[clang] [Clang] Fix a DeclContext mismatch when parsing nested lambda parameters (PR #112177)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

<details>
<summary>Changes</summary>

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,

```cpp
struct Foo {};

template &lt;int, Foo f&gt; struct Arr {};

constexpr void foo() {
  constexpr Foo F;
  [&amp;]&lt;int I&gt;() {
     [&amp;](Arr&lt;I, F&gt;) {};
   }.template operator()&lt;42&gt;();
}
```

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
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

``````````

</details>


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