[Clang][Sema] Do not add implicit 'const' when matching constexpr function template explicit specializations after C++14 (#92449)

Clang incorrectly accepts the following when using C++14 or later:
```
struct A {
  template<typename T>
  void f() const;

  template<>
  constexpr void f<int>();
};
```
Non-static member functions declared `constexpr` are only implicitly
`const` in C++11. This patch makes clang reject the explicit
specialization of `f` in language modes after C++11.

Author: sdkrystian

clang/docs/ReleaseNotes.rst

@@ -747,6 +747,8 @@ Bug Fixes to C++ Support
 - Clang no longer transforms dependent qualified names into implicit class member access expressions
   until it can be determined whether the name is that of a non-static member.
 - Clang now correctly diagnoses when the current instantiation is used as an incomplete base class.
+- Clang no longer treats ``constexpr`` class scope function template specializations of non-static members
+  as implicitly ``const`` in language modes after C++11.
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplate.cpp

@@ -10255,15 +10255,20 @@ bool Sema::CheckFunctionTemplateSpecialization(
                                 Ovl->getDeclContext()->getRedeclContext()))
         continue;
 
+      QualType FT = FD->getType();
+      // C++11 [dcl.constexpr]p8:
+      //   A constexpr specifier for a non-static member function that is not
+      //   a constructor declares that member function to be const.
+      //
       // When matching a constexpr member function template specialization
       // against the primary template, we don't yet know whether the
       // specialization has an implicit 'const' (because we don't know whether
       // it will be a static member function until we know which template it
-      // specializes), so adjust it now assuming it specializes this template.
-      QualType FT = FD->getType();
-      if (FD->isConstexpr()) {
-        CXXMethodDecl *OldMD =
-          dyn_cast<CXXMethodDecl>(FunTmpl->getTemplatedDecl());
+      // specializes). This rule was removed in C++14.
+      if (auto *NewMD = dyn_cast<CXXMethodDecl>(FD);
+          !getLangOpts().CPlusPlus14 && NewMD && NewMD->isConstexpr() &&
+          !isa<CXXConstructorDecl, CXXDestructorDecl>(NewMD)) {
+        auto *OldMD = dyn_cast<CXXMethodDecl>(FunTmpl->getTemplatedDecl());
         if (OldMD && OldMD->isConst()) {
           const FunctionProtoType *FPT = FT->castAs<FunctionProtoType>();
           FunctionProtoType::ExtProtoInfo EPI = FPT->getExtProtoInfo();

clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p1.cpp

@@ -89,6 +89,9 @@ struct S {
   template<typename T> constexpr T f(); // expected-warning 0-1{{C++14}} expected-note 0-1{{candidate}}
   template <typename T>
   T g() const; // expected-note-re {{candidate template ignored: could not match 'T (){{( __attribute__\(\(thiscall\)\))?}} const' against 'char (){{( __attribute__\(\(thiscall\)\))?}}'}}
+#if __cplusplus >= 201402L
+  // expected-note@-2 {{candidate template ignored: could not match 'T () const' against 'int ()'}}
+#endif
 };
 
 // explicit specialization can differ in constepxr
@@ -100,13 +103,17 @@ template <> notlit S::f() const { return notlit(); }
 #if __cplusplus >= 201402L
 // expected-error@-2 {{no function template matches}}
 #endif
-template <> constexpr int S::g() { return 0; } // expected-note {{previous}}
+template <> constexpr int S::g() { return 0; }
 #if __cplusplus < 201402L
 // expected-warning@-2 {{C++14}}
+// expected-note@-3 {{previous}}
 #else
-// expected-error@-4 {{does not match any declaration in 'S'}}
+// expected-error@-5 {{no function template matches function template specialization 'g'}}
+#endif
+template <> int S::g() const;
+#if __cplusplus < 201402L
+// expected-error@-2 {{non-constexpr declaration of 'g<int>' follows constexpr declaration}}
 #endif
-template <> int S::g() const; // expected-error {{non-constexpr declaration of 'g<int>' follows constexpr declaration}}
 // specializations can drop the 'constexpr' but not the implied 'const'.
 template <> char S::g() { return 0; } // expected-error {{no function template matches}}
 template <> double S::g() const { return 0; } // ok

clang/test/CXX/temp/temp.spec/temp.expl.spec/p12.cpp

@@ -0,0 +1,70 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -verify=expected,cxx11 %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++14 -verify=expected,since-cxx14 %s
+
+struct A {
+  template<typename T>
+  void f0();
+
+  template<>
+  constexpr void f0<short>(); // cxx11-error {{conflicting types for 'f0'}}
+                              // cxx11-note@-1 {{previous declaration is here}}
+                              // cxx11-warning@-2 {{'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const'}}
+
+  template<typename T>
+  void f1() const; // since-cxx14-note 2{{candidate template ignored: could not match 'void () const' against 'void ()'}}
+
+  template<>
+  constexpr void f1<short>(); // since-cxx14-error {{no function template matches function template specialization 'f1'}}
+                              // cxx11-warning@-1 {{'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const'}}
+};
+
+template<>
+constexpr void A::f0<long>(); // cxx11-error {{conflicting types for 'f0'}}
+                              // cxx11-note@-1 {{previous declaration is here}}
+                              // cxx11-warning@-2 {{'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const'}}
+
+template<>
+constexpr void A::f1<long>(); // since-cxx14-error {{no function template matches function template specialization 'f1'}}
+                              // cxx11-warning@-1 {{'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const'}}
+
+// FIXME: It's unclear whether [temp.expl.spec]p12 is intended to apply to
+// members of a class template explicitly specialized for an implicitly
+// instantiated specialization of that template.
+template<typename T>
+struct B {
+  void g0(); // since-cxx14-note {{previous declaration is here}}
+             // cxx11-note@-1 {{member declaration does not match because it is not const qualified}}
+
+  void g1() const; // since-cxx14-note {{member declaration does not match because it is const qualified}}
+                   // cxx11-note@-1 {{previous declaration is here}}
+
+  template<typename U>
+  void h0(); // since-cxx14-note {{previous declaration is here}}
+
+  template<typename U>
+  void h1() const; // cxx11-note {{previous declaration is here}}
+};
+
+template<>
+constexpr void B<short>::g0(); // since-cxx14-error {{constexpr declaration of 'g0' follows non-constexpr declaration}}
+                               // cxx11-error@-1 {{out-of-line declaration of 'g0' does not match any declaration in 'B<short>'}}
+                               // cxx11-warning@-2 {{'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const'}}
+
+template<>
+constexpr void B<short>::g1(); // since-cxx14-error {{out-of-line declaration of 'g1' does not match any declaration in 'B<short>'}}
+                               // cxx11-error@-1 {{constexpr declaration of 'g1' follows non-constexpr declaration}}
+                               // cxx11-warning@-2 {{'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const'}}
+
+template<>
+template<typename U>
+constexpr void B<long>::h0(); // since-cxx14-error {{constexpr declaration of 'h0' follows non-constexpr declaration}}
+                              // cxx11-error@-1 {{out-of-line declaration of 'h0' does not match any declaration in 'B<long>'}}
+                              // cxx11-warning@-2 {{'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const'}}
+
+template<>
+template<typename U>
+constexpr void B<long>::h1(); // since-cxx14-error {{out-of-line declaration of 'h1' does not match any declaration in 'B<long>'}}
+                              // cxx11-error@-1 {{constexpr declaration of 'h1' follows non-constexpr declaration}}
+                              // cxx11-warning@-2 {{'constexpr' non-static member function will not be implicitly 'const' in C++14; add 'const'}}
+
+