[clang] Fix consteval initializers of temporaries

[Fznamznon]

Summary

When a potential immediate invocation is met, it is immediately wrapped by a
ConstantExpr. There is also a TreeTransform that removes this ConstantExpr
wrapper when corresponding expression evaluation context is popped.
So, in case initializer was an immediate invocation, CXXTemporaryObjectExpr
was wrapped by a ConstantExpr, and that caused additional unnecessary
CXXFunctionalCastExpr to be added, which later confused the TreeTransform
that rebuilds immediate invocations, so it was adding unnecessary
constructor call.

Fixes https://github.com/llvm/llvm-project/issues/60286

[aaron.ballman]

The changes LGTM (though I had a small nit), but you should add a release note when landing.

clang/lib/Sema/SemaExprCXX.cpp

1593–1595

This will need reformatting, but because we can use C++17 features, we can be slightly fancier here. :-)

NB: I think we don't need to use dyn_cast_or_null here or on the predicate above. We're calling isa<Whatever>(Inner) immediately below and that will assert if Inner was null, so it seems like we're already relying on Inner being nonnull.

[Fznamznon]

Apply the suggestion

[Fznamznon]

clang/lib/Sema/SemaExprCXX.cpp

1593–1595

That makes sense, thank you.
Although, I can't make CE const because Inner is not const.

[aaron.ballman]

clang/lib/Sema/SemaExprCXX.cpp

1593–1595

Ah, that's fine, no worries about the const then. That suggestion is mostly a reflex for me at this point. :-D

[Fznamznon]

Rebase, maybe this will get "pre-commit" started

[aaron.ballman]

LGTM! Precommit CI is still falling over because of the libcxx bot. That issue is being actively investigated, but we don't have an ETA on it being resolved. I think these changes are sufficiently safe to land and watch for post-commit failures.

clang/docs/ReleaseNotes.rst

 - Fix coroutines issue where ``get_return_object()`` result was always eargerly
   converted to the return type. Eager initialization (allowing RVO) is now only
   perfomed when these types match, otherwise deferred initialization is used,
   enabling short-circuiting coroutines use cases. This fixes
   (`#56532 <https://github.com/llvm/llvm-project/issues/56532>`_) in
   antecipation of `CWG2563 <https://cplusplus.github.io/CWG/issues/2563.html>_`.
 - Fix highlighting issue with ``_Complex`` and initialization list with more than
   2 items. (`#61518 <https://github.com/llvm/llvm-project/issues/61518>`_)
+- Fix false-positive diagnostic issued for consteval initializers of temporary
+  objects.
+  (`#60286 <https://github.com/llvm/llvm-project/issues/60286>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to C++ Support

clang/lib/Sema/SemaExprCXX.cpp

Sema::BuildCXXTypeConstructExpr(TypeSourceInfo *TInfo,
   ExprResult Result = InitSeq.Perform(*this, Entity, Kind, Exprs);
 
   if (Result.isInvalid())
     return Result;
 
   Expr *Inner = Result.get();
   if (CXXBindTemporaryExpr *BTE = dyn_cast_or_null<CXXBindTemporaryExpr>(Inner))
     Inner = BTE->getSubExpr();
+  if (auto *CE = dyn_cast<ConstantExpr>(Inner);
+      CE && CE->isImmediateInvocation())
+    Inner = CE->getSubExpr();
   if (!isa<CXXTemporaryObjectExpr>(Inner) &&
       !isa<CXXScalarValueInitExpr>(Inner)) {
     // If we created a CXXTemporaryObjectExpr, that node also represents the
     // functional cast. Otherwise, create an explicit cast to represent
     // the syntactic form of a functional-style cast that was used here.
     //
     // FIXME: Creating a CXXFunctionalCastExpr around a CXXConstructExpr
     // would give a more consistent AST representation than using a

clang/test/SemaCXX/cxx2a-consteval.cpp

constexpr auto lstatic = []() static consteval { // expected-error {{cannot take address of consteval call operator of '(lambda at}} \
                                        // expected-warning {{extension}}
         return 3;
   };
   constexpr int (*f2)(void) = lstatic; // expected-error {{constexpr variable 'f2' must be initialized by a constant expression}} \
                                        // expected-note  {{pointer to a consteval declaration is not a constant expression}}
 
 }
 }
+
+namespace GH60286 {
+
+struct A {
+  int i = 0;
+
+  consteval A() {}
+  A(const A&) { i = 1; }
+  consteval int f() { return i; }
+};
+
+constexpr auto B = A{A{}}.f();
+static_assert(B == 0);
+
+}