C++: Add getTemplateClass to DeductionGuide

cpp/downgrades/e197626a6ebccd052d5c891975fccf8aebcc9803/upgrade.properties

@@ -0,0 +1,3 @@
+description: Add relation between deduction guides and class templates
+compatibility: full
+deduction_guide_for_class.rel: delete

cpp/ql/lib/change-notes/2024-08-01-deduction-guide.md

@@ -0,0 +1,4 @@
+---
+category: feature
+---
+* A `getTemplateClass` predicate was added to the `DeductionGuide` class to get the class template for which the deduction guide is a guide.

cpp/ql/lib/semmle/code/cpp/Function.qll

@@ -898,4 +898,11 @@ class UserDefinedLiteral extends Function {
  */
 class DeductionGuide extends Function {
   DeductionGuide() { functions(underlyingElement(this), _, 8) }
+
+  /**
+   * Gets the class template for which this is a deduction guide.
+   */
+  TemplateClass getTemplateClass() {
+    deduction_guide_for_class(underlyingElement(this), unresolveElement(result))
+  }
 }

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -409,6 +409,11 @@ function_defaulted(unique int id: @function ref);
 
 function_prototyped(unique int id: @function ref)
 
+deduction_guide_for_class(
+    int id: @function ref,
+    int class_template: @usertype ref
+)
+
 member_function_this_type(
     unique int id: @function ref,
     int this_type: @type ref

cpp/ql/lib/upgrades/9629fc87dab7dbed0771bf5ce22bce4d7f943b52/upgrade.properties

@@ -0,0 +1,2 @@
+description: Add relation between deduction guides and class templates
+compatibility: partial

cpp/ql/test/library-tests/deduction_guides/test.cpp

@@ -0,0 +1,18 @@
+// semmle-extractor-options: -std=c++20
+
+template<typename T>
+struct C {
+    C(const T);
+};
+
+C(const double)  -> C<int>;
+
+template<typename T>
+C(const T)  -> C<int>;
+
+void test() {
+    new C<char>(0);
+    new C<int>(0);
+    new C(0.0f);
+    new C(0.0);
+}

cpp/ql/test/library-tests/deduction_guides/test.expected

@@ -0,0 +1,3 @@
+| file://:0:0:0:0 | C | test.cpp:4:8:4:8 | C<T> |
+| test.cpp:5:5:5:5 | (unnamed deduction guide) | test.cpp:4:8:4:8 | C<T> |
+| test.cpp:11:1:11:1 | C | test.cpp:4:8:4:8 | C<T> |

cpp/ql/test/library-tests/deduction_guides/test.ql

@@ -0,0 +1,4 @@
+import cpp
+
+from DeductionGuide d
+select d, d.getTemplateClass()