[clang] Implement CWG1878 "`operator auto` template" #78103

Endilll · 2024-01-14T16:50:27Z

C++14 introduced deduced return type for regular functions, but shortly after 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

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.

llvmbot · 2024-01-14T16:50:55Z

@llvm/pr-subscribers-clang

Author: Vlad Serebrennikov (Endilll)

Changes

C++14 introduced deduced return type for regular functions, but shortly after 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

Full diff: https://github.com/llvm/llvm-project/pull/78103.diff

6 Files Affected:

(modified) clang/docs/ReleaseNotes.rst (+2)
(modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2-1)
(modified) clang/lib/Sema/SemaDeclCXX.cpp (+9-1)
(modified) clang/test/CXX/drs/dr18xx.cpp (+21-5)
(modified) clang/test/SemaCXX/deduced-return-type-cxx14.cpp (+22-6)
(modified) clang/www/cxx_dr_status.html (+1-1)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3cbce1be1594376..271e641a5ccce0f 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -560,6 +560,8 @@ 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>`_)
 
 
 Improvements to Clang's time-trace
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 1a79892e40030ae..de99d753a13a6cd 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/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|"
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index 36e53c684ac4dc3..613dccbf445f91d 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -11323,8 +11323,16 @@ Decl *Sema::ActOnConversionDeclarator(CXXConversionDecl *Conversion) {
   }
 
   if (FunctionTemplateDecl *ConversionTemplate
-                                = Conversion->getDescribedFunctionTemplate())
+                                = Conversion->getDescribedFunctionTemplate()) {
+    if (ConvType->isUndeducedAutoType()) {
+      Diag(Conversion->getTypeSpecStartLoc(), diag::err_auto_not_allowed)
+        << Conversion->getTypeSourceInfo()->getTypeLoc().getContainedAutoTypeLoc().getSourceRange()
+        << llvm::to_underlying(Conversion->getConversionType()->getAs<AutoType>()->getKeyword())
+        << /* in declaration of conversion function template */ 24;
+    }
+
     return ConversionTemplate;
+  }
 
   return Conversion;
 }
diff --git a/clang/test/CXX/drs/dr18xx.cpp b/clang/test/CXX/drs/dr18xx.cpp
index 175c39e8b73abbe..8be94ed49e639d2 100644
--- a/clang/test/CXX/drs/dr18xx.cpp
+++ b/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__)
@@ -317,6 +317,22 @@ namespace dr1872 { // dr1872: 9
 #endif
 }
 
+namespace dr1878 { // dr1878: 18
+#if __cplusplus >= 201402L
+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 decltype(auto)() const { return unsigned(); }
+  // since-cxx14-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
+};
+#endif
+}
+
 namespace dr1881 { // dr1881: 7
   struct A { int a : 4; };
   struct B : A { int b : 3; };
diff --git a/clang/test/SemaCXX/deduced-return-type-cxx14.cpp b/clang/test/SemaCXX/deduced-return-type-cxx14.cpp
index eac9c587869f552..6533a9e561f248f 100644
--- a/clang/test/SemaCXX/deduced-return-type-cxx14.cpp
+++ b/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-cxx14,cxx20_23,cxx23    %s
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify=expected,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-cxx14,cxx14_20,cxx20_23 %s
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify=expected,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 +686,19 @@ auto f(auto x) { // cxx14-error {{'auto' not allowed in function prototype}}
 }
 
 }
+
+struct DeducedTargetTypeOfConversionFunction {
+  operator auto() const { return char(); }
+  operator const auto() const { return float(); }
+  operator decltype(auto)() const { return double(0); }
+
+  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 decltype(auto)() const { return unsigned(); }
+  // since-cxx14-error@-1 {{'decltype(auto)' not allowed in declaration of conversion function template}}
+};
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index 397bf1357d3cb36..c0b481936942304 100755
--- a/clang/www/cxx_dr_status.html
+++ b/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>

github-actions · 2024-01-14T16:52:42Z

✅ With the latest revision this PR passed the C/C++ code formatter.

cor3ntin · 2024-01-14T17:11:13Z

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

+
+struct DeducedTargetTypeOfConversionFunction {
+  operator auto() const { return char(); }
+  operator const auto() const { return float(); }


can you add tests with references?

clang/lib/Sema/SemaDeclCXX.cpp

Add tests with constrained auto and references, adapt diagnostic emitter accordingly.

cor3ntin

LGTM, thanks
Feel free to refactor err_auto_not_allowed in a separate PR

cor3ntin · 2024-01-18T14:46:40Z

clang/lib/Sema/SemaDeclCXX.cpp

+      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;


Suggested change

<< /* in declaration of conversion function template */ 24;

<< /* in declaration of conversion function template= */ 24;

shafik · 2024-01-21T00:22:42Z

clang/lib/Sema/SemaDeclCXX.cpp

+      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;


warn_cconv_unsupported uses CallingConventionIgnoredReason enum to specify which one.

Not sure if this is something we want to use more often or not.

Endilll added c++14 clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Jan 14, 2024

Endilll requested review from cor3ntin and AaronBallman January 14, 2024 16:50

llvmbot added the clang Clang issues not falling into any other category label Jan 14, 2024

Run clang-format

3c43f0f

cor3ntin reviewed Jan 14, 2024

View reviewed changes

Endilll added 4 commits January 17, 2024 14:38

Simplify extracting source range info for return type

119ab09

Address Corentin's feedback

5558f55

Add tests with constrained auto and references, adapt diagnostic emitter accordingly.

Merge remote-tracking branch 'upstream/main' into cwg1878

55ecf13

Add auto * test and simplify diag emitting logic

85a1ca2

cor3ntin approved these changes Jan 18, 2024

View reviewed changes

Apply Corentin's fix

54bba24

Endilll merged commit 9eb0f86 into llvm:main Jan 20, 2024
5 checks passed

Endilll deleted the cwg1878 branch January 20, 2024 08:23

shafik reviewed Jan 21, 2024

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[clang] Implement CWG1878 "`operator auto` template" #78103

[clang] Implement CWG1878 "`operator auto` template" #78103

Endilll commented Jan 14, 2024

llvmbot commented Jan 14, 2024

github-actions bot commented Jan 14, 2024 •

edited

cor3ntin Jan 14, 2024

Endilll Jan 18, 2024

cor3ntin left a comment

cor3ntin Jan 18, 2024

shafik Jan 21, 2024

	<< /* in declaration of conversion function template */ 24;
	<< /* in declaration of conversion function template= */ 24;

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

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

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Endilll commented Jan 14, 2024

llvmbot commented Jan 14, 2024

github-actions bot commented Jan 14, 2024 • edited

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[clang] Implement CWG1878 "`operator auto` template" #78103

[clang] Implement CWG1878 "`operator auto` template" #78103

github-actions bot commented Jan 14, 2024 •

edited