[Clang] Correctly determine constexprness of dependent lambdas. (#124468)

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

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 lambdas in dependent contexts. (#GH97958) (#GH114234)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

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
+  // and is not dependent, 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)

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