[Sema] Diagnose by-value copy constructors in template instantiations #130866

Megan0704-1 · 2025-03-12T00:11:11Z

This PR ensures Clang diagnoses by-value copy constructors in implicitly instantiated class templates (e.g., A<int, int>(A<int, int>)), per [class.copy.ctor].

Changes:

Remove TSK_ImplicitInstantiation check in SemaDeclCXX.cpp.
Add !isFunctionTemplateSpecialization() to skip templated constructors.
Add regression tests.

Fixes llvm#80963 Previously, Clang skipped diagnosing a constructor if it was implicitly instantiated from a template class (TSK_ImplicitInstantiation). This allowed ill-formed “copy” constructors taking the class by value (e.g. A(A)) to slip through without a diagnostic. However, the C++ standard mandates that copy constructors must take their class type parameter by reference (e.g., A(const A&)). Furthermore, a constructor template that *would* form a copy-by-value signature is not treated as a copy constructor and should never be chosen for copying. This patch replaces the check on TSK_ImplicitInstantiation with a check to see if the constructor is a function template specialization (i.e., isFunctionTemplateSpecialization()). That ensures proper diagnosis of non-template copy-by-value constructors, while still allowing valid template constructors that might appear to have a copy-like signature but should be SFINAEd out or simply not selected as a copy constructor.

github-actions · 2025-03-12T00:11:32Z

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llvmbot · 2025-03-12T00:12:04Z

@llvm/pr-subscribers-clang

Author: Kuo, Mei-Chun (Megan0704-1)

Changes

Fixes #80963

This PR ensures Clang diagnoses by-value copy constructors in implicitly instantiated class templates (e.g., A<int, int>(A<int, int>)), per [class.copy.ctor].

Changes:

Remove TSK_ImplicitInstantiation check in SemaDeclCXX.cpp.
Add !isFunctionTemplateSpecialization() to skip templated constructors.
Add regression tests.

Full diff: https://github.com/llvm/llvm-project/pull/130866.diff

2 Files Affected:

(modified) clang/lib/Sema/SemaDeclCXX.cpp (+2-2)
(added) clang/test/SemaCXX/copy-ctor-template.cpp (+22)

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 96aac7871db1e..1c62a551ee732 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -10921,8 +10921,8 @@ void Sema::CheckConstructor(CXXConstructorDecl *Constructor) {
   //   parameters have default arguments.
   if (!Constructor->isInvalidDecl() &&
       Constructor->hasOneParamOrDefaultArgs() &&
-      Constructor->getTemplateSpecializationKind() !=
-          TSK_ImplicitInstantiation) {
+      !Constructor->isFunctionTemplateSpecialization()
+          ) {
     QualType ParamType = Constructor->getParamDecl(0)->getType();
     QualType ClassTy = Context.getTagDeclType(ClassDecl);
     if (Context.getCanonicalType(ParamType).getUnqualifiedType() == ClassTy) {
diff --git a/clang/test/SemaCXX/copy-ctor-template.cpp b/clang/test/SemaCXX/copy-ctor-template.cpp
new file mode 100644
index 0000000000000..a46a167038cf7
--- /dev/null
+++ b/clang/test/SemaCXX/copy-ctor-template.cpp
@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+template<class T, class V>
+struct A{
+    A();
+    A(A&);
+    A(A<V, T>); // expected-error{{copy constructor must pass its first argument by reference}}
+};
+
+void f() {
+    A<int, int> a = A<int, int>(); // expected-note{{in instantiation of template class 'A<int, int>'}}
+}
+
+template<class T, class V>
+struct B{
+    B();
+    template<class U> B(U); // No error (templated constructor)
+};
+
+void g() {
+    B<int, int> b = B<int, int>(); // should use implicit copy constructor
+}

Megan0704-1 · 2025-03-12T00:21:10Z

@erichkeane, @shafik
Could you please take a look and let me know if it looks good to you? Thank you very much!

cor3ntin

Thanks for working on this

Can you add a changelog entry in clang/docs/ReleaseNotes.rst (in the bug fixes to c++ section)?

Thanks

cor3ntin · 2025-03-12T07:01:30Z

clang/test/SemaCXX/copy-ctor-template.cpp

+
+void g() {
+    B<int, int> b = B<int, int>(); // should use implicit copy constructor
+}


Can you add a test for @hubert-reinterpretcast comment here
#80963 (comment)

Can you add tests for something like
A<int, double> a = A<double, int>(); // not a copy constructor

Thanks for the review! I’ll add tests for the A<int, double> case and update the PR shortly.

Megan0704-1 · 2025-03-12T08:25:21Z

Thanks for your feedback and review!! I’ve updated the changelog and the test file to include the rvalue-to-lvalue case and converting constructor scenario. Please let me know if there’s anything else!

cor3ntin

LGTM. Thanks for fixing this issue!

Will you need me to merge this for you?

Megan0704-1 · 2025-03-12T09:18:44Z

Thank you for reviewing and approving the PR!
Yes, please merge it for me. Thanks again for all your help!

cor3ntin · 2025-03-12T12:51:45Z

@Megan0704-1 Can you look into the test failures? thanks

Adjust the test to account for new diagnostics emitted when a class template constructor takes the class by value after instantiation. - Add expected-error annotations to lines 142, 159, 172 (invalid by-value constructors when T = U). - Add expected-note annotations to lines 147, 164, 176 (template instantiation points). - Remove expected-error from line 76 (valid template specialization).

Megan0704-1 · 2025-03-12T21:20:18Z

@cor3ntin
Thank you for flagging the test failures!

I’ve investigated them, and here’s what I found:
Failed Tests in constructor-template.cpp:

The failures occurred because the test wasn’t annotated to expect the new diagnostics introduced by the fix (e.g., A<int, int> instantiations now emit errors for invalid by-value copy constructors).

Fix it by updating the test to:

Add expected-error to lines 142, 159, 172 (invalid by-value constructors when T = U).
Add expected-note to lines 147, 164, 176 (template instantiation points).
Remove an incorrect expected-error from line 76 (valid template specialization).

Please Let me know if further adjustments are needed~

github-actions · 2025-03-13T04:53:02Z

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github-actions · 2025-03-13T04:55:01Z

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff c2ed840ed94d3412c7c0bdd9ed84cac6fe0afb57 790d151975c9ce4f5f823484d100d9460077b971 --extensions cpp -- clang/test/SemaCXX/copy-ctor-template.cpp clang/lib/Sema/SemaDeclCXX.cpp clang/test/SemaTemplate/constructor-template.cpp

View the diff from clang-format here.

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 1c62a551ee..00b4006b5e 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -10921,8 +10921,7 @@ void Sema::CheckConstructor(CXXConstructorDecl *Constructor) {
   //   parameters have default arguments.
   if (!Constructor->isInvalidDecl() &&
       Constructor->hasOneParamOrDefaultArgs() &&
-      !Constructor->isFunctionTemplateSpecialization()
-          ) {
+      !Constructor->isFunctionTemplateSpecialization()) {
     QualType ParamType = Constructor->getParamDecl(0)->getType();
     QualType ClassTy = Context.getTagDeclType(ClassDecl);
     if (Context.getCanonicalType(ParamType).getUnqualifiedType() == ClassTy) {

shafik · 2025-03-13T05:12:07Z

clang/lib/Sema/SemaDeclCXX.cpp

-      Constructor->getTemplateSpecializationKind() !=
-          TSK_ImplicitInstantiation) {
+      !Constructor->isFunctionTemplateSpecialization()
+          ) {


Did you run clang-format on this? I am surprised it would leave the ) on the line like that but maybe I am off.

Thank you for catching this, and I am so sorry for the oversight.
I forgot to run clang-format before merging.
Should I submitt a follow-up PR to fix the formatting issues?

I missed it too, sorry.

yes, feel free to submit a new pr. thanks

@cor3ntin

This PR fixes formatting issues in `constructor-template.cpp` introduced in #130866. Changes: - Ran `clang-format` to adhere to LLVM style guidelines. - No functional changes. CC: @cor3ntin @shafik Thanks

…llvm#130866) Fixes llvm#80963 This PR ensures Clang diagnoses by-value copy constructors in implicitly instantiated class templates (e.g., `A<int, int>(A<int, int>)`), per [class.copy.ctor]. Changes: - Remove `TSK_ImplicitInstantiation` check in `SemaDeclCXX.cpp`. - Add `!isFunctionTemplateSpecialization()` to skip templated constructors. - Add regression tests.

@cor3ntin

This PR fixes formatting issues in `constructor-template.cpp` introduced in llvm#130866. Changes: - Ran `clang-format` to adhere to LLVM style guidelines. - No functional changes. CC: @cor3ntin @shafik Thanks

zygoloid · 2025-03-24T18:10:58Z

This error is produced for the following code:

template <typename KeyT>
class SetView {
  SetView(SetView<std::remove_const_t<KeyT>> other_view)
    requires(!std::same_as<KeyT, std::remove_const_t<KeyT>>);
};

... which is never an eligible copy constructor. Is that intended? If this is in line with the language rules, perhaps we should file a core issue.

[Edit: I've mailed the core reflector.]

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Mar 12, 2025

Megan0704-1 mentioned this pull request Mar 12, 2025

Clang fails to detect illegal copy constructor with template class as its parameter #80963

Closed

cor3ntin reviewed Mar 12, 2025

View reviewed changes

Megan0704-1 added 2 commits March 12, 2025 00:47

[Docs] Add release note for llvm#80963 fix

4fe1d93

[Sema] Fix test diagnostics for copy-ctor-template

12d9237

Megan0704-1 force-pushed the fix-80963 branch from 5ea66a6 to 12d9237 Compare March 12, 2025 08:24

cor3ntin approved these changes Mar 12, 2025

View reviewed changes

cor3ntin merged commit fe0d3e3 into llvm:main Mar 13, 2025
7 of 11 checks passed

Megan0704-1 deleted the fix-80963 branch March 13, 2025 05:11

shafik reviewed Mar 13, 2025

View reviewed changes

Megan0704-1 restored the fix-80963 branch March 13, 2025 05:27

Megan0704-1 mentioned this pull request Mar 13, 2025

[NFC] Fix formatting for #80963 #131100

Merged

cor3ntin mentioned this pull request May 14, 2025

[clang] Clang should detect illegal copy constructor with template class as its parameter #81251

Closed

[Sema] Diagnose by-value copy constructors in template instantiations #130866

[Sema] Diagnose by-value copy constructors in template instantiations #130866

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