[clang-tidy] Handle template instantiations in modenize-use-default check

[malcolm.parsons]

Summary

Duplicate fixes were being created for explicit template instantiations
of out-of-line constructors or destructors.

Fixes PR30921.

[aaron.ballman]

test/clang-tidy/modernize-use-default.cpp

142 ↗

(On Diff #77728)

Would it be better if the = default were on the inline declaration rather than the out-of-line definition? If so, that might be a reasonable FIXME to add to this test (assuming you don't feel like fixing it as part of this patch).

Also, does the = delete check suffer from the same issue?

[malcolm.parsons]

test/clang-tidy/modernize-use-default.cpp

142 ↗

(On Diff #77728)

There are valid reasons for having out-of-line definitions, even when defaulted.
The delete check only triggers if there is no definition. Template instantiation can't copy a definition that doesn't exist.

[aaron.ballman]

LGTM!

test/clang-tidy/modernize-use-default.cpp

142 ↗

(On Diff #77728)

Hmm, I suppose that is true. Thanks!

[malcolm.parsons]

test/clang-tidy/modernize-use-default.cpp

142 ↗

(On Diff #77728)

Scratch that.

The = delete check doesn't create conflicting duplicate fixes, but it does have an issue.
The fixes for template instantiations are correct for the operators, but not for the constructors.
This is because the AST has the wrong source range for the template instantiation's constructors.

The template's copy constructor's source range includes the parameters:
CXXConstructorDecl 0x27bc9c0 <line:31:3, col:58> col:3 PositivePrivateTemplate<T> 'void (const PositivePrivateTemplate<T> &)'

The template instantiation's copy constructor's source range does not:
CXXConstructorDecl 0x27bd560 <line:31:3> col:3 PositivePrivateTemplate 'void (const struct PositivePrivateTemplate<int> &)'

I'll make another patch for this.

clang-tools-extra/trunk/clang-tidy/modernize/UseDefaultCheck.cpp

void UseDefaultCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *SpecialFunctionDecl =
       Result.Nodes.getNodeAs<CXXMethodDecl>(SpecialFunction);
 
   // Discard explicitly deleted/defaulted special member functions and those
   // that are not user-provided (automatically generated).
   if (SpecialFunctionDecl->isDeleted() ||
       SpecialFunctionDecl->isExplicitlyDefaulted() ||
       SpecialFunctionDecl->isLateTemplateParsed() ||
+      SpecialFunctionDecl->isTemplateInstantiation() ||
       !SpecialFunctionDecl->isUserProvided() || !SpecialFunctionDecl->hasBody())
     return;
 
   const auto *Body = dyn_cast<CompoundStmt>(SpecialFunctionDecl->getBody());
   if (!Body)
     return;
 
   // If there are comments inside the body, don't do the change.

clang-tools-extra/trunk/test/clang-tidy/modernize-use-default.cpp

public:
   Temp() {}
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: use '= default'
   // CHECK-FIXES: Temp() = default;
   ~Temp() {}
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: use '= default'
   // CHECK-FIXES: ~Temp() = default;
 };
 
+// Class template out of line with explicit instantiation.
+template <class T>
+class TempODef {
+public:
+  TempODef();
+  ~TempODef();
+};
+
+template <class T>
+TempODef<T>::TempODef() {}
+// CHECK-MESSAGES: :[[@LINE-2]]:1: warning: use '= default'
+// CHECK-FIXES: TempODef<T>::TempODef() = default;
+template <class T>
+TempODef<T>::~TempODef() {}
+// CHECK-MESSAGES: :[[@LINE-2]]:1: warning: use '= default'
+// CHECK-FIXES: TempODef<T>::~TempODef() = default;
+
+template class TempODef<int>;
+template class TempODef<double>;
+
 // Non user-provided constructor/destructor.
 struct Imp {
   int Int;
 };
 void g() {
   Imp *PtrImp = new Imp();
   PtrImp->~Imp();
   delete PtrImp;