[clangd] IncludeCleaner: Don't consider the definition as usage for function forward declarations

[kbobyrev]

Make the logic correct.

[kbobyrev]

Only get function redecls as used for definition.

[kadircet]

thanks, lgtm!

clang-tools-extra/clangd/IncludeCleaner.cpp

42

i've got a feeling that this might have nasty side effects in the presence of functions with defaulted-arguments, but they're pretty obscure (like declare a function with a default parameter in a header, then in the main file both include the header and have a forward declaration of the function, now that forward declaration can't have the default parameter because the one in the header has. but who does that really). so i suppose it won't result in any false positives in practice.

clang-tools-extra/clangd/IncludeCleaner.cpp

bool VisitMemberExpr(MemberExpr *ME) {
     return true;
   }
 
   bool VisitTagType(TagType *TT) {
     add(TT->getDecl());
     return true;
   }
 
+  bool VisitFunctionDecl(FunctionDecl *FD) {
+    // Function definition will require redeclarations to be included.
+    if (FD == FD->getDefinition())
+      add(FD);
+    return true;
+  }
+
   bool VisitCXXConstructExpr(CXXConstructExpr *CCE) {
     add(CCE->getConstructor());
     return true;
   }
 
   bool VisitTemplateSpecializationType(TemplateSpecializationType *TST) {
     if (isNew(TST)) {
       add(TST->getTemplateName().getAsTemplateDecl()); // Primary template.

clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp

TestCase Cases[] = {
           "class ^X; class ^X{}; class ^X;",
           "X *y;",
       },
       // Constructor
       {
           "struct ^X { ^X(int) {} int ^foo(); };",
           "auto x = X(42); auto y = x.foo();",
       },
+      // Function
+      {
+          "void ^foo();",
+          "void foo() {}",
+      },
+      {
+          "void foo() {}",
+          "void foo();",
+      },
+      {
+          "inline void ^foo() {}",
+          "void bar() { foo(); }",
+      },
       // Static function
       {
           "struct ^X { static bool ^foo(); }; bool X::^foo() {}",
           "auto b = X::foo();",
       },
       // TemplateRecordDecl
       {
           "template <typename> class ^X;",