[clang] Implement CWG1878 "operator auto template" (#78103)

C++14 introduced deduced return type for regular functions, but shortly after [CWG1878](https://wg21.link/cwg1878) was filed and resolved to disallow deduced return types in conversion function templates. So this patch diagnoses such usage of deduced return type in C++14 mode onwards.

Fixes #51776

clang/docs/ReleaseNotes.rst

@@ -575,6 +575,8 @@ Improvements to Clang's diagnostics
 - Clang now diagnoses unexpanded packs within the template argument lists of function template specializations.
 - Clang now diagnoses attempts to bind a bitfield to an NTTP of a reference type as erroneous
   converted constant expression and not as a reference to subobject.
+- Clang now diagnoses ``auto`` and ``decltype(auto)`` in declarations of conversion function template
+  (`CWG1878: <https://cplusplus.github.io/CWG/issues/1878.html>`_)
 - Clang now diagnoses the requirement that non-template friend declarations with requires clauses
   and template friend declarations with a constraint that depends on a template parameter from an
   enclosing template must be a definition.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2411,7 +2411,8 @@ def err_auto_not_allowed : Error<
   "|in type allocated by 'new'|in K&R-style function parameter"
   "|in template parameter|in friend declaration|in function prototype that is "
   "not a function declaration|in requires expression parameter"
-  "|in array declaration}1">;
+  "|in array declaration"
+  "|in declaration of conversion function template}1">;
 def err_dependent_deduced_tst : Error<
   "typename specifier refers to "
   "%select{class template|function template|variable template|alias template|"

clang/lib/Sema/SemaDeclCXX.cpp

@@ -11321,9 +11321,20 @@ Decl *Sema::ActOnConversionDeclarator(CXXConversionDecl *Conversion) {
       << ClassType << ConvType;
   }
 
-  if (FunctionTemplateDecl *ConversionTemplate
-                                = Conversion->getDescribedFunctionTemplate())
+  if (FunctionTemplateDecl *ConversionTemplate =
+          Conversion->getDescribedFunctionTemplate()) {
+    if (const auto *ConvTypePtr = ConvType->getAs<PointerType>()) {
+      ConvType = ConvTypePtr->getPointeeType();
+    }
+    if (ConvType->isUndeducedAutoType()) {
+      Diag(Conversion->getTypeSpecStartLoc(), diag::err_auto_not_allowed)
+          << getReturnTypeLoc(Conversion).getSourceRange()
+          << llvm::to_underlying(ConvType->getAs<AutoType>()->getKeyword())
+          << /* in declaration of conversion function template= */ 24;
+    }
+
     return ConversionTemplate;
+  }
 
   return Conversion;
 }

clang/test/CXX/drs/dr18xx.cpp

@@ -1,10 +1,10 @@
 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify=expected,cxx98-14,cxx98 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify=expected,cxx98-14,cxx11-17,since-cxx11 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify=expected,cxx98-14,cxx11-17,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx17,cxx11-17,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify=expected,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,cxx98-14,cxx11-17,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,since-cxx17,cxx11-17,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify=expected,since-cxx14,since-cxx17,since-cxx20,since-cxx11,since-cxx14 -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 
 #if __cplusplus == 199711L
 #define static_assert(...) __extension__ _Static_assert(__VA_ARGS__)
@@ -319,6 +319,41 @@ namespace dr1872 { // dr1872: 9
 #endif
 }
 
+namespace dr1878 { // dr1878: 18
+#if __cplusplus >= 201402L
+#if __cplusplus >= 202002L
+template <typename T>
+concept C = true;
+#endif
+
+struct S {
+  template <typename T>
+  operator auto() const { return short(); }
+  // since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
+  template <typename T>
+  operator const auto() const { return int(); }
+  // since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
+  template <typename T>
+  operator const auto&() const { return char(); }
+  // since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
+  template <typename T>
+  operator const auto*() const { return long(); }
+  // since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
+  template <typename T>
+  operator decltype(auto)() const { return unsigned(); }
+  // since-cxx14-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
+#if __cplusplus >= 202002L
+  template <typename T>
+  operator C auto() const { return float(); }
+  // since-cxx20-error@-1 {{'auto' not allowed in declaration of conversion function template}}
+  template <typename T>
+  operator C decltype(auto)() const { return double(); }
+  // since-cxx20-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
+#endif
+};
+#endif
+}
+
 namespace dr1881 { // dr1881: 7
   struct A { int a : 4; };
   struct B : A { int b : 3; };

clang/test/SemaCXX/deduced-return-type-cxx14.cpp

@@ -1,11 +1,11 @@
-// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,cxx20_23,cxx23    %s
-// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,cxx20_23,cxx23    %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,since-cxx20,since-cxx14,cxx20_23,cxx23    %s
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,since-cxx20,since-cxx14,cxx20_23,cxx23    %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
 
-// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,cxx14_20,cxx20_23 %s
-// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,cxx14_20,cxx20_23 %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,since-cxx20,since-cxx14,cxx14_20,cxx20_23 %s
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,since-cxx20,since-cxx14,cxx14_20,cxx20_23 %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
 
-// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify=expected,cxx14_20,cxx14    %s
-// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify=expected,cxx14_20,cxx14    %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
+// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify=expected,since-cxx14,cxx14_20,cxx14    %s
+// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify=expected,since-cxx14,cxx14_20,cxx14    %s -fdelayed-template-parsing -DDELAYED_TEMPLATE_PARSING
 
 auto f(); // expected-note {{previous}}
 int f(); // expected-error {{differ only in their return type}}
@@ -686,3 +686,41 @@ auto f(auto x) { // cxx14-error {{'auto' not allowed in function prototype}}
 }
 
 }
+
+#if __cplusplus >= 202002L
+template <typename T>
+concept C = true;
+#endif
+
+struct DeducedTargetTypeOfConversionFunction {
+  operator auto() const { return char(); }
+  operator const auto() const { return float(); }
+  operator const auto&() const { return int(); }
+  // expected-warning@-1 {{returning reference to local temporary object}}
+  operator decltype(auto)() const { return double(); }
+#if __cplusplus >= 202002L
+  operator C auto() const { return unsigned(); }
+  operator C decltype(auto)() const { return long(); }
+#endif
+
+  template <typename T>
+  operator auto() const { return short(); }
+  // since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
+  template <typename T>
+  operator const auto() const { return int(); }
+  // since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
+  template <typename T>
+  operator const auto&() const { return char(); }
+  // since-cxx14-error@-1 {{'auto' not allowed in declaration of conversion function template}}
+  template <typename T>
+  operator decltype(auto)() const { return unsigned(); }
+  // since-cxx14-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
+#if __cplusplus >= 202002L
+  template <typename T>
+  operator C auto() const { return float(); }
+  // since-cxx20-error@-1 {{'auto' not allowed in declaration of conversion function template}}
+  template <typename T>
+  operator C decltype(auto)() const { return double(); }
+  // since-cxx20-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
+#endif
+};

clang/www/cxx_dr_status.html

@@ -11076,7 +11076,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1878.html">1878</a></td>
     <td>CD4</td>
     <td><TT>operator auto</TT> template</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 18</td>
   </tr>
   <tr id="1879">
     <td><a href="https://cplusplus.github.io/CWG/issues/1879.html">1879</a></td>