C++: Add test for C++20 implicit array sizes

Implement NewArrayExpr.getArraySize()

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

@@ -949,6 +949,16 @@ class NewArrayExpr extends NewOrNewArrayExpr, @new_array_expr {
    * gives nothing, as the 10 is considered part of the type.
    */
   Expr getExtent() { result = this.getChild(2) }
+
+  /**
+   * Gets the number of elements in the array, if available.
+   * 
+   * For example, `new int[]{1,2,3}` has an array size of 3.
+   */
+  int getArraySize() {
+    result = this.getAllocatedType().(ArrayType).getArraySize() or
+    result = this.getInitializer().(ArrayAggregateLiteral).getArraySize()
+  }
 }
 
 private class TDeleteOrDeleteArrayExpr = @delete_expr or @delete_array_expr;

cpp/ql/test/library-tests/array_sizes/arr2.expected

@@ -1,5 +1,11 @@
 | file://:0:0:0:0 | char[6] | 6 |
 | file://:0:0:0:0 | char[26] | 26 |
+| file://:0:0:0:0 | char[] |  |
+| file://:0:0:0:0 | const char[6] | 6 |
+| file://:0:0:0:0 | double[0] | 0 |
+| file://:0:0:0:0 | double[3] | 3 |
+| file://:0:0:0:0 | double[4] | 4 |
+| file://:0:0:0:0 | double[] |  |
 | file://:0:0:0:0 | int[1] | 1 |
 | file://:0:0:0:0 | int[11] | 11 |
 | file://:0:0:0:0 | long[] |  |

cpp/ql/test/library-tests/array_sizes/implicit_sizes.cpp

@@ -0,0 +1,10 @@
+// semmle-extractor-options: -std=c++20
+double a1[]{1,2,3};
+double* p1 = new double[]{1,2,3};
+double* p2 = new double[0]{};
+double* p3 = new double[]{};
+char c[]{"Hello"};
+char* d = new char[]{"Hello"};
+double a2[](1,2,3);
+double* p4 = new double[](1,2,3);
+double* p5 = new double[4]{1,2};  // Size mismatch

cpp/ql/test/library-tests/array_sizes/new.expected

@@ -0,0 +1,6 @@
+| implicit_sizes.cpp:3:14:3:32 | new[] | 3 |
+| implicit_sizes.cpp:4:14:4:28 | new[] | 0 |
+| implicit_sizes.cpp:5:14:5:27 | new[] | 0 |
+| implicit_sizes.cpp:7:11:7:29 | new[] | 6 |
+| implicit_sizes.cpp:9:14:9:32 | new[] | 3 |
+| implicit_sizes.cpp:10:14:10:31 | new[] | 4 |

cpp/ql/test/library-tests/array_sizes/new.ql

@@ -0,0 +1,4 @@
+import cpp
+
+from NewArrayExpr nae
+select nae, nae.getArraySize()