[clang][C++20] Fix clang/clangd assert/crash after compilation errors

[DmitryPolukhin]

Summary

After compilation errors, expression a transformation result may not be usable.
It triggers an assert in RemoveNestedImmediateInvocation and SIGSEGV in case of
builds without asserts. This issue significantly affects clangd because source
may not be valid during typing. Tests cases that I attached was reduce from huge
C++ translation unit.

Test Plan: check-clang

[DmitryPolukhin]

clang-format

[aaron.ballman]

LGTM! Please add a release note for the fix when you land the changes.

clang/test/SemaCXX/remove-nested-immediate-invocation-crash.cpp

5

This surely wins an award for "best line noise of the year" or something. :-D

[DmitryPolukhin]

In D133948#3793085, @aaron.ballman wrote:

LGTM! Please add a release note for the fix when you land the changes.

Thank you for prompt review! Release notes update in D134112.
Also I found existing issues about this crash https://github.com/llvm/llvm-project/issues/53628.

clang/lib/Sema/SemaExpr.cpp

static void RemoveNestedImmediateInvocation(
   /// can only occur for the top-level CXXConstructExpr because it is used
   /// nowhere in the expression being transformed therefore will not be rebuilt.
   /// Setting AllowSkippingFirstCXXConstructExpr to false will prevent from
   /// skipping the first CXXConstructExpr.
   if (isa<CXXConstructExpr>(It->getPointer()->IgnoreImplicit()))
     Transformer.AllowSkippingFirstCXXConstructExpr = false;
 
   ExprResult Res = Transformer.TransformExpr(It->getPointer()->getSubExpr());
-  assert(Res.isUsable());
+  // The result may not be usable in case of previous compilation errors.
+  // In this case evaluation of the expression may result in crash so just
+  // don't do anything further with the result.
+  if (Res.isUsable()) {
     Res = SemaRef.MaybeCreateExprWithCleanups(Res);
     It->getPointer()->setSubExpr(Res.get());
   }
+}
 
 static void
 HandleImmediateInvocations(Sema &SemaRef,
                            Sema::ExpressionEvaluationContextRecord &Rec) {
   if ((Rec.ImmediateInvocationCandidates.size() == 0 &&
        Rec.ReferenceToConsteval.size() == 0) ||
       SemaRef.RebuildingImmediateInvocation)
     return;

clang/test/SemaCXX/remove-nested-immediate-invocation-crash.cpp

+// RUN: not %clang_cc1 -fsyntax-only -verify -std=gnu++20 -ferror-limit 19 %s
+// Creduced test case for the crash in RemoveNestedImmediateInvocation after compliation errros.
+
+a, class b {                               template < typename c>                 consteval b(c
+} template <typename...> using d = b;
+auto e(d<>) -> int:;
+}
+f
+}
+g() {
+                    auto h = "":(::i(e(h))