[clang] [Sema] Diagnose by-value copy constructors in template instantiations (PR #130866)

[via cfe-commits]

https://github.com/Megan0704-1 updated https://github.com/llvm/llvm-project/pull/130866

>From 80e764fcfa1912e9d3771f4edb354569741010b7 Mon Sep 17 00:00:00 2001
From: Megan <megankuo at asu.edu>
Date: Tue, 11 Mar 2025 17:09:04 -0700
Subject: [PATCH 1/3] [Sema] Diagnose by-value copy constructors in template
 instantiations
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Fixes #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.
---
 clang/lib/Sema/SemaDeclCXX.cpp            |  4 ++--
 clang/test/SemaCXX/copy-ctor-template.cpp | 22 ++++++++++++++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/SemaCXX/copy-ctor-template.cpp

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
+}

>From 4fe1d934e7a688c346d5218e213decc67b261d70 Mon Sep 17 00:00:00 2001
From: Megan <megankuo at asu.edu>
Date: Wed, 12 Mar 2025 00:47:56 -0700
Subject: [PATCH 2/3] [Docs] Add release note for #80963 fix

---
 clang/docs/ReleaseNotes.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 004f78f22ac36..8bae0d6e99ae0 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -293,6 +293,7 @@ Bug Fixes to Attribute Support
 Bug Fixes to C++ Support
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
+- Clang now diagnoses copy constructors taking the class by value in template instantiations. (#GH130866)
 - Clang is now better at keeping track of friend function template instance contexts. (#GH55509)
 - Clang now prints the correct instantiation context for diagnostics suppressed
   by template argument deduction.

>From 12d9237d7a0650328f6167b1951195cd851f6234 Mon Sep 17 00:00:00 2001
From: Megan <megankuo at asu.edu>
Date: Wed, 12 Mar 2025 01:24:35 -0700
Subject: [PATCH 3/3] [Sema] Fix test diagnostics for copy-ctor-template

---
 clang/test/SemaCXX/copy-ctor-template.cpp | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/clang/test/SemaCXX/copy-ctor-template.cpp b/clang/test/SemaCXX/copy-ctor-template.cpp
index a46a167038cf7..c58bd7c0c5e10 100644
--- a/clang/test/SemaCXX/copy-ctor-template.cpp
+++ b/clang/test/SemaCXX/copy-ctor-template.cpp
@@ -2,13 +2,20 @@
 
 template<class T, class V>
 struct A{
-    A();
-    A(A&);
+    A(); // expected-note{{candidate constructor not viable: requires 0 arguments, but 1 was provided}}
+    A(A&); // expected-note{{candidate constructor not viable: expects an lvalue for 1st argument}} 
     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>'}}
+    A<int, int> a = A<int, int>(); // expected-note{{in instantiation of template class 'A<int, int>'}} 
+    A<int, double> a1 = A<double, int>(); // No error (not a copy constructor)
+}
+
+// Test rvalue-to-lvalue conversion in copy constructor
+A<int, int> &&a(void);
+void g() {
+    A<int, int> a2 = a(); // expected-error{{no matching constructor}}
 }
 
 template<class T, class V>
@@ -17,6 +24,6 @@ struct B{
     template<class U> B(U); // No error (templated constructor)
 };
 
-void g() {
+void h() {
     B<int, int> b = B<int, int>(); // should use implicit copy constructor
 }