C++: Expose trailing return type presence.

Author: sashabu

cpp/change-notes/2021-08-10-has-trailing-return-type.md

@@ -0,0 +1,2 @@
+lgtm,codescanning
+* Added `Function.hasTrailingReturnType` predicate to check whether a function was declared with a trailing return type.

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

@@ -151,6 +151,20 @@ class Function extends Declaration, ControlFlowNode, AccessHolder, @function {
    */
   predicate isNaked() { getAnAttribute().hasName("naked") }
 
+  /**
+   * Holds if this function has a trailing return type.
+   *
+   * Note that this is true whether or not deduction took place. For example,
+   * this holds for both `e` and `f`, but not `g` or `h`:
+   * ```
+   * auto e() -> int { return 0; }
+   * auto f() -> auto { return 0; }
+   * auto g() { return 0; }
+   * int h() { return 0; }
+   * ```
+   */
+  predicate hasTrailingReturnType() { this.hasSpecifier("has_trailing_return_type") }
+
   /** Gets the return type of this function. */
   Type getType() { function_return_type(underlyingElement(this), unresolveElement(result)) }
 

cpp/ql/test/library-tests/clang_ms/element.expected

@@ -97,6 +97,7 @@
 | file://:0:0:0:0 | forceinline |
 | file://:0:0:0:0 | fp_offset |
 | file://:0:0:0:0 | gp_offset |
+| file://:0:0:0:0 | has_trailing_return_type |
 | file://:0:0:0:0 | implicit_int |
 | file://:0:0:0:0 | inline |
 | file://:0:0:0:0 | int |

cpp/ql/test/library-tests/conditions/elements.expected

@@ -68,6 +68,7 @@
 | file://:0:0:0:0 | forceinline |
 | file://:0:0:0:0 | fp_offset |
 | file://:0:0:0:0 | gp_offset |
+| file://:0:0:0:0 | has_trailing_return_type |
 | file://:0:0:0:0 | implicit_int |
 | file://:0:0:0:0 | initializer for <error> |
 | file://:0:0:0:0 | inline |

cpp/ql/test/library-tests/templates/instantiations_functions/elements.expected

@@ -129,6 +129,7 @@
 | file://:0:0:0:0 | gp_offset |
 | file://:0:0:0:0 | grammar_helper_base & |
 | file://:0:0:0:0 | grammar_helper_base && |
+| file://:0:0:0:0 | has_trailing_return_type |
 | file://:0:0:0:0 | implicit_int |
 | file://:0:0:0:0 | inline |
 | file://:0:0:0:0 | int |

cpp/ql/test/library-tests/unnamed/elements.expected

@@ -59,6 +59,7 @@
 | file://:0:0:0:0 | forceinline | Other |
 | file://:0:0:0:0 | fp_offset | Other |
 | file://:0:0:0:0 | gp_offset | Other |
+| file://:0:0:0:0 | has_trailing_return_type | Other |
 | file://:0:0:0:0 | implicit_int | Other |
 | file://:0:0:0:0 | inline | Other |
 | file://:0:0:0:0 | int | Other |