Tokenize the method reference sign of Visual X#

modules/syntax/src/main/kotlin/com/progmasoft/xide/syntax/VisualXSharpTokenizer.kt

@@ -16,7 +16,8 @@ package com.progmasoft.xide.syntax
  * - a comment whose text starts with `|` or `!` is a documentation comment;
  * - `[[` and `[=[` start a raw string with the same long-bracket rule, in which nothing is an escape;
  * - a quoted string may continue over line breaks and has backslash escapes;
- * - `'` separates digit groups inside a number and otherwise starts a character literal.
+ * - `'` separates digit groups inside a number and otherwise starts a character literal;
+ * - `::` is one sign, the method reference of `Type::Method`, and not two colons.
  */
 object VisualXSharpTokenizer : Tokenizer {
   /** The reserved words of the language, as the compiler's lexer lists them. */
@@ -32,7 +33,7 @@ object VisualXSharpTokenizer : Tokenizer {
   private val operators: List<String> =
     listOf(
       "...", "<<=", ">>=", "**=", "//=", "??=",
-      "+=", "-=", "*=", "/=", "%=", "&=", "|=", "^=", "??", "?:", "==", "\\=", "<=", ">=", "&&", "||", "//", "->",
+      "+=", "-=", "*=", "/=", "%=", "&=", "|=", "^=", "??", "?:", "::", "==", "\\=", "<=", ">=", "&&", "||", "//", "->",
       "**", "++",
       "=", "+", "-", "*", "/", "%", "&", "|", "^", "<", ">", "!", "?", "\\",
     )

modules/syntax/src/test/kotlin/com/progmasoft/xide/syntax/VisualXSharpTokenizerTest.kt

@@ -126,6 +126,30 @@ class VisualXSharpTokenizerTest {
     tokenizer.assertPieces("!mask", O to "!", I to "mask")
   }
 
+  @Test
+  fun aMethodReferenceIsOneSignBetweenTwoNames() {
+    tokenizer.assertPieces(
+      "auto next = Counter::Next;",
+      K to "auto", I to "next", O to "=", I to "Counter", O to "::", I to "Next", P to ";",
+    )
+    tokenizer.assertPieces(
+      "Apply(Demo.Counter::Next)",
+      I to "Apply", P to "(", I to "Demo", P to ".", I to "Counter", O to "::", I to "Next", P to ")",
+    )
+    // One colon is still the punctuation of a conditional.
+    tokenizer.assertPieces("a ? b : c", I to "a", O to "?", I to "b", P to ":", I to "c")
+    tokenizer.assertPieces("a :: : b", I to "a", O to "::", P to ":", I to "b")
+  }
+
+  @Test
+  fun aConditionalSelectsStrings() {
+    tokenizer.assertPieces(
+      "String kind = count > 0 ? \"some\" : \"none\";",
+      I to "String", I to "kind", O to "=", I to "count", O to ">", N to "0", O to "?", S to "\"some\"",
+      P to ":", S to "\"none\"", P to ";",
+    )
+  }
+
   @Test
   fun everyCompilerKeywordIsAKeyword() {
     for (keyword in VisualXSharpTokenizer.keywords) {