[clang] [Clang] Handle instantiating captures in addInstantiatedCapturesToScope() (PR #128478)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

<details>
<summary>Changes</summary>

`addInstantiatedCapturesToScope()` might be called when transforming a lambda body. In this situation, it would look into all the lambda's parents and figure out all the instantiated captures. However, the instantiated captures are not visible from lambda's class decl until the lambda is rebuilt (i.e. after the lambda body transform). So this patch corrects that by also examining the `LambdaScopeInfo`, serving as a workaround for not having deferred lambda body instantiation in Clang 20, to avoid regressing some real-world use cases.

Fixes #<!-- -->128175

---
Full diff: https://github.com/llvm/llvm-project/pull/128478.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaConcept.cpp (+24-1) 
- (modified) clang/test/SemaTemplate/concepts-lambda.cpp (+18) 


``````````diff
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 8a77cbf8c9477..7fff29f5ca167 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -711,9 +711,32 @@ bool Sema::addInstantiatedCapturesToScope(
 
   unsigned Instantiated = 0;
 
+  // FIXME: This is a workaround for not having deferred lambda body
+  // instantiation.
+  // When transforming a lambda's body, if we encounter another call to an
+  // inline lambda that contains a constraint expression, we add all of the
+  // parent lambda's instantiated captures to the current instantiation scope to
+  // facilitate constraint evaluation. However, these captures don't appear in
+  // the CXXRecordDecl until after the lambda expression is rebuilt, so we
+  // pull them out from the corresponding LSI.
+  LambdaScopeInfo *InstantiatingScope = nullptr;
+  if (LambdaPattern->capture_size() && !LambdaClass->capture_size()) {
+    for (FunctionScopeInfo *Scope : llvm::reverse(FunctionScopes)) {
+      auto *LSI = dyn_cast<LambdaScopeInfo>(Scope);
+      if (!LSI ||
+          LSI->CallOperator->getTemplateInstantiationPattern() != PatternDecl)
+        continue;
+      InstantiatingScope = LSI;
+      break;
+    }
+    assert(InstantiatingScope);
+  }
+
   auto AddSingleCapture = [&](const ValueDecl *CapturedPattern,
                               unsigned Index) {
-    ValueDecl *CapturedVar = LambdaClass->getCapture(Index)->getCapturedVar();
+    ValueDecl *CapturedVar =
+        InstantiatingScope ? InstantiatingScope->Captures[Index].getVariable()
+                           : LambdaClass->getCapture(Index)->getCapturedVar();
     assert(CapturedVar->isInitCapture());
     Scope.InstantiatedLocal(CapturedPattern, CapturedVar);
   };
diff --git a/clang/test/SemaTemplate/concepts-lambda.cpp b/clang/test/SemaTemplate/concepts-lambda.cpp
index 306f86cfcb28f..dcb09c76d26b6 100644
--- a/clang/test/SemaTemplate/concepts-lambda.cpp
+++ b/clang/test/SemaTemplate/concepts-lambda.cpp
@@ -307,3 +307,21 @@ void test() {
 }
 
 }
+
+namespace GH128175 {
+
+template <class> void f() {
+  [i{0}] {
+    [&] {
+      [&] {
+        []()
+          requires true
+        {}();
+      }();
+    }();
+  }();
+}
+
+template void f<int>();
+
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/128478