[clang] Report narrowing conversions with const references (#75332)

Fixes #63151

---------

Co-authored-by: Erich Keane <[email protected]>

Author: Fznamznon

clang/docs/ReleaseNotes.rst

@@ -521,6 +521,9 @@ Improvements to Clang's diagnostics
           |               ~~~~~~~^~~~~
 
 - Clang now diagnoses definitions of friend function specializations, e.g. ``friend void f<>(int) {}``.
+- Clang now diagnoses narrowing conversions involving const references.
+  (`#63151: <https://github.com/llvm/llvm-project/issues/63151>`_).
+
 
 Improvements to Clang's time-trace
 ----------------------------------

clang/lib/Sema/SemaInit.cpp

@@ -4432,7 +4432,8 @@ static void TryReferenceInitializationCore(Sema &S,
                                            Qualifiers T1Quals,
                                            QualType cv2T2, QualType T2,
                                            Qualifiers T2Quals,
-                                           InitializationSequence &Sequence);
+                                           InitializationSequence &Sequence,
+                                           bool TopLevelOfInitList);
 
 static void TryValueInitialization(Sema &S,
                                    const InitializedEntity &Entity,
@@ -4486,7 +4487,8 @@ static void TryReferenceListInitialization(Sema &S,
     if (RefRelationship >= Sema::Ref_Related) {
       // Try to bind the reference here.
       TryReferenceInitializationCore(S, Entity, Kind, Initializer, cv1T1, T1,
-                                     T1Quals, cv2T2, T2, T2Quals, Sequence);
+                                     T1Quals, cv2T2, T2, T2Quals, Sequence,
+                                     /*TopLevelOfInitList=*/true);
       if (Sequence)
         Sequence.RewrapReferenceInitList(cv1T1, InitList);
       return;
@@ -4945,11 +4947,11 @@ static void CheckCXX98CompatAccessibleCopy(Sema &S,
                                            Expr *CurInitExpr);
 
 /// Attempt reference initialization (C++0x [dcl.init.ref])
-static void TryReferenceInitialization(Sema &S,
-                                       const InitializedEntity &Entity,
+static void TryReferenceInitialization(Sema &S, const InitializedEntity &Entity,
                                        const InitializationKind &Kind,
                                        Expr *Initializer,
-                                       InitializationSequence &Sequence) {
+                                       InitializationSequence &Sequence,
+                                       bool TopLevelOfInitList) {
   QualType DestType = Entity.getType();
   QualType cv1T1 = DestType->castAs<ReferenceType>()->getPointeeType();
   Qualifiers T1Quals;
@@ -4967,7 +4969,8 @@ static void TryReferenceInitialization(Sema &S,
 
   // Delegate everything else to a subfunction.
   TryReferenceInitializationCore(S, Entity, Kind, Initializer, cv1T1, T1,
-                                 T1Quals, cv2T2, T2, T2Quals, Sequence);
+                                 T1Quals, cv2T2, T2, T2Quals, Sequence,
+                                 TopLevelOfInitList);
 }
 
 /// Determine whether an expression is a non-referenceable glvalue (one to
@@ -4990,7 +4993,8 @@ static void TryReferenceInitializationCore(Sema &S,
                                            Qualifiers T1Quals,
                                            QualType cv2T2, QualType T2,
                                            Qualifiers T2Quals,
-                                           InitializationSequence &Sequence) {
+                                           InitializationSequence &Sequence,
+                                           bool TopLevelOfInitList) {
   QualType DestType = Entity.getType();
   SourceLocation DeclLoc = Initializer->getBeginLoc();
 
@@ -5264,7 +5268,8 @@ static void TryReferenceInitializationCore(Sema &S,
       Sequence.SetFailed(InitializationSequence::FK_ReferenceInitFailed);
     return;
   } else {
-    Sequence.AddConversionSequenceStep(ICS, TempEntity.getType());
+    Sequence.AddConversionSequenceStep(ICS, TempEntity.getType(),
+                                       TopLevelOfInitList);
   }
 
   //        [...] If T1 is reference-related to T2, cv1 must be the
@@ -6228,7 +6233,8 @@ void InitializationSequence::InitializeFrom(Sema &S,
     else if (isa<InitListExpr>(Args[0]))
       SetFailed(FK_ParenthesizedListInitForReference);
     else
-      TryReferenceInitialization(S, Entity, Kind, Args[0], *this);
+      TryReferenceInitialization(S, Entity, Kind, Args[0], *this,
+                                 TopLevelOfInitList);
     return;
   }
 
@@ -10431,7 +10437,7 @@ static void DiagnoseNarrowingInInitList(Sema &S,
                                         : diag::warn_init_list_type_narrowing)
         << PostInit->getSourceRange()
         << PreNarrowingType.getLocalUnqualifiedType()
-        << EntityType.getLocalUnqualifiedType();
+        << EntityType.getNonReferenceType().getLocalUnqualifiedType();
     break;
 
   case NK_Constant_Narrowing:
@@ -10442,7 +10448,7 @@ static void DiagnoseNarrowingInInitList(Sema &S,
                : diag::warn_init_list_constant_narrowing)
         << PostInit->getSourceRange()
         << ConstantValue.getAsString(S.getASTContext(), ConstantType)
-        << EntityType.getLocalUnqualifiedType();
+        << EntityType.getNonReferenceType().getLocalUnqualifiedType();
     break;
 
   case NK_Variable_Narrowing:
@@ -10453,7 +10459,7 @@ static void DiagnoseNarrowingInInitList(Sema &S,
                : diag::warn_init_list_variable_narrowing)
         << PostInit->getSourceRange()
         << PreNarrowingType.getLocalUnqualifiedType()
-        << EntityType.getLocalUnqualifiedType();
+        << EntityType.getNonReferenceType().getLocalUnqualifiedType();
     break;
   }
 

clang/test/SemaCXX/GH63151.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+
+struct A { A(const unsigned &x) {} };
+
+void foo(int p) {
+  A a { -1 }; // expected-error {{constant expression evaluates to -1 which cannot be narrowed to type 'unsigned int'}}
+  A b { 0 };
+  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}}
+}