[clang] [clang] Implement CWG3135 - constexpr structured bindings with prvalues from tuples (PR #191880)

[Yanzuo Liu via cfe-commits]

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@@ -1380,47 +1380,52 @@ static bool checkTupleLikeDecomposition(Sema &S,
       return true;
     Expr *Init = E.get();
 
-    //   Given the type T designated by std::tuple_element<i - 1, E>::type,
+    //   Given the type T designated by std::tuple_element<i - 1, E>::type
     QualType T = getTupleLikeElementType(S, Loc, I, DecompType);
     if (T.isNull())
       return true;
 
-    //   each vi is a variable of type "reference to T" initialized with the
-    //   initializer, where the reference is an lvalue reference if the
-    //   initializer is an lvalue and an rvalue reference otherwise
-    QualType RefType =
-        S.BuildReferenceType(T, E.get()->isLValue(), Loc, B->getDeclName());
-    if (RefType.isNull())
+    // C++26 [dcl.struct.bind]p7:
+    //   and the type Ui, defined as Ti if the initializer is a prvalue,
+    //   as "lvalue reference to Ti" if the initializer is an lvalue,
+    //   or as "rvalue reference to Ti" otherwise
+    // "defined as Ti if the initializer is a prvalue" was introduced by CWG3135
+    QualType U = E.get()->isPRValue()
+                     ? T
+                     : S.BuildReferenceType(T, E.get()->isLValue(), Loc,
+                                            B->getDeclName());
+    if (U.isNull())
       return true;
 
     // Don't give this VarDecl a TypeSourceInfo, since this is a synthesized
     // entity and this type was never written in source code.
-    auto *RefVD =
+    auto *BindingVD =
         VarDecl::Create(S.Context, Src->getDeclContext(), Loc, Loc,
-                        B->getDeclName().getAsIdentifierInfo(), RefType,
+                        B->getDeclName().getAsIdentifierInfo(), U,
                         /*TInfo=*/nullptr, Src->getStorageClass());
-    RefVD->setLexicalDeclContext(Src->getLexicalDeclContext());
-    RefVD->setTSCSpec(Src->getTSCSpec());
-    RefVD->setImplicit();
+    BindingVD->setLexicalDeclContext(Src->getLexicalDeclContext());
+    BindingVD->setTSCSpec(Src->getTSCSpec());
+    BindingVD->setImplicit();
     if (Src->isInlineSpecified())
-      RefVD->setInlineSpecified();
-    RefVD->getLexicalDeclContext()->addHiddenDecl(RefVD);
+      BindingVD->setInlineSpecified();
+    BindingVD->getLexicalDeclContext()->addHiddenDecl(BindingVD);
 
-    InitializedEntity Entity = InitializedEntity::InitializeBinding(RefVD);
-    InitializationKind Kind = InitializationKind::CreateCopy(Loc, Loc);
+    InitializedEntity Entity = InitializedEntity::InitializeBinding(BindingVD);
+    InitializationKind Kind =
+        E.get()->isPRValue() ? InitializationKind::CreateDirect(Loc, Loc, Loc)
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zwuis wrote:

They are semantically different. Copy initialization ignores member functions with `explicit`.

Please fix assertion failures or leave a FIXME comment.

https://github.com/llvm/llvm-project/pull/191880