[AST] Don't track lambda captures when checking a potential constant expression.

Fixes PR36054.

Differential revision: https://reviews.llvm.org/D45194

llvm-svn: 329244

clang/lib/AST/ExprConstant.cpp

@@ -4309,9 +4309,15 @@ static bool HandleFunctionCall(SourceLocation CallLoc,
     This->moveInto(Result);
     return true;
   } else if (MD && isLambdaCallOperator(MD)) {
-    // We're in a lambda; determine the lambda capture field maps.
-    MD->getParent()->getCaptureFields(Frame.LambdaCaptureFields,
-                                      Frame.LambdaThisCaptureField);
+    // We're in a lambda; determine the lambda capture field maps unless we're
+    // just constexpr checking a lambda's call operator. constexpr checking is
+    // done before the captures have been added to the closure object (unless
+    // we're inferring constexpr-ness), so we don't have access to them in this
+    // case. But since we don't need the captures to constexpr check, we can
+    // just ignore them.
+    if (!Info.checkingPotentialConstantExpression())
+      MD->getParent()->getCaptureFields(Frame.LambdaCaptureFields,
+                                        Frame.LambdaThisCaptureField);
   }
 
   StmtResult Ret = {Result, ResultSlot};
@@ -5201,10 +5207,17 @@ bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
   // 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)) {
+  if (Info.CurrentCall && isLambdaCallOperator(Info.CurrentCall->Callee) &&
+      isa<DeclRefExpr>(E) &&
+      cast<DeclRefExpr>(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 (Info.checkingPotentialConstantExpression())
-        return false;
       // Start with 'Result' referring to the complete closure object...
       Result = *Info.CurrentCall->This;
       // ... then update it to refer to the field of the closure object

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

@@ -270,4 +270,37 @@ namespace ns1_test_lvalue_type {
 
 } // end ns test_lambda_is_cce
 
+namespace PR36054 {
+constexpr int fn() {
+  int Capture = 42;
+  return [=]() constexpr { return Capture; }();
+}
+
+static_assert(fn() == 42, "");
+
+template <class T>
+constexpr int tfn() {
+  int Capture = 42;
+  return [=]() constexpr { return Capture; }();
+}
+
+static_assert(tfn<int>() == 42, "");
+
+constexpr int gfn() {
+  int Capture = 42;
+  return [=](auto P) constexpr { return Capture + P; }(58);
+}
+
+static_assert(gfn() == 100, "");
+
+constexpr bool OtherCaptures() {
+  int Capture = 42;
+  constexpr auto Outer = [](auto P) constexpr { return 42 + P; };
+  auto Inner = [&](auto O) constexpr { return O(58) + Capture; };
+  return Inner(Outer) == 142;
+}
+
+static_assert(OtherCaptures(), "");
+} // namespace PR36054
+
 #endif // ndef CPP14_AND_EARLIER