[Clang] Correctly determine constexprness of dependent lambdas.

[cor3ntin]

We skipped checking if a lambda is constexpr if the parent context was dependent, even if the lambda itself wasn't (and there is no other opportunity to establish constexprness)

Fixes #114234
Fixes #97958

[llvmbot]

@llvm/pr-subscribers-clang

Author: cor3ntin (cor3ntin)

Changes

We skipped checking if a lambda is constexpr if the parent context was dependent, even if the lambda itself wasn't (and there is no other opportunity to establish constexprness)

---

Full diff: https://github.com/llvm/llvm-project/pull/124468.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/Sema/SemaLambda.cpp (+6-6)
• (modified) clang/test/SemaCXX/cxx1z-constexpr-lambdas.cpp (+24)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index e9fffddd507c66..6083efdea09ddb 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -991,6 +991,7 @@ Bug Fixes to C++ Support
 - Fixed assertions or false compiler diagnostics in the case of C++ modules for
   lambda functions or inline friend functions defined inside templates (#GH122493).
 - Clang now rejects declaring an alias template with the same name as its template parameter. (#GH123423)
+- Correctly determine the implicit constexprness of dependent lambdas. (#GH97958) (#GH114234)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 87b3ca53cefaf2..88693dace45e95 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -2239,18 +2239,18 @@ ExprResult Sema::BuildLambdaExpr(SourceLocation StartLoc, SourceLocation EndLoc,
 
   Cleanup.mergeFrom(LambdaCleanup);
 
-  LambdaExpr *Lambda = LambdaExpr::Create(Context, Class, IntroducerRange,
-                                          CaptureDefault, CaptureDefaultLoc,
-                                          ExplicitParams, ExplicitResultType,
-                                          CaptureInits, EndLoc,
-                                          ContainsUnexpandedParameterPack);
+  LambdaExpr *Lambda =
+      LambdaExpr::Create(Context, Class, IntroducerRange, CaptureDefault,
+                         CaptureDefaultLoc, ExplicitParams, ExplicitResultType,
+                         CaptureInits, EndLoc, ContainsUnexpandedParameterPack);
+
   // If the lambda expression's call operator is not explicitly marked constexpr
   // and we are not in a dependent context, analyze the call operator to infer
   // its constexpr-ness, suppressing diagnostics while doing so.
   if (getLangOpts().CPlusPlus17 && !CallOperator->isInvalidDecl() &&
       !CallOperator->isConstexpr() &&
       !isa<CoroutineBodyStmt>(CallOperator->getBody()) &&
-      !Class->getDeclContext()->isDependentContext()) {
+      !Class->isDependentContext()) {
     CallOperator->setConstexprKind(
         CheckConstexprFunctionDefinition(CallOperator,
                                          CheckConstexprKind::CheckValid)
diff --git a/clang/test/SemaCXX/cxx1z-constexpr-lambdas.cpp b/clang/test/SemaCXX/cxx1z-constexpr-lambdas.cpp
index 6a1f48bf7958fd..0c20dd9dc58c63 100644
--- a/clang/test/SemaCXX/cxx1z-constexpr-lambdas.cpp
+++ b/clang/test/SemaCXX/cxx1z-constexpr-lambdas.cpp
@@ -349,3 +349,27 @@ static_assert(OtherCaptures(), "");
 } // namespace PR36054
 
 #endif // ndef CPP14_AND_EARLIER
+
+
+#if __cpp_constexpr >= 201907L
+namespace GH114234 {
+template <auto Arg>
+auto g() { return Arg; }
+
+template <typename>
+auto f() {
+    []<typename>() {
+        g<[] { return 123; }()>();
+    }.template operator()<int>();
+}
+
+void test() { f<int>(); }
+}
+
+namespace GH97958 {
+static_assert(
+  []<int I=0>() -> decltype([]{ return true; })
+  { return {}; }()());
+}
+
+#endif

[Sirraide]

Makes sense. After thinking about this for a bit, I don’t think there is a reason why we’d ever need to care about the parent context at all for the purpose of determining constexprness, is there?

[zyn0217]

Thanks