[include-cleaner] Report references to operator calls as implicit

[kadircet]

Summary

Missing these references can result in false negatives in the used-ness
analysis and break builds.

[hokein]

clang-tools-extra/include-cleaner/lib/WalkAST.cpp

102–110

I will be more defensive for nullptr (llvm::cast doesn't allow nullptr), use the cast_if_present in case where S->getCalleeDecl() returns a nullptr

[kadircet]

• Update to treat operator calls to members as "explicit" fater offline discussions. This better aligns with what we do for regular member exprs.

[hokein]

still LG.

clang-tools-extra/include-cleaner/lib/WalkAST.cpp

if (llvm::isa_and_present<UsingShadowDecl, TypeAliasTemplateDecl>(ND))
       return ND;
     // Fallback to primary template for dependent instantiations.
     return RD ? RD : ND;
   }
 
 public:
   ASTWalker(DeclCallback Callback) : Callback(Callback) {}
 
+  // Operators are almost always ADL extension points and by design references
+  // to them doesn't count as uses (generally the type should provide them, so
+  // ignore them).
+  // Unless we're using an operator defined as a member, in such cases treat
+  // this as a regular reference.
   bool TraverseCXXOperatorCallExpr(CXXOperatorCallExpr *S) {
     if (!WalkUpFromCXXOperatorCallExpr(S))
       return false;
-
-    // Operators are always ADL extension points, by design references to them
-    // doesn't count as uses (generally the type should provide them).
-    // Don't traverse the callee.
-
+    if (auto *CD = S->getCalleeDecl()) {
+      if (llvm::isa<CXXMethodDecl>(CD)) {
+        // Treat this as a regular member reference.
+        report(S->getOperatorLoc(), getMemberProvider(S->getArg(0)->getType()));
+      } else {
+        report(S->getOperatorLoc(), llvm::dyn_cast<NamedDecl>(CD),
+               RefType::Implicit);
+      }
+    }
     for (auto *Arg : S->arguments())
       if (!TraverseStmt(Arg))
         return false;
     return true;
   }
 
   bool VisitDeclRefExpr(DeclRefExpr *DRE) {
     report(DRE->getLocation(), DRE->getFoundDecl());

clang-tools-extra/include-cleaner/unittests/WalkASTTest.cpp

TEST(WalkAST, ConstructExprs) {
   testWalk("struct $implicit^S {};", "S ^t;");
   testWalk("struct $implicit^S { S(); };", "S ^t;");
   testWalk("struct $implicit^S { S(int); };", "S ^t(42);");
   testWalk("struct $implicit^S { S(int); };", "S t = ^42;");
   testWalk("namespace ns { struct S{}; } using ns::$implicit^S;", "S ^t;");
 }
 
 TEST(WalkAST, Operator) {
-  // References to operators are not counted as uses.
-  testWalk("struct string {}; int operator+(string, string);",
-           "int k = string() ^+ string();");
-  testWalk("struct string {int operator+(string); }; ",
-           "int k = string() ^+ string();");
-  testWalk("struct string { friend int operator+(string, string); }; ",
+  // Operator calls are marked as implicit references as they're ADL-used and
+  // type should be providing them.
+  testWalk(
+      "struct string { friend int $implicit^operator+(string, string); }; ",
+  // Unless they're members, we treat them as regular member expr calls.
+  testWalk("struct $explicit^string {int operator+(string); }; ",
            "int k = string() ^+ string();");
+  // Make sure usage is attributed to the alias.
+  testWalk(
+      "struct string {int operator+(string); }; using $explicit^foo = string;",
+      "int k = foo() ^+ string();");
 }
 
 TEST(WalkAST, VarDecls) {
   // Definition uses declaration, not the other way around.
   testWalk("extern int $explicit^x;", "int ^x = 1;");
   testWalk("int x = 1;", "extern int ^x;");
 }