C++: Add heuristic for GNU autoconf config files

cpp/ql/lib/change-notes/2026-04-07-autoconf.md

@@ -0,0 +1,4 @@
+---
+category: feature
+---
+* Added a subclass `AutoconfConfigureTestFile` of `ConfigurationTestFile` that represents files created by GNU autoconf configure scripts to test the build configuration.

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

@@ -42,3 +42,10 @@ class MesonPrivateTestFile extends ConfigurationTestFile {
     )
   }
 }
+
+/**
+ * A file created by a GNU autoconf configure script to test the system configuration.
+ */
+class AutoconfConfigureTestFile extends ConfigurationTestFile {
+  AutoconfConfigureTestFile() { this.getBaseName().regexpMatch("conftest[0-9]*\\.c(pp)?") }
+}

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/autoconf/ExprHasNoEffect.expected

@@ -0,0 +1,2 @@
+| conftest.c.c:4:3:4:8 | call to strlen | This expression has no effect (because $@ has no external side effects). | conftest.h:3:8:3:13 | strlen | strlen |
+| conftest_abc.c:4:3:4:8 | call to strlen | This expression has no effect (because $@ has no external side effects). | conftest.h:3:8:3:13 | strlen | strlen |

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/autoconf/ExprHasNoEffect.qlref

@@ -0,0 +1 @@
+Likely Bugs/Likely Typos/ExprHasNoEffect.ql

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/autoconf/conftest.c

@@ -0,0 +1,6 @@
+#include "conftest.h"
+
+int main2() {
+  strlen(""); // GOOD: conftest files are ignored
+  return 0;
+}

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/autoconf/conftest.c.c

@@ -0,0 +1,6 @@
+#include "conftest.h"
+
+int main3() {
+  strlen(""); // BAD: not a `conftest` file, as `conftest` is not directly followed by the extension or a sequence of numbers.
+  return 0;
+}

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/autoconf/conftest.cpp

@@ -0,0 +1,6 @@
+#include "conftest.h"
+
+int main4() {
+  strlen(""); // GOOD: conftest files are ignored
+  return 0;
+}

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/autoconf/conftest.h

@@ -0,0 +1,3 @@
+typedef long long size_t;
+
+size_t strlen(const char *s);

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/autoconf/conftest123.c

@@ -0,0 +1,6 @@
+#include "conftest.h"
+
+int main5() {
+  strlen(""); // GOOD: conftest files are ignored
+  return 0;
+}

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/autoconf/conftest_abc.c

@@ -0,0 +1,6 @@
+#include "conftest.h"
+
+int main1() {
+  strlen(""); // BAD: not a `conftest` file, as `conftest` is not directly followed by the extension or a sequence of numbers.
+  return 0;
+}