[Sema] Add -Wc++11-narrowing-const-reference

llvm#75332 diagnosed narrowing
involving const reference. Our depot has hundreds if not thousands of
breakages
(llvm#75332 (comment)).
Add a subgroup of -Wc++11-narrowing to help users gradually fix their
issues without regressing the existing -Wc++11-narrowing diagnostics.

Author: MaskRay

clang/include/clang/Basic/DiagnosticGroups.td

@@ -348,7 +348,8 @@ def CXX98CompatPedantic : DiagGroup<"c++98-compat-pedantic",
                                      CXXPre20CompatPedantic,
                                      CXXPre23CompatPedantic]>;
 
-def CXX11Narrowing : DiagGroup<"c++11-narrowing">;
+def CXX11NarrowingConstReference : DiagGroup<"c++11-narrowing-const-reference">;
+def CXX11Narrowing : DiagGroup<"c++11-narrowing", [CXX11NarrowingConstReference]>;
 
 def CXX11WarnInconsistentOverrideDestructor :
   DiagGroup<"inconsistent-missing-destructor-override">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6158,12 +6158,24 @@ def err_illegal_initializer_type : Error<"illegal initializer type %0">;
 def ext_init_list_type_narrowing : ExtWarn<
   "type %0 cannot be narrowed to %1 in initializer list">,
   InGroup<CXX11Narrowing>, DefaultError, SFINAEFailure;
+// *_narrowing_const_reference diagnostics have the same messages, but are
+// controlled by -Wc++11-narrowing-const-reference for narrowing involving a
+// const reference.
+def ext_init_list_type_narrowing_const_reference : ExtWarn<
+  "type %0 cannot be narrowed to %1 in initializer list">,
+  InGroup<CXX11NarrowingConstReference>, DefaultError, SFINAEFailure;
 def ext_init_list_variable_narrowing : ExtWarn<
   "non-constant-expression cannot be narrowed from type %0 to %1 in "
   "initializer list">, InGroup<CXX11Narrowing>, DefaultError, SFINAEFailure;
+def ext_init_list_variable_narrowing_const_reference : ExtWarn<
+  "non-constant-expression cannot be narrowed from type %0 to %1 in "
+  "initializer list">, InGroup<CXX11NarrowingConstReference>, DefaultError, SFINAEFailure;
 def ext_init_list_constant_narrowing : ExtWarn<
   "constant expression evaluates to %0 which cannot be narrowed to type %1">,
   InGroup<CXX11Narrowing>, DefaultError, SFINAEFailure;
+def ext_init_list_constant_narrowing_const_reference : ExtWarn<
+  "constant expression evaluates to %0 which cannot be narrowed to type %1">,
+  InGroup<CXX11NarrowingConstReference>, DefaultError, SFINAEFailure;
 def warn_init_list_type_narrowing : Warning<
   "type %0 cannot be narrowed to %1 in initializer list in C++11">,
   InGroup<CXX11Narrowing>, DefaultIgnore;

clang/lib/Sema/SemaInit.cpp

@@ -10410,40 +10410,53 @@ static void DiagnoseNarrowingInInitList(Sema &S,
     // No narrowing occurred.
     return;
 
-  case NK_Type_Narrowing:
+  case NK_Type_Narrowing: {
     // This was a floating-to-integer conversion, which is always considered a
     // narrowing conversion even if the value is a constant and can be
     // represented exactly as an integer.
-    S.Diag(PostInit->getBeginLoc(), NarrowingErrs(S.getLangOpts())
-                                        ? diag::ext_init_list_type_narrowing
-                                        : diag::warn_init_list_type_narrowing)
+    QualType T = EntityType.getNonReferenceType();
+    S.Diag(PostInit->getBeginLoc(),
+           NarrowingErrs(S.getLangOpts())
+               ? (T == EntityType
+                      ? diag::ext_init_list_type_narrowing
+                      : diag::ext_init_list_type_narrowing_const_reference)
+               : diag::warn_init_list_type_narrowing)
         << PostInit->getSourceRange()
         << PreNarrowingType.getLocalUnqualifiedType()
-        << EntityType.getNonReferenceType().getLocalUnqualifiedType();
+        << T.getLocalUnqualifiedType();
     break;
+  }
 
-  case NK_Constant_Narrowing:
+  case NK_Constant_Narrowing: {
     // A constant value was narrowed.
+    QualType T = EntityType.getNonReferenceType();
     S.Diag(PostInit->getBeginLoc(),
            NarrowingErrs(S.getLangOpts())
-               ? diag::ext_init_list_constant_narrowing
+               ? (T == EntityType
+                      ? diag::ext_init_list_constant_narrowing
+                      : diag::ext_init_list_constant_narrowing_const_reference)
                : diag::warn_init_list_constant_narrowing)
         << PostInit->getSourceRange()
         << ConstantValue.getAsString(S.getASTContext(), ConstantType)
         << EntityType.getNonReferenceType().getLocalUnqualifiedType();
     break;
+  }
 
-  case NK_Variable_Narrowing:
+  case NK_Variable_Narrowing: {
     // A variable's value may have been narrowed.
+    QualType T = EntityType.getNonReferenceType();
     S.Diag(PostInit->getBeginLoc(),
            NarrowingErrs(S.getLangOpts())
-               ? diag::ext_init_list_variable_narrowing
+               ? (T == EntityType
+                      ? diag::ext_init_list_variable_narrowing
+                      : diag::ext_init_list_variable_narrowing_const_reference)
                : diag::warn_init_list_variable_narrowing)
         << PostInit->getSourceRange()
         << PreNarrowingType.getLocalUnqualifiedType()
         << EntityType.getNonReferenceType().getLocalUnqualifiedType();
     break;
   }
+  }
 
   SmallString<128> StaticCast;
   llvm::raw_svector_ostream OS(StaticCast);

clang/test/CXX/dcl.decl/dcl.init/dcl.init.list/p7-0x.cpp

@@ -1,4 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -std=c++11 -triple x86_64-apple-macosx10.6.7 -verify %s
+// The following narrowing does not involve const references, so
+// -Wno-c++11-narrowing-const-reference does not suppress the errors.
+// RUN: %clang_cc1 -fsyntax-only -std=c++11 -triple x86_64-apple-macosx10.6.7 -Wno-c++11-narrowing-const-reference -verify %s
 
 // Verify that narrowing conversions in initializer lists cause errors in C++0x
 // mode.

clang/test/SemaCXX/GH63151.cpp

@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
-
+// RUN: %clang_cc1 -fsyntax-only -Wno-c++11-narrowing-const-reference -verify=allow %s
 
 struct A { A(const unsigned &x) {} };
 
@@ -9,4 +9,5 @@ void foo(int p) {
   A c { p }; // expected-error {{non-constant-expression cannot be narrowed from type 'int' to 'unsigned int' in initializer list}}
   A d { 0.5 }; // expected-error {{type 'double' cannot be narrowed to 'unsigned int' in initializer list}}
                // expected-warning@-1 {{implicit conversion from 'double' to 'unsigned int' changes value from 0.5 to 0}}
+               // allow-warning@-2 {{implicit conversion from 'double' to 'unsigned int' changes value from 0.5 to 0}}
 }