Make assert condition in ScanInterpolatedStringPunctuation more precise.

Fixes #6341.

src/Compilers/VisualBasic/Portable/Scanner/ScannerInterpolatedString.vb

@@ -31,7 +31,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Syntax.InternalSyntax
 
             ' Another } may follow the close brace of an interpolation if the interpolation lacked a format clause.
             ' This is because the normal escaping rules only apply when parsing the format string.
-            Debug.Assert(Not CanGet(1) OrElse Peek(offset + 1) <> c OrElse IsRightCurlyBracket(c), "Escape sequence not detected.")
+            Debug.Assert(Not CanGet(offset + 1) OrElse Peek(offset + 1) <> c OrElse Not (IsLeftCurlyBracket(c) OrElse IsDoubleQuote(c)), "Escape sequence not detected.")
 
             Dim scanTrailingTrivia As Boolean
 

src/Compilers/VisualBasic/Test/Syntax/Parser/InterpolatedStringParsingTests.vb

 ' Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+Imports Roslyn.Test.Utilities
+
 Public Class InterpolatedStringParsingTests
     Inherits BasicTestBase
 
@@ -858,4 +860,66 @@ End Module")
 End Module")
     End Sub
 
+    <Fact, WorkItem(6341, "https://github.com/dotnet/roslyn/issues/6341")>
+    Public Sub LineBreakInInterpolation_1()
+        Parse(
+"Module Program
+    Sub Main()
+        Dim x = $""{ " + vbCr + vbCr + "1 
+
+}""
+    End Sub
+End Module"
+        ).AssertTheseDiagnostics(
+<expected>
+BC30625: 'Module' statement must end with a matching 'End Module'.
+Module Program
+~~~~~~~~~~~~~~
+BC30026: 'End Sub' expected.
+    Sub Main()
+    ~~~~~~~~~~
+BC30201: Expression expected.
+        Dim x = $"{ 
+                    ~
+BC30370: '}' expected.
+        Dim x = $"{ 
+                    ~
+BC30801: Labels that are numbers must be followed by colons.
+1 
+~~
+BC30648: String constants must end with a double quote.
+}"
+ ~~
+</expected>)
+
+    End Sub
+
+    <Fact, WorkItem(6341, "https://github.com/dotnet/roslyn/issues/6341")>
+    Public Sub LineBreakInInterpolation_2()
+        Parse(
+"Module Program
+    Sub Main()
+        Dim x = $""{ 1 " + vbCr + vbCr + " 
+
+}""
+    End Sub
+End Module"
+        ).AssertTheseDiagnostics(
+<expected>
+BC30625: 'Module' statement must end with a matching 'End Module'.
+Module Program
+~~~~~~~~~~~~~~
+BC30026: 'End Sub' expected.
+    Sub Main()
+    ~~~~~~~~~~
+BC30370: '}' expected.
+        Dim x = $"{ 1 
+                      ~
+BC30648: String constants must end with a double quote.
+}"
+ ~~
+</expected>)
+
+    End Sub
+
 End Class