C++: Fix FPs for cpp/unused-static-function in files that were not extracted completely

cpp/ql/src/Best Practices/Unused Entities/UnusedStaticFunctions.ql

@@ -13,16 +13,32 @@
 
 import cpp
 
+pragma[noinline]
+predicate possiblyIncompleteFile(File f) {
+  exists(Diagnostic d | d.getFile() = f and d.getSeverity() >= 3)
+}
+
 predicate immediatelyReachableFunction(Function f) {
-  not f.isStatic() or
-  exists(BlockExpr be | be.getFunction() = f) or
-  f instanceof MemberFunction or
-  f instanceof TemplateFunction or
-  f.getFile() instanceof HeaderFile or
-  f.getAnAttribute().hasName("constructor") or
-  f.getAnAttribute().hasName("destructor") or
-  f.getAnAttribute().hasName("used") or
+  not f.isStatic()
+  or
+  exists(BlockExpr be | be.getFunction() = f)
+  or
+  f instanceof MemberFunction
+  or
+  f instanceof TemplateFunction
+  or
+  f.getFile() instanceof HeaderFile
+  or
+  f.getAnAttribute().hasName("constructor")
+  or
+  f.getAnAttribute().hasName("destructor")
+  or
+  f.getAnAttribute().hasName("used")
+  or
   f.getAnAttribute().hasName("unused")
+  or
+  // a compiler error in the same file suggests we may be missing data
+  possiblyIncompleteFile(f.getFile())
 }
 
 predicate immediatelyReachableVariable(Variable v) {

cpp/ql/src/change-notes/2022-09-21-unused-static-function.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* Fixed false positives from the "Unused static function" (`cpp/unused-static-function`) query in files that had errors during compilation.

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticFunctions/extraction_error.c

@@ -0,0 +1,15 @@
+// semmle-extractor-options: --expect_errors
+
+static void my_function1_called() {} // GOOD
+static void my_function2_called_after_error() {} // GOOD
+static void my_function3_not_called() {} // BAD [NOT DETECTED]
+
+int main(void) {
+	my_function1_called();
+
+--- compilation stops here because this line is not valid C code ---
+
+	my_function2_called_after_error();
+
+	return 0;
+}

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticFunctions/unused_static_functions.cpp

@@ -33,3 +33,16 @@ static void f6(void);
 static void f5(void) { f6(); }
 static void f6(void) { f5(); }
 
+// f7 and f8 are reachable from `function_caller` 
+static int f7() { return 1; } // GOOD
+static void f8() { } // GOOD
+
+void function_caller()
+{
+	auto my_lambda = []() {
+		return f7();
+	}();
+
+	f8();
+}
+