[clang] Implement consteval for captured structured bindings. (#147615)

127bf44 implemented most of the
infrastructure for capturing structured bindings in lambdas, but missed
one piece: constant evaluation of such lambdas. Refactor the code to
handle this case.

Fixes #145956.

Author: efriedma-quic

clang/docs/ReleaseNotes.rst

@@ -145,6 +145,8 @@ C++20 Feature Support
 - Fixed a crash with a defaulted spaceship (``<=>``) operator when the class
   contains a member declaration of vector type. Vector types cannot yet be
   compared directly, so this causes the operator to be deleted. (#GH137452)
+- Implement constant evaluation of lambdas that capture structured bindings.
+  (#GH145956)
 
 C++17 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -8951,39 +8951,39 @@ static bool EvaluateLValue(const Expr *E, LValue &Result, EvalInfo &Info,
 }
 
 bool LValueExprEvaluator::VisitDeclRefExpr(const DeclRefExpr *E) {
-  const NamedDecl *D = E->getDecl();
-  if (isa<FunctionDecl, MSGuidDecl, TemplateParamObjectDecl,
-          UnnamedGlobalConstantDecl>(D))
-    return Success(cast<ValueDecl>(D));
-  if (const VarDecl *VD = dyn_cast<VarDecl>(D))
-    return VisitVarDecl(E, VD);
-  if (const BindingDecl *BD = dyn_cast<BindingDecl>(D))
-    return Visit(BD->getBinding());
-  return Error(E);
-}
+  const ValueDecl *D = E->getDecl();
 
-bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
   // If we are within a lambda's call operator, check whether the 'VD' referred
   // to within 'E' actually represents a lambda-capture that maps to a
   // data-member/field within the closure object, and if so, evaluate to the
   // field or what the field refers to.
   if (Info.CurrentCall && isLambdaCallOperator(Info.CurrentCall->Callee) &&
-      isa<DeclRefExpr>(E) &&
-      cast<DeclRefExpr>(E)->refersToEnclosingVariableOrCapture()) {
+      E->refersToEnclosingVariableOrCapture()) {
     // We don't always have a complete capture-map when checking or inferring if
     // the function call operator meets the requirements of a constexpr function
     // - but we don't need to evaluate the captures to determine constexprness
     // (dcl.constexpr C++17).
     if (Info.checkingPotentialConstantExpression())
       return false;
 
-    if (auto *FD = Info.CurrentCall->LambdaCaptureFields.lookup(VD)) {
+    if (auto *FD = Info.CurrentCall->LambdaCaptureFields.lookup(D)) {
       const auto *MD = cast<CXXMethodDecl>(Info.CurrentCall->Callee);
       return HandleLambdaCapture(Info, E, Result, MD, FD,
                                  FD->getType()->isReferenceType());
     }
   }
 
+  if (isa<FunctionDecl, MSGuidDecl, TemplateParamObjectDecl,
+          UnnamedGlobalConstantDecl>(D))
+    return Success(cast<ValueDecl>(D));
+  if (const VarDecl *VD = dyn_cast<VarDecl>(D))
+    return VisitVarDecl(E, VD);
+  if (const BindingDecl *BD = dyn_cast<BindingDecl>(D))
+    return Visit(BD->getBinding());
+  return Error(E);
+}
+
+bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
   CallStackFrame *Frame = nullptr;
   unsigned Version = 0;
   if (VD->hasLocalStorage()) {

clang/test/SemaCXX/cxx1z-constexpr-lambdas.cpp

@@ -373,3 +373,31 @@ static_assert(
 }
 
 #endif
+
+#ifndef CPP14_AND_EARLIER
+namespace GH145956 {
+  constexpr int f() {
+    struct Pair { int first; int second; };
+    Pair p = {1, 2};
+    auto const& [key, value] = p;
+    return [&] { return key; }();
+#if __cpp_constexpr < 202002L
+    // expected-warning@-2 {{captured structured bindings are a C++20 extension}}
+    // expected-note@-4 {{'key' declared here}}
+#endif
+  }
+  static_assert(f() == 1);
+  constexpr auto retlambda() {
+    struct Pair { int first; int second; };
+    Pair p = {1, 2};
+    auto const& [key, value] = p;
+    return [=] { return key; };
+#if __cpp_constexpr < 202002L
+    // expected-warning@-2 {{captured structured bindings are a C++20 extension}}
+    // expected-note@-4 {{'key' declared here}}
+#endif
+  }
+  constexpr auto lambda = retlambda();
+  static_assert(lambda() == 1);
+}
+#endif