[Clang] prevent assertion failure by avoiding required literal type checking in C context

[a-tarasyuk]

Fixes #101304

[llvmbot]

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

Changes

Fixes #101304

---

Full diff: https://github.com/llvm/llvm-project/pull/101426.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/lib/Sema/SemaType.cpp (+2-2)
• (modified) clang/test/Sema/constexpr.c (+4)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3c2e0282d1c72..c5ec3dfc11bbe 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -156,6 +156,7 @@ Bug Fixes in This Version
 
 - Fixed the definition of ``ATOMIC_FLAG_INIT`` in ``<stdatomic.h>`` so it can
   be used in C++.
+- Fixed a failed assertion when casting type declarations that require complete types. (#GH101304).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 6fa39cdccef2b..75e1b116e2043 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -9267,14 +9267,14 @@ bool Sema::RequireLiteralType(SourceLocation Loc, QualType T,
   if (!RT)
     return true;
 
-  const CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());
-
   // A partially-defined class type can't be a literal type, because a literal
   // class type must have a trivial destructor (which can't be checked until
   // the class definition is complete).
   if (RequireCompleteType(Loc, ElemType, diag::note_non_literal_incomplete, T))
     return true;
 
+  const CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());
+
   // [expr.prim.lambda]p3:
   //   This class type is [not] a literal type.
   if (RD->isLambda() && !getLangOpts().CPlusPlus17) {
diff --git a/clang/test/Sema/constexpr.c b/clang/test/Sema/constexpr.c
index 8286cd2107d2f..0d2ddabd4c072 100644
--- a/clang/test/Sema/constexpr.c
+++ b/clang/test/Sema/constexpr.c
@@ -357,3 +357,7 @@ void infsNaNs() {
   constexpr double db5 = LD_SNAN; // expected-error {{constexpr initializer evaluates to nan which is not exactly representable in type 'const double'}}
   constexpr double db6 = INF;
 }
+
+constexpr struct S9 s9 = {  }; // expected-error {{constexpr variable cannot have non-literal type 'const struct S9'}} \
+                               // expected-note {{incomplete type 'const struct S9' is not a literal type}} \
+                               // expected-note {{forward declaration of 'struct S9'}}

[shafik]

This looks good but you need to add a bit more details to your summary, more specifically what case caused the bug to emerge.

I am also curious why this does not show up in C++, we obtain a similar diagnostic but no crash. If we don't have a C++ test similar to this we should add one.

[MitalAshok]

I think the issue is here somewhere:

llvm-project/clang/lib/Sema/SemaDecl.cpp

Lines 8753 to 8764 in 7088a5e

	if (getLangOpts().C23 && NewVD->isConstexpr() &&
	CheckC23ConstexprVarType(*this, NewVD->getLocation(), T)) {
	NewVD->setInvalidDecl();
	return;
	}
	if (NewVD->isConstexpr() && !T->isDependentType() &&
	RequireLiteralType(NewVD->getLocation(), T,
	diag::err_constexpr_var_non_literal)) {
	NewVD->setInvalidDecl();
	return;
	}

An incomplete struct passes CheckC23ConstexprVarType (which it should, incomplete types should fail later) so it goes on to the next check. We should only check RequireLiteralType in C++. The cast isn't failing because it's expecting a complete type, it's failing because it's expecting a CXXRecordDecl when we only have a C RecordDecl.

[Fznamznon]

LGTM

[AaronBallman]

LGTM!

This seems like a pretty reasonable candidate to backport to 19.x as well: https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches