[Sema] Add -Wc++11-narrowing-const-reference (#76094)

https://github.com/llvm/llvm-project/pull/75332 diagnosed narrowing
involving const reference. Our depot has hundreds if not thousands of
breakages

(https://github.com/llvm/llvm-project/pull/75332#issuecomment-1864757240).
Add a subgroup of -Wc++11-narrowing to help users gradually fix their
issues without regressing the existing -Wc++11-narrowing diagnostics.

1 files changed, 22 insertions, 9 deletions

diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index f768d27..cc9db5d 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -10411,40 +10411,53 @@ 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);