[clang] deprecate frelaxed-template-template-args, make it on by default

A resolution to the ambiguity issues created by P0522, which is a DR solving
CWG 150, did not come as expected, so we are just going to accept the change,
and watch how users digest it.

For now we deprecate the flag with a warning, and make it on by default.
We don't remove the flag completely in order to give users a chance to
work around any problems by disabling it.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D109496

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -355,7 +355,7 @@ def warn_drv_diagnostics_hotness_requires_pgo : Warning<
 def warn_drv_clang_unsupported : Warning<
   "the clang compiler does not support '%0'">;
 def warn_drv_deprecated_arg : Warning<
-  "argument '%0' is deprecated, use '%1' instead">, InGroup<Deprecated>;
+  "argument '%0' is deprecated%select{|, use '%2' instead}1">, InGroup<Deprecated>;
 def warn_drv_assuming_mfloat_abi_is : Warning<
   "unknown platform, assuming -mfloat-abi=%0">;
 def warn_ignoring_ftabstop_value : Warning<

clang/include/clang/Basic/LangOptions.def

@@ -149,7 +149,7 @@ LANGOPT(NoMathBuiltin     , 1, 0, "disable math builtin functions")
 LANGOPT(GNUAsm            , 1, 1, "GNU-style inline assembly")
 LANGOPT(Coroutines        , 1, 0, "C++20 coroutines")
 LANGOPT(DllExportInlines  , 1, 1, "dllexported classes dllexport inline methods")
-LANGOPT(RelaxedTemplateTemplateArgs, 1, 0, "C++17 relaxed matching of template template arguments")
+LANGOPT(RelaxedTemplateTemplateArgs, 1, 1, "C++17 relaxed matching of template template arguments")
 
 LANGOPT(DoubleSquareBracketAttributes, 1, 0, "'[[]]' attributes extension for all language standard modes")
 

clang/include/clang/Driver/Options.td

@@ -2335,9 +2335,9 @@ defm application_extension : BoolFOption<"application-extension",
   PosFlag<SetTrue, [CC1Option], "Restrict code to those available for App Extensions">,
   NegFlag<SetFalse>>;
 defm relaxed_template_template_args : BoolFOption<"relaxed-template-template-args",
-  LangOpts<"RelaxedTemplateTemplateArgs">, DefaultFalse,
-  PosFlag<SetTrue, [CC1Option], "Enable C++17 relaxed template template argument matching">,
-  NegFlag<SetFalse>>;
+  LangOpts<"RelaxedTemplateTemplateArgs">, DefaultTrue,
+  PosFlag<SetTrue>,
+  NegFlag<SetFalse, [CC1Option], "Disable C++17 relaxed template template argument matching">>;
 defm sized_deallocation : BoolFOption<"sized-deallocation",
   LangOpts<"SizedDeallocation">, DefaultFalse,
   PosFlag<SetTrue, [CC1Option], "Enable C++14 sized global deallocation functions">,

clang/lib/Driver/SanitizerArgs.cpp

@@ -683,7 +683,8 @@ SanitizerArgs::SanitizerArgs(const ToolChain &TC,
         Arg->claim();
         if (LegacySanitizeCoverage != 0) {
           D.Diag(diag::warn_drv_deprecated_arg)
-              << Arg->getAsString(Args) << "-fsanitize-coverage=trace-pc-guard";
+              << Arg->getAsString(Args) << true
+              << "-fsanitize-coverage=trace-pc-guard";
         }
         continue;
       }
@@ -718,11 +719,11 @@ SanitizerArgs::SanitizerArgs(const ToolChain &TC,
   // enabled.
   if (CoverageFeatures & CoverageTraceBB)
     D.Diag(clang::diag::warn_drv_deprecated_arg)
-        << "-fsanitize-coverage=trace-bb"
+        << "-fsanitize-coverage=trace-bb" << true
         << "-fsanitize-coverage=trace-pc-guard";
   if (CoverageFeatures & Coverage8bitCounters)
     D.Diag(clang::diag::warn_drv_deprecated_arg)
-        << "-fsanitize-coverage=8bit-counters"
+        << "-fsanitize-coverage=8bit-counters" << true
         << "-fsanitize-coverage=trace-pc-guard";
 
   int InsertionPointTypes = CoverageFunc | CoverageBB | CoverageEdge;
@@ -732,7 +733,7 @@ SanitizerArgs::SanitizerArgs(const ToolChain &TC,
   if ((CoverageFeatures & InsertionPointTypes) &&
       !(CoverageFeatures & InstrumentationTypes)) {
     D.Diag(clang::diag::warn_drv_deprecated_arg)
-        << "-fsanitize-coverage=[func|bb|edge]"
+        << "-fsanitize-coverage=[func|bb|edge]" << true
         << "-fsanitize-coverage=[func|bb|edge],[trace-pc-guard|trace-pc]";
   }
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -6391,12 +6391,14 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                     options::OPT_fno_assume_sane_operator_new))
     CmdArgs.push_back("-fno-assume-sane-operator-new");
 
-  // -frelaxed-template-template-args is off by default, as it is a severe
-  // breaking change until a corresponding change to template partial ordering
-  // is provided.
-  if (Args.hasFlag(options::OPT_frelaxed_template_template_args,
-                   options::OPT_fno_relaxed_template_template_args, false))
-    CmdArgs.push_back("-frelaxed-template-template-args");
+  // -frelaxed-template-template-args is deprecated and on by default.
+  if (Arg *A =
+          Args.getLastArg(options::OPT_frelaxed_template_template_args,
+                          options::OPT_fno_relaxed_template_template_args)) {
+    D.Diag(diag::warn_drv_deprecated_arg) << A->getAsString(Args) << false;
+    if (A->getOption().matches(options::OPT_fno_relaxed_template_template_args))
+      CmdArgs.push_back("-fno-relaxed-template-template-args");
+  }
 
   // -fsized-deallocation is off by default, as it is an ABI-breaking change for
   // most platforms.

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -1890,12 +1890,12 @@ void tools::checkAMDGPUCodeObjectVersion(const Driver &D,
 
   // Emit warnings for legacy options even if they are overridden.
   if (Args.hasArg(options::OPT_mno_code_object_v3_legacy))
-    D.Diag(diag::warn_drv_deprecated_arg) << "-mno-code-object-v3"
-                                          << "-mcode-object-version=2";
+    D.Diag(diag::warn_drv_deprecated_arg)
+        << "-mno-code-object-v3" << true << "-mcode-object-version=2";
 
   if (Args.hasArg(options::OPT_mcode_object_v3_legacy))
-    D.Diag(diag::warn_drv_deprecated_arg) << "-mcode-object-v3"
-                                          << "-mcode-object-version=3";
+    D.Diag(diag::warn_drv_deprecated_arg)
+        << "-mcode-object-v3" << true << "-mcode-object-version=3";
 
   if (auto *CodeObjArg = getAMDGPUCodeObjectArgument(D, Args)) {
     if (CodeObjArg->getOption().getID() ==

clang/lib/Sema/SemaTemplate.cpp

@@ -7407,9 +7407,7 @@ bool Sema::CheckTemplateTemplateArgument(TemplateTemplateParmDecl *Param,
   // C++1z [temp.arg.template]p3: (DR 150)
   //   A template-argument matches a template template-parameter P when P
   //   is at least as specialized as the template-argument A.
-  // FIXME: We should enable RelaxedTemplateTemplateArgs by default as it is a
-  //  defect report resolution from C++17 and shouldn't be introduced by
-  //  concepts.
+  // FIXME: RelaxedTemplateTemplateArgs is deprecated, should be always on.
   if (getLangOpts().RelaxedTemplateTemplateArgs) {
     // Quick check for the common case:
     //   If P contains a parameter pack, then A [...] matches P if each of A's

clang/test/CXX/temp/temp.arg/temp.arg.template/p3-2a.cpp

@@ -1,4 +1,4 @@
-// RUN:  %clang_cc1 -std=c++2a -frelaxed-template-template-args -verify %s
+// RUN:  %clang_cc1 -std=c++2a -verify %s
 
 template<typename T> concept C = T::f();
 // expected-note@-1{{similar constraint}}

clang/test/Driver/frelaxed-template-template-args.cpp

@@ -0,0 +1,5 @@
+// RUN: %clang -fsyntax-only -frelaxed-template-template-args %s 2>&1 | FileCheck --check-prefix=CHECK-ON %s
+// RUN: %clang -fsyntax-only -fno-relaxed-template-template-args %s 2>&1 | FileCheck --check-prefix=CHECK-OFF %s
+
+// CHECK-ON:  warning: argument '-frelaxed-template-template-args' is deprecated [-Wdeprecated]
+// CHECK-OFF: warning: argument '-fno-relaxed-template-template-args' is deprecated [-Wdeprecated]

clang/test/Lexer/cxx-features.cpp

@@ -5,7 +5,7 @@
 // RUN: %clang_cc1 -std=c++20 -fcxx-exceptions -fsized-deallocation -verify %s
 // RUN: %clang_cc1 -std=c++2b -fcxx-exceptions -fsized-deallocation -verify %s
 //
-// RUN: %clang_cc1 -std=c++17 -fcxx-exceptions -fsized-deallocation -frelaxed-template-template-args -DRELAXED_TEMPLATE_TEMPLATE_ARGS=1 -verify %s
+// RUN: %clang_cc1 -std=c++17 -fcxx-exceptions -fsized-deallocation -fno-relaxed-template-template-args -DNO_RELAXED_TEMPLATE_TEMPLATE_ARGS=1 -verify %s
 // RUN: %clang_cc1 -std=c++17 -fcxx-exceptions -fsized-deallocation -DCONCEPTS_TS=1 -verify %s
 // RUN: %clang_cc1 -std=c++14 -fno-rtti -fno-threadsafe-statics -verify %s -DNO_EXCEPTIONS -DNO_RTTI -DNO_THREADSAFE_STATICS -fsized-deallocation
 // RUN: %clang_cc1 -std=c++14 -fcoroutines-ts -DNO_EXCEPTIONS -DCOROUTINES -verify -fsized-deallocation %s
@@ -195,9 +195,9 @@
 #error "wrong value for __cpp_nontype_template_args"
 #endif
 
-#if defined(RELAXED_TEMPLATE_TEMPLATE_ARGS) \
-    ? check(template_template_args, 0, 0, 0, 201611, 201611, 201611) \
-    : check(template_template_args, 0, 0, 0, 0, 0, 0)
+#if defined(NO_RELAXED_TEMPLATE_TEMPLATE_ARGS) \
+    ? check(template_template_args, 0, 0, 0, 0, 0, 0) \
+    : check(template_template_args, 201611, 201611, 201611, 201611, 201611, 201611)
 #error "wrong value for __cpp_template_template_args"
 #endif
 

clang/test/SemaTemplate/deduction.cpp

@@ -60,12 +60,14 @@ struct Replace<const T, Arg1, Arg2> {
 // Replacement of templates
 template<template<typename> class TT, typename T1, typename Arg1, typename Arg2>
 struct Replace<TT<T1>, Arg1, Arg2> {
+// expected-note@-1 2 {{partial specialization matches}}
   typedef TT<typename Replace<T1, Arg1, Arg2>::type> type;
 };
 
 template<template<typename, typename> class TT, typename T1, typename T2,
          typename Arg1, typename Arg2>
 struct Replace<TT<T1, T2>, Arg1, Arg2> {
+// expected-note@-1 2 {{partial specialization matches}}
   typedef TT<typename Replace<T1, Arg1, Arg2>::type,
              typename Replace<T2, Arg1, Arg2>::type> type;
 };
@@ -79,8 +81,19 @@ struct Replace<TT<T1, _2>, Arg1, Arg2> {
 
 int array0[is_same<Replace<_1, int, float>::type, int>::value? 1 : -1];
 int array1[is_same<Replace<const _1, int, float>::type, const int>::value? 1 : -1];
+
 int array2[is_same<Replace<vector<_1>, int, float>::type, vector<int> >::value? 1 : -1];
+// expected-error@-1 {{ambiguous partial specializations of 'Replace<vector<_1>, int, float>'}}
+// FIXME: Some bad error recovery from the parser here:
+// expected-error@-3 {{expected '(' for function-style cast or type construction}}
+// expected-error@-4 {{no member named 'value' in the global namespace}}
+
 int array3[is_same<Replace<vector<const _1>, int, float>::type, vector<const int> >::value? 1 : -1];
+// expected-error@-1 {{ambiguous partial specializations of 'Replace<vector<const _1>, int, float>'}}
+// FIXME: Some bad error recovery from the parser here:
+// expected-error@-3 {{expected '(' for function-style cast or type construction}}
+// expected-error@-4 {{no member named 'value' in the global namespace}}
+
 int array4[is_same<Replace<vector<int, _2>, double, float>::type, vector<int, float> >::value? 1 : -1];
 
 // PR5911

clang/test/SemaTemplate/default-arguments.cpp

@@ -112,15 +112,14 @@ template<typename T, template<typename> class X = T::template apply>
 int array4[is_same<X4<add_pointer>, 
                    X4<add_pointer, add_pointer::apply> >::value? 1 : -1];
 
-template<int> struct X5 {}; // expected-note{{has a different type 'int'}}
+template<int> struct X5 {};
 template<long> struct X5b {};
 template<typename T, 
-         template<T> class B = X5> // expected-error{{template template argument has different}} \
-                                   // expected-note{{previous non-type template parameter}}
+         template<T> class B = X5>
   struct X6 {};
 
 X6<int> x6a;
-X6<long> x6b; // expected-note{{while checking a default template argument}}
+X6<long> x6b;
 X6<long, X5b> x6c;
 
 

clang/test/SemaTemplate/instantiate-template-template-parm.cpp

@@ -20,30 +20,29 @@ apply<add_reference, int>::type ir = i;
 apply<add_reference, float>::type fr = i; // expected-error{{non-const lvalue reference to type 'float' cannot bind to a value of unrelated type 'int'}}
 
 // Template template parameters
-template<int> struct B; // expected-note{{has a different type 'int'}}
+template<int> struct B;
 
 template<typename T, 
-         template<T Value> class X> // expected-error{{cannot have type 'float'}} \
-                                    // expected-note{{with type 'long'}}
+         template<T Value> class X> // expected-error{{cannot have type 'float'}}
 struct X0 { };
 
 X0<int, B> x0b1;
 X0<float, B> x0b2; // expected-note{{while substituting}}
-X0<long, B> x0b3; // expected-error{{template template argument has different template parameters}}
+X0<long, B> x0b3;
 
-template<template<int V> class TT> // expected-note{{parameter with type 'int'}}
+template<template<int V> class TT>
 struct X1 { };
 
 template<typename T, template<T V> class TT>
 struct X2 {
-  X1<TT> x1; // expected-error{{has different template parameters}}
+  X1<TT> x1;
 };
 
 template<int V> struct X3i { };
-template<long V> struct X3l { }; // expected-note{{different type 'long'}}
+template<long V> struct X3l { };
 
 X2<int, X3i> x2okay;
-X2<long, X3l> x2bad; // expected-note{{instantiation}}
+X2<long, X3l> x2bad;
 
 template <typename T, template <T, T> class TT, class R = TT<1, 2> >
 struct Comp {

clang/test/SemaTemplate/nested-template.cpp

@@ -112,18 +112,16 @@ template struct X1<int>::B<bool>;
 // Template template parameters
 template<typename T>
 struct X2 {
-  template<template<class U, T Value> class>  // expected-error{{cannot have type 'float'}} \
-                                              // expected-note{{previous non-type template}}
+  template<template<class U, T Value> class>  // expected-error{{cannot have type 'float'}}
     struct Inner { };
 };
 
-template<typename T, 
-         int Value> // expected-note{{template non-type parameter}}
+template<typename T, int Value>
   struct X2_arg;
 
 X2<int>::Inner<X2_arg> x2i1;
 X2<float> x2a; // expected-note{{instantiation}}
-X2<long>::Inner<X2_arg> x2i3; // expected-error{{template template argument has different}}
+X2<long>::Inner<X2_arg> x2i3;
 
 namespace PR10896 {
   template<typename TN>

clang/test/SemaTemplate/temp_arg_template.cpp

@@ -6,11 +6,11 @@ template<template<typename T> class X> struct A; // expected-note 2{{previous te
 
 template<template<typename T, int I> class X> struct B; // expected-note{{previous template template parameter is here}}
 
-template<template<int I> class X> struct C;  // expected-note 2{{previous non-type template parameter with type 'int' is here}}
+template<template<int I> class X> struct C;  // expected-note {{previous non-type template parameter with type 'int' is here}}
 
 template<class> struct X; // expected-note{{too few template parameters in template template argument}}
 template<int N> struct Y; // expected-note{{template parameter has a different kind in template argument}}
-template<long N> struct Ylong; // expected-note{{template non-type parameter has a different type 'long' in template argument}}
+template<long N> struct Ylong;
 template<const int &N> struct Yref; // expected-note{{template non-type parameter has a different type 'const int &' in template argument}}
 
 namespace N {
@@ -27,7 +27,7 @@ A<Y> *a4; // expected-error{{template template argument has different template p
 A<TooMany> *a5; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
 B<X> *a6; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
 C<Y> *a7;
-C<Ylong> *a8; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
+C<Ylong> *a8;
 C<Yref> *a9; // expected-error{{template template argument has different template parameters than its corresponding template template parameter}}
 
 template<typename T> void f(int);

clang/test/SemaTemplate/temp_arg_template_cxx1z.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z -frelaxed-template-template-args %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++1z %s
 
 // expected-note@temp_arg_template_cxx1z.cpp:* 1+{{}}
 

clang/www/cxx_status.html

@@ -596,7 +596,7 @@ <h2 id="cxx17">C++17 implementation status</h2>
 <p>You can use Clang in C++17 mode with the <code>-std=c++17</code> option
 (use <code>-std=c++1z</code> in Clang 4 and earlier).</p>
 
-<details open>
+<details>
 <summary>List of features and minimum Clang version with support</summary>
 
 <table width="689" border="1" cellspacing="0">
@@ -813,8 +813,8 @@ <h2 id="cxx17">C++17 implementation status</h2>
     <!-- Issaquah 2016 papers -->
     <tr>
       <td>Matching template template parameters to compatible arguments</td>
-      <td><a href="https://wg21.link/p0522r0">P0522R0</a></td>
-      <td class="partial" align="center">Partial <a href="#p0522">(10)</a></td>
+      <td><a href="https://wg21.link/p0522r0">P0522R0</a> (<a href="#dr">DR</a>)</td>
+      <td class="full" align="center">Clang 4 <a href="#p0522">(10)</a></td>
     </tr>
     <tr>
       <td>Removing deprecated dynamic exception specifications</td>
@@ -842,13 +842,10 @@ <h2 id="cxx17">C++17 implementation status</h2>
 reverse construction order in that ABI.
 This is not fully supported during constant expression evaluation until Clang 12.
 </span><br>
-<span id="p0522">(10): Despite being the resolution to a Defect Report, this
-feature is disabled by default in all language versions, and can be enabled
-explicitly with the flag <tt>-frelaxed-template-template-args</tt> in Clang 4
-onwards.
-The change to the standard lacks a corresponding change for template partial
-ordering, resulting in ambiguity errors for reasonable and previously-valid
-code. This issue is expected to be rectified soon.
+<span id="p0522">(10): Prior to Clang 14, this feature is not enabled by
+default, but can be enabled with <tt>-frelaxed-template-template-args</tt>.
+Starting from Clang 14, the flag is deprecated and will be removed in a future
+version.
 </span>
 </p>
 </details>