[clang-tidy] Limit non-Strict mode to public functions

[mehdi_amini]

Summary

The rational to avoid applying the warning/fix in non-Strict more to
functions with an empty body is that there is "no place for a bug to hide".
However for private functions, the parameters can be entirely eliminated
and all the call sites improved.

[aaron.ballman]

clang-tools-extra/clang-tidy/misc/UnusedParametersCheck.cpp

188

This is looking at the linkage of the function, not at its access control; is that intended?

clang-tools-extra/docs/clang-tidy/checks/misc-unused-parameters.rst

43

Call sites are not always visible for protected functions, so this seems a bit suspicious. The changes are missing test coverage for that situation.

clang-tools-extra/test/clang-tidy/checkers/misc-unused-parameters.cpp

157–159

I think this demonstrates a bad fix -- this changes code meaning from being a converting constructor to being a default constructor, which are very different things.

[mehdi_amini]

clang-tools-extra/docs/clang-tidy/checks/misc-unused-parameters.rst

43

You're using public for "access control" while I was using the linkage aspect: my reasoning is that if a method isn't "externally visible" from the current translation unit, you see all the call-sites. This is orthogonal to public/private/protected as far as I know.

I am likely missing a check for "isDefinedInMainFile" (or whatever the api is called) to not flag included headers.

clang-tools-extra/test/clang-tidy/checkers/misc-unused-parameters.cpp

157–159

Oh you're right: so we can't do it for a Ctor with a single parameter...

But we also can't do it for a Ctor with two parameters as it'll turn it into a converting ctor. Unless you can eliminate both parameters, in which case it is become a default ctor (which can conflict with an existing one, in which case it can be just deleted?)

[aaron.ballman]

clang-tools-extra/docs/clang-tidy/checks/misc-unused-parameters.rst

43

You're using public for "access control" while I was using the linkage aspect: my reasoning is that if a method isn't "externally visible" from the current translation unit, you see all the call-sites.

Oh, thanks for pointing out that I was confused! I'm not used to "public" when describing linkage, usually that's "external" or "externally visible". Any appetite for rewording in those terms? Something like "On the other hand, for functions with internal linkage, all the call sites are visible and parameters can be safely removed."

clang-tools-extra/test/clang-tidy/checkers/misc-unused-parameters.cpp

157–159

Yeah, I think we need to not transform ctors at all currently.

[mehdi_amini]

clang-tools-extra/docs/clang-tidy/checks/misc-unused-parameters.rst

43

Sure: happy to reword.

(We use public/private for symbol visibility at a module level in MLIR unfortunately, I've been "contaminated" ;) ).

clang-tools-extra/test/clang-tidy/checkers/misc-unused-parameters.cpp

157–159

In this case I can fix the existing bug by disabling the fixit (as discussed in D116513) and abandon this revision I think.

clang-tools-extra/clang-tidy/misc/UnusedParametersCheck.cpp

if (const auto *Method = dyn_cast<CXXMethodDecl>(Function))
     if (Method->isLambdaStaticInvoker())
       return;
   for (unsigned I = 0, E = Function->getNumParams(); I != E; ++I) {
     const auto *Param = Function->getParamDecl(I);
     if (Param->isUsed() || Param->isReferenced() || !Param->getDeclName() ||
         Param->hasAttr<UnusedAttr>())
       continue;
 
-    // In non-strict mode ignore function definitions with empty bodies
+    // In non-strict mode ignore public function definitions with empty bodies
     // (constructor initializer counts for non-empty body).
-    if (StrictMode ||
+    if (StrictMode || !Function->isExternallyVisible() ||
         (Function->getBody()->child_begin() !=
          Function->getBody()->child_end()) ||
         (isa<CXXConstructorDecl>(Function) &&
          cast<CXXConstructorDecl>(Function)->getNumCtorInitializers() > 0))
       warnOnUnusedParameter(Result, Function, I);
   }
 }
 
 } // namespace misc
 } // namespace tidy
 } // namespace clang

clang-tools-extra/docs/clang-tidy/checks/misc-unused-parameters.rst

 
 Options
 -------
 
 .. option:: StrictMode
 
    When `false` (default value), the check will ignore trivially unused parameters,
    i.e. when the corresponding function has an empty body (and in case of
-   constructors - no constructor initializers). When the function body is empty,
-   an unused parameter is unlikely to be unnoticed by a human reader, and
-   there's basically no place for a bug to hide.
+   constructors - no constructor initializers) and the definition is public. When
+   the function body is empty, an unused parameter is unlikely to be unnoticed by
+   a human reader, and there's basically no place for a bug to hide. On the other
+   hand for non-public functions, all the call-sites are visible and the parameter
+   can be eliminated entirely.

clang-tools-extra/test/clang-tidy/checkers/misc-unused-parameters.cpp

// CHECK-FIXES: static void h(int  /*i*/[]) {;}
   static void s(void (*fn)()) {;}
 // CHECK-MESSAGES: :[[@LINE-1]]:24: warning
 // CHECK-FIXES: static void s(void (* /*fn*/)()) {;}
 };
 
 namespace {
 class C {
 public:
+// CHECK-FIXES: C() {}
+  C(int i) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:9: warning
+
   void f(int i);
 // CHECK-FIXES: void f();
   void g(int i) {;}
 // CHECK-MESSAGES: :[[@LINE-1]]:14: warning
 // CHECK-FIXES: void g() {;}
   void h(int i) {;}
 // CHECK-MESSAGES: :[[@LINE-1]]:14: warning
 // CHECK-FIXES: void h(int  /*i*/) {;}

 void C::f(int i) {;}
 // CHECK-MESSAGES: :[[@LINE-1]]:15: warning
 // CHECK-FIXES: void C::f() {;}
 
 template <typename T>
 void useFunction(T t);
 
 void someMoreCallSites() {
-  C c;
+  C c(42);
   c.f(1);
 // CHECK-FIXES: c.f();
   c.g(1);
 // CHECK-FIXES: c.g();
 
   useFunction(&C::h);
   useFunction(&C::s);
   useFunction(&C::u);