[CLANG] Fixes the crash on the use of nested requirements in require expressions

[ebinjose02]

Fixes #165386
Nested requirements in requires-expressions must be constant expressions. Previously, using an invented parameter in a nested requirement caused a crash. Now emit a clear diagnostic and recover.

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Ebin Jose (ebinjose02)

Changes

Fixes #165386
Nested requirements in requires-expressions must be constant expressions. Previously, using an invented parameter in a nested requirement caused a crash. Now emit a clear diagnostic and recover.

---

Full diff: https://github.com/llvm/llvm-project/pull/169876.diff

3 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2)
• (modified) clang/lib/Sema/SemaExprCXX.cpp (+9-1)
• (added) clang/test/SemaCXX/requires-nested-non-constant.cpp (+11)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 4a145fd71eedd..8083fde95f2ab 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3190,6 +3190,8 @@ def note_ambiguous_atomic_constraints_similar_expression : Note<
 def err_unsupported_placeholder_constraint : Error<
   "constrained placeholder types other than simple 'auto' on non-type template "
   "parameters not supported yet">;
+def err_nested_requirement_not_constant : Error<
+  "nested requirement is not a constant expression">;
 
 def err_template_different_requires_clause : Error<
   "requires clause differs in template redeclaration">;
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index d6f70e728be29..c8c36fce846a8 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -7922,7 +7922,15 @@ concepts::Requirement *Sema::ActOnNestedRequirement(Expr *Constraint) {
 
 concepts::NestedRequirement *
 Sema::BuildNestedRequirement(Expr *Constraint) {
-  ConstraintSatisfaction Satisfaction;
+  if (!Constraint->isValueDependent() &&
+      !Constraint->isInstantiationDependent()) {
+    Expr::EvalResult Result;
+    if (!Constraint->EvaluateAsConstantExpr(Result, Context)) {
+      Diag(Constraint->getExprLoc(), diag::err_nested_requirement_not_constant);
+      return nullptr;
+    }
+  }
+    ConstraintSatisfaction Satisfaction;
   if (!Constraint->isInstantiationDependent() &&
       CheckConstraintSatisfaction(nullptr, AssociatedConstraint(Constraint),
                                   /*TemplateArgs=*/{},
diff --git a/clang/test/SemaCXX/requires-nested-non-constant.cpp b/clang/test/SemaCXX/requires-nested-non-constant.cpp
new file mode 100644
index 0000000000000..acb516392e616
--- /dev/null
+++ b/clang/test/SemaCXX/requires-nested-non-constant.cpp
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+
+template <class C> class A {
+  void f() {
+    auto result = []() constexpr {
+      return requires (int x) {
+        requires (x > 0) && (x < 10); // expected-error {{nested requirement is not a constant expression}}
+      };
+    }();
+  }
+};

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff origin/main HEAD --extensions cpp -- clang/test/SemaCXX/requires-nested-non-constant.cpp clang/lib/Sema/SemaExprCXX.cpp --diff_from_common_commit

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The reproduction instructions above might return results for more than one PR
in a stack if you are using a stacked PR workflow. You can limit the results by
changing origin/main to the base branch/commit you want to compare against.
⚠️

View the diff from clang-format here.

diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index c8c36fce8..490ce09e7 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -7930,7 +7930,7 @@ Sema::BuildNestedRequirement(Expr *Constraint) {
       return nullptr;
     }
   }
-    ConstraintSatisfaction Satisfaction;
+  ConstraintSatisfaction Satisfaction;
   if (!Constraint->isInstantiationDependent() &&
       CheckConstraintSatisfaction(nullptr, AssociatedConstraint(Constraint),
                                   /*TemplateArgs=*/{},