While working on the new Type Name Parser API (#97566) I've discovered some testing gaps.

Namely:

1. Reporting parse error for a backslash that was used to escape a character that is not legal to escape inside a type name:

   runtime/src/libraries/Common/src/System/Reflection/TypeNameParser.cs

   Lines 326 to 331 in 7185df8

   	if (!NeedsEscapingInTypeName(c))
   	{
   	// If we got here, a backslash was used to escape a character that is not legal to escape inside a type name.
   	ParseError();
   	return null;
   	}

2. Ignore leading '.' for type names without namespace:
   

   runtime/src/libraries/Common/src/System/Reflection/TypeNameParser.cs

   Lines 200 to 207 in 7185df8

   	// Compat: Ignore leading '.' for type names without namespace. .NET Framework historically ignored leading '.' here. It is likely
   	// that code out there depends on this behavior. For example, type names formed by concatenating namespace and name, without checking for
   	// empty namespace (bug), are going to have superfluous leading '.'.
   	// This behavior means that types that start with '.' are not round-trippable via type name.
   	static string ApplyLeadingDotCompatQuirk(string typeName)
   	{
   	#if NETCOREAPP
   	return (typeName.StartsWith('.') && !typeName.AsSpan(1).Contains('.')) ? typeName.Substring(1) : typeName;

3. Supporting all kinds of leading whitespaces (existing test use only (space character)):
   

   runtime/src/libraries/Common/src/System/Reflection/TypeNameParser.cs

   Lines 439 to 450 in 7185df8

   	// The type name parser has a strange attitude towards whitespace. It throws away whitespace between punctuation tokens and whitespace
   	// preceding identifiers or assembly names (and this cannot be escaped away). But whitespace between the end of an identifier
   	// and the punctuation that ends it is *not* ignored.
   	//
   	// In other words, GetType(" Foo") searches for "Foo" but GetType("Foo ") searches for "Foo ".
   	//
   	// Whitespace between the end of an assembly name and the punction mark that ends it is also not ignored by this parser,
   	// but this is irrelevant since the assembly name is then turned over to AssemblyName for parsing, which *does* ignore trailing whitespace.
   	//
   	private void SkipWhiteSpace()
   	{
   	while (_index < _input.Length && char.IsWhiteSpace(_input[_index]))

4. Type names with escape characters in their name (illegal for C# and F#, but valid for IL)
   

   runtime/src/libraries/Common/src/System/Reflection/TypeNameParser.cs

   Lines 628 to 631 in 7185df8

   	private static ReadOnlySpan<char> CharsToEscape => "\\[]+*&,";
   	private static bool NeedsEscapingInTypeName(char c)
   	=> CharsToEscape.Contains(c);

   
   cc @GrabYourPitchforks @jeffhandley