 +362,8 @@ namespace Microsoft.PowerFx.Core.Parser
                 }
                 else if (_curs.TidCur == TokKind.Equ)
                 {
+                    var equalToken = _curs.TokCur;
+                    
                     var declaration = script.Substring(declarationStart, _curs.TokCur.Span.Min - declarationStart);
                     _curs.TokMove();
                     definitionBeforeTrivia.Add(ParseTrivia());
@@ -385,6 +387,12 @@ namespace Microsoft.PowerFx.Core.Parser
                         definitionBeforeTrivia.Add(ParseTrivia());
                         var result = ParseExpr(Precedence.None);
 
+                        if (result is TypeLiteralNode _)
+                        {
+                            CreateError(equalToken, TexlStrings.ErrUserDefinedTypeIncorrectSyntax);
+                            continue;
+                        }
+
                         namedFormulas.Add(new NamedFormula(thisIdentifier.As<IdentToken>(), new Formula(result.GetCompleteSpan().GetFragment(script), result), _startingIndex, attribute));
                         userDefinitionSourceInfos.Add(new UserDefinitionSourceInfo(index++, UserDefinitionType.NamedFormula, thisIdentifier.As<IdentToken>(), declaration, new SourceList(definitionBeforeTrivia), GetExtraTriviaSourceList()));
                         definitionBeforeTrivia = new List<ITexlSource>();

src/strings/PowerFxResources.en-US.resx

@@ -4846,4 +4846,11 @@
     <value>The 'User-defined types' experimental feature is not enabled.</value>
     <comment>Error message returned when user uses User-defined types with the User-defined types feature flag turned off.</comment>
   </data>
+  <data name="ErrUserDefinedTypeIncorrectSyntax" xml:space="preserve">
+    <value>Incorrect syntax: '=' cannot be used to declare user-defined types. Use ':=' instead.</value>
+    <comment>
+      Error message returned when user uses '=' syntax to define User-defined type.
+      Some examples for valid named type declarations - "Point := Type({x: Number, y: Number});" , "T1 := Type(Number);" , "T2 := Type([Boolean]);".
+    </comment>
+  </data>
 </root>

src/tests/Microsoft.PowerFx.Core.Tests.Shared/ParseTests.cs

@@ -1014,5 +1014,17 @@ namespace Microsoft.PowerFx.Core.Tests
             Assert.Equal(expectErrors, parseResult.HasErrors);
             Assert.Equal(isImperative, parseResult.UDFs.First().IsImperative);
         }
+
+        [Theory]
+        [InlineData("A = Type(Number);", 0)]
+        [InlineData("A = \"hello\";B = Type([Boolean]); C = 5;", 2)]
+        public void TestTypeLiteralInNamedFormula(string script, int namedFormulaCount)
+        {
+            var parserOptions = new ParserOptions();
+            var parseResult = UserDefinitions.Parse(script, parserOptions, Features.PowerFxV1);
+            Assert.True(parseResult.HasErrors);
+            Assert.Equal(namedFormulaCount, parseResult.NamedFormulas.Count());
+            Assert.Contains(parseResult.Errors, e => e.MessageKey.Contains("ErrUserDefinedTypeIncorrectSyntax"));
+        }
     }
 }