C#: Add library support for list- and slice patterns.

csharp/ql/lib/semmle/code/csharp/exprs/Expr.qll

@@ -321,6 +321,18 @@ private predicate hasChildPattern(ControlFlowElement pm, Expr child) {
     mid instanceof @tuple_expr and
     child = mid.getAChildExpr()
   )
+  or
+  exists(Expr mid |
+    hasChildPattern(pm, mid) and
+    mid instanceof @list_pattern_expr and
+    child = mid.getAChildExpr()
+  )
+  or
+  exists(Expr mid |
+    hasChildPattern(pm, mid) and
+    mid instanceof @slice_pattern_expr and
+    child = mid.getAChildExpr()
+  )
 }
 
 /**
@@ -506,6 +518,26 @@ class PositionalPatternExpr extends PatternExpr, @positional_pattern_expr {
   override string getAPrimaryQlClass() { result = "PositionalPatternExpr" }
 }
 
+/** A list pattern. For example `[1, 2, int y]` in `x is [1, 2, int y]`. */
+class ListPatternExpr extends PatternExpr, @list_pattern_expr {
+  override string toString() { result = "[ ... ]" }
+
+  /** Gets the `n`th pattern. */
+  PatternExpr getPattern(int n) { result = this.getChild(n) }
+
+  override string getAPrimaryQlClass() { result = "ListPatternExpr" }
+}
+
+/** A slice pattern. For example `..` in `x is [1, .., 2]. */
+class SlicePatternExpr extends PatternExpr, @slice_pattern_expr {
+  override string toString() { result = ".." }
+
+  /** Gets the subpattern, if any. */
+  PatternExpr getPattern() { result = this.getChild(0) }
+
+  override string getAPrimaryQlClass() { result = "SlicePatternExpr" }
+}
+
 /** A unary pattern. For example, `not 1`. */
 class UnaryPatternExpr extends PatternExpr, @unary_pattern_expr {
   /** Gets the underlying pattern. */