Fix bug when trailing and leading underscores are not the same (#22141)

Merging on behalf of @alrz. Thanks

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

@@ -933,9 +933,8 @@ private bool ScanInteger()
         }
 
         // Allows underscores in integers, except at beginning for decimal and end
-        private void ScanNumericLiteralSingleInteger(ref bool underscoreInWrongPlace, ref bool usedUnderscore, bool isHex, bool isBinary)
+        private void ScanNumericLiteralSingleInteger(ref bool underscoreInWrongPlace, ref bool usedUnderscore, ref bool firstCharWasUnderscore, bool isHex, bool isBinary)
         {
-            bool firstCharWasUnderscore = false;
             if (TextWindow.PeekChar() == '_')
             {
                 if (isHex || isBinary)
@@ -971,13 +970,7 @@ private void ScanNumericLiteralSingleInteger(ref bool underscoreInWrongPlace, re
                 TextWindow.AdvanceChar();
             }
 
-            if (firstCharWasUnderscore)
-            {
-                CheckFeatureAvailability(MessageID.IDS_FeatureLeadingDigitSeparator);
-                // No need for cascading feature error
-                usedUnderscore = false;
-            }
-            else if (lastCharWasUnderscore)
+            if (lastCharWasUnderscore)
             {
                 underscoreInWrongPlace = true;
             }
@@ -998,6 +991,7 @@ private bool ScanNumericLiteral(ref TokenInfo info)
             bool hasLSuffix = false;
             bool underscoreInWrongPlace = false;
             bool usedUnderscore = false;
+            bool firstCharWasUnderscore = false;
 
             ch = TextWindow.PeekChar();
             if (ch == '0')
@@ -1020,7 +1014,7 @@ private bool ScanNumericLiteral(ref TokenInfo info)
             {
                 // It's OK if it has no digits after the '0x' -- we'll catch it in ScanNumericLiteral
                 // and give a proper error then.
-                ScanNumericLiteralSingleInteger(ref underscoreInWrongPlace, ref usedUnderscore, isHex, isBinary);
+                ScanNumericLiteralSingleInteger(ref underscoreInWrongPlace, ref usedUnderscore, ref firstCharWasUnderscore, isHex, isBinary);
 
                 if ((ch = TextWindow.PeekChar()) == 'L' || ch == 'l')
                 {
@@ -1050,7 +1044,7 @@ private bool ScanNumericLiteral(ref TokenInfo info)
             }
             else
             {
-                ScanNumericLiteralSingleInteger(ref underscoreInWrongPlace, ref usedUnderscore, isHex: false, isBinary: false);
+                ScanNumericLiteralSingleInteger(ref underscoreInWrongPlace, ref usedUnderscore, ref firstCharWasUnderscore, isHex: false, isBinary: false);
 
                 if (this.ModeIs(LexerMode.DebuggerSyntax) && TextWindow.PeekChar() == '#')
                 {
@@ -1071,7 +1065,7 @@ private bool ScanNumericLiteral(ref TokenInfo info)
                         _builder.Append(ch);
                         TextWindow.AdvanceChar();
 
-                        ScanNumericLiteralSingleInteger(ref underscoreInWrongPlace, ref usedUnderscore, isHex: false, isBinary: false);
+                        ScanNumericLiteralSingleInteger(ref underscoreInWrongPlace, ref usedUnderscore, ref firstCharWasUnderscore, isHex: false, isBinary: false);
                     }
                     else if (_builder.Length == 0)
                     {
@@ -1102,7 +1096,7 @@ private bool ScanNumericLiteral(ref TokenInfo info)
                     }
                     else
                     {
-                        ScanNumericLiteralSingleInteger(ref underscoreInWrongPlace, ref usedUnderscore, isHex: false, isBinary: false);
+                        ScanNumericLiteralSingleInteger(ref underscoreInWrongPlace, ref usedUnderscore, ref firstCharWasUnderscore, isHex: false, isBinary: false);
                     }
                 }
 
@@ -1174,7 +1168,11 @@ private bool ScanNumericLiteral(ref TokenInfo info)
             {
                 this.AddError(MakeError(start, TextWindow.Position - start, ErrorCode.ERR_InvalidNumber));
             }
-            if (usedUnderscore)
+            else if (firstCharWasUnderscore)
+            {
+                CheckFeatureAvailability(MessageID.IDS_FeatureLeadingDigitSeparator);
+            }
+            else if (usedUnderscore)
             {
                 CheckFeatureAvailability(MessageID.IDS_FeatureDigitSeparator);
             }
@@ -1198,7 +1196,10 @@ private bool ScanNumericLiteral(ref TokenInfo info)
                 default:
                     if (string.IsNullOrEmpty(valueText))
                     {
-                        this.AddError(MakeError(ErrorCode.ERR_InvalidNumber));
+                        if (!underscoreInWrongPlace)
+                        {
+                            this.AddError(MakeError(ErrorCode.ERR_InvalidNumber));
+                        }
                         val = 0; //safe default
                     }
                     else

src/Compilers/CSharp/Test/Syntax/LexicalAndXml/LexicalTests.cs

@@ -2815,6 +2815,17 @@ public void TestNumericWithBadUnderscores()
             Assert.Equal("error CS1013: Invalid number", errors[0].ToString(EnsureEnglishUICulture.PreferredOrNull));
             Assert.Equal(text, token.Text);
 
+            text = "0x_2_";
+            token = LexToken(text, _options72);
+
+            Assert.NotNull(token);
+            Assert.Equal(SyntaxKind.NumericLiteralToken, token.Kind());
+            errors = token.Errors();
+            Assert.Equal(1, errors.Length);
+            Assert.Equal((int)ErrorCode.ERR_InvalidNumber, errors[0].Code);
+            Assert.Equal("error CS1013: Invalid number", errors[0].ToString(EnsureEnglishUICulture.PreferredOrNull));
+            Assert.Equal(text, token.Text);
+
             text = "1E+_2";
             token = LexToken(text, _options72);
 

src/Compilers/VisualBasic/Test/Syntax/Scanner/ScannerTests.vb

@@ -1298,6 +1298,14 @@ End If]]>.Value,
         Assert.Equal(1, errors.Count)
         Assert.Equal(30035, errors.First().Code)
         Assert.Equal(0, CInt(tk.Value))
+
+        Str = "&H_2_"
+        tk = ScanOnce(Str)
+        Assert.Equal(SyntaxKind.IntegerLiteralToken, tk.Kind)
+        errors = tk.Errors()
+        Assert.Equal(1, errors.Count)
+        Assert.Equal(30035, errors.First().Code)
+        Assert.Equal(0, CInt(tk.Value))
     End Sub
 
     <Fact>