[clang] [CLANG] Fixes the crash on the use of nested requirements in require expressions (PR #169876)

[Ebin Jose via cfe-commits]

https://github.com/ebinjose02 updated https://github.com/llvm/llvm-project/pull/169876

>From af9b88da83a7f2954a3a8ddc5f4796f2017ceaec Mon Sep 17 00:00:00 2001
From: ebinjose02 <ebin371 at gmail.com>
Date: Fri, 28 Nov 2025 06:46:40 +0000
Subject: [PATCH 1/3] 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.

---
 clang/include/clang/Basic/DiagnosticSemaKinds.td    |  2 ++
 clang/lib/Sema/SemaExprCXX.cpp                      |  8 ++++++++
 clang/test/SemaCXX/requires-nested-non-constant.cpp | 11 +++++++++++
 3 files changed, 21 insertions(+)
 create mode 100644 clang/test/SemaCXX/requires-nested-non-constant.cpp

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..490ce09e75208 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -7922,6 +7922,14 @@ concepts::Requirement *Sema::ActOnNestedRequirement(Expr *Constraint) {
 
 concepts::NestedRequirement *
 Sema::BuildNestedRequirement(Expr *Constraint) {
+  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),
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}}
+      };
+    }();
+  }
+};

>From 2de5ef4d02f9d0aed61cebacb14517381aeeb9e7 Mon Sep 17 00:00:00 2001
From: ebinjose02 <ebin371 at gmail.com>
Date: Tue, 2 Dec 2025 09:22:10 +0000
Subject: [PATCH 2/3] Modified implementation to use instantationscope

---
 clang/include/clang/Basic/DiagnosticSemaKinds.td    |  2 --
 clang/lib/Sema/SemaExprCXX.cpp                      | 10 ++--------
 clang/test/SemaCXX/requires-nested-non-constant.cpp |  6 ++++--
 3 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 8083fde95f2ab..4a145fd71eedd 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3190,8 +3190,6 @@ 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 490ce09e75208..f915caffa3487 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -7922,16 +7922,10 @@ concepts::Requirement *Sema::ActOnNestedRequirement(Expr *Constraint) {
 
 concepts::NestedRequirement *
 Sema::BuildNestedRequirement(Expr *Constraint) {
-  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;
+  LocalInstantiationScope Scope(*this);
   if (!Constraint->isInstantiationDependent() &&
+      !Constraint->isValueDependent() &&
       CheckConstraintSatisfaction(nullptr, AssociatedConstraint(Constraint),
                                   /*TemplateArgs=*/{},
                                   Constraint->getSourceRange(), Satisfaction))
diff --git a/clang/test/SemaCXX/requires-nested-non-constant.cpp b/clang/test/SemaCXX/requires-nested-non-constant.cpp
index acb516392e616..b5b8bdbf5a9dc 100644
--- a/clang/test/SemaCXX/requires-nested-non-constant.cpp
+++ b/clang/test/SemaCXX/requires-nested-non-constant.cpp
@@ -3,8 +3,10 @@
 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}}
+      return requires (int x) { // expected-note {{declared here}}
+        requires (x > 0) && (x < 10); // expected-error {{substitution into constraint expression resulted in a non-constant expression}} \
+                                       // expected-note {{while checking the satisfaction of nested requirement requested here}} \
+                                       // expected-note {{function parameter 'x' with unknown value cannot be used in a constant expression}}
       };
     }();
   }

>From 113a1afe2d9fb3d1652dbd1bc49e71d678ea94b4 Mon Sep 17 00:00:00 2001
From: ebinjose02 <ebin371 at gmail.com>
Date: Wed, 3 Dec 2025 04:50:32 +0000
Subject: [PATCH 3/3] Added documentation in ReleaseNotes.rst

---
 clang/docs/ReleaseNotes.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index c6a79ed71ca2f..4404ddc9f7e45 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -724,6 +724,8 @@ Crash and bug fixes
   ``[[assume(expr)]]`` attribute was enclosed in parentheses.  (#GH151529)
 - Fixed a crash when parsing ``#embed`` parameters with unmatched closing brackets. (#GH152829)
 - Fixed a crash when compiling ``__real__`` or ``__imag__`` unary operator on scalar value with type promotion. (#GH160583)
+- Fixed a crash when evaluating nested requirements in requires-expressions that
+  reference invented parameters. (#GH166325)
 
 Improvements
 ^^^^^^^^^^^^