[Clang] Fix a DeclContext mismatch when parsing nested lambda parameters

[zyn0217]

When parsing its function parameters, we don't change the CurContext to the lambda's function declaration. However, CheckIfAnyEnclosingLambdasMustCaptureAnyPotentialCaptures() has not yet adapted to such behavior when nested lambdas come into play. Consider the following case,

struct Foo {};

template <int, Foo f> struct Arr {};

constexpr void foo() {
  constexpr Foo F;
  [&]<int I>() {
     [&](Arr<I, F>) {};
   }.template operator()<42>();
}

As per [basic.def.odr]p5.2, the use of F constitutes an ODR-use. And per [basic.def.odr]p10, F should be ODR-usable in that interleaving scope.

We failed to accept the case because the call to tryCaptureVariable() in getStackIndexOfNearestEnclosingCaptureCapableLambda() suggested that F is needlessly captureable. That was due to a missed handling for AfterParameterList in FunctionScopeIndexToStopAt, where it still presumed DC and LSI matched.

Fixes #47400
Fixes #90896

[llvmbot]

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

Changes

When parsing its function parameters, we don't change the CurContext to the lambda's function declaration. However, CheckIfAnyEnclosingLambdasMustCaptureAnyPotentialCaptures() has not yet adapted to such behavior when nested lambdas come into play. Consider the following case,

struct Foo {};

template <int, Foo f> struct Arr {};

constexpr void foo() {
  constexpr Foo F;
  [&]<int I>() {
     [&](Arr<I, F>) {};
   }.template operator()<42>();
}

As per [basic.def.odr]p5.2, the use of F constitutes an ODR-use. And per [basic.def.odr]p10, F should be ODR-usable in that interleaving scope.

We failed to accept the case because the call to tryCaptureVariable() in getStackIndexOfNearestEnclosingCaptureCapableLambda() suggested that F is needlessly captureable. That was due to a missed handling for AfterParameterList in FunctionScopeIndexToStopAt, where it still presumed DC and LSI matched.

Fixes #47400
Fixes #90896

---

Full diff: https://github.com/llvm/llvm-project/pull/112177.diff

4 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/Sema/SemaExpr.cpp (+10)
• (modified) clang/lib/Sema/SemaExprCXX.cpp (+1-1)
• (modified) clang/test/SemaCXX/lambda-capture-type-deduction.cpp (+23)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8d02cc3eae9fd9..3d6da43cf02f69 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -487,6 +487,8 @@ Bug Fixes to C++ Support
 - Clang now uses the correct set of template argument lists when comparing the constraints of
   out-of-line definitions and member templates explicitly specialized for a given implicit instantiation of
   a class template. (#GH102320)
+- Fixed a bug in lambda captures where ``constexpr`` class-type objects were not properly considered ODR-used in
+  certain situations. (#GH47400), (#GH90896)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index f930a21ea870ec..0075ec86823920 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -18843,7 +18843,17 @@ bool Sema::tryCaptureVariable(
   // We need to sync up the Declaration Context with the
   // FunctionScopeIndexToStopAt
   if (FunctionScopeIndexToStopAt) {
+    assert(!FunctionScopes.empty() && "No function scopes to stop at?");
     unsigned FSIndex = FunctionScopes.size() - 1;
+    // When we're parsing the lambda parameter list, the current DeclContext is
+    // NOT the lambda but its parent. So move away the current LSI before
+    // aligning DC and FunctionScopeIndexToStopAt.
+    if (auto *LSI = dyn_cast<LambdaScopeInfo>(FunctionScopes[FSIndex]);
+        LSI && !LSI->AfterParameterList)
+      --FSIndex;
+    assert(MaxFunctionScopesIndex <= FSIndex &&
+           "FunctionScopeIndexToStopAt should be no larger than FSIndex into "
+           "FunctionScopes.");
     while (FSIndex != MaxFunctionScopesIndex) {
       DC = getLambdaAwareParentOfDeclContext(DC);
       --FSIndex;
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index d490452e91c3bb..09ab83dcf21c22 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -8679,7 +8679,7 @@ static void CheckIfAnyEnclosingLambdasMustCaptureAnyPotentialCaptures(
   while (isa_and_nonnull<CapturedDecl>(DC))
     DC = DC->getParent();
   assert(
-      CurrentLSI->CallOperator == DC &&
+      (CurrentLSI->CallOperator == DC || !CurrentLSI->AfterParameterList) &&
       "The current call operator must be synchronized with Sema's CurContext");
 #endif // NDEBUG
 
diff --git a/clang/test/SemaCXX/lambda-capture-type-deduction.cpp b/clang/test/SemaCXX/lambda-capture-type-deduction.cpp
index 7bf36a6a9cab73..253d0f429abbed 100644
--- a/clang/test/SemaCXX/lambda-capture-type-deduction.cpp
+++ b/clang/test/SemaCXX/lambda-capture-type-deduction.cpp
@@ -297,3 +297,26 @@ void __trans_tmp_1() {
 }
 
 }
+
+namespace GH47400 {
+
+struct Foo {};
+
+template <int, Foo> struct Arr {};
+
+template <int> struct S {};
+
+constexpr bool foo() {
+  constexpr Foo f;
+  [&]<int is>() {
+    [&](Arr<is, f>) {}({}); // f constitutes an ODR-use
+  }.template operator()<42>();
+
+  constexpr int C = 1;
+  [] {
+    [](S<C>) { }({}); // ... while C doesn't
+  }();
+  return true;
+}
+
+} // namespace GH47400