Explain which assertion failed during consteval

Author: jj-marr

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -162,6 +162,8 @@ def err_ice_too_large : Error<
   "integer constant expression evaluates to value %0 that cannot be "
   "represented in a %1-bit %select{signed|unsigned}2 integer type">;
 def err_expr_not_string_literal : Error<"expression is not a string literal">;
+def note_constexpr_assert_failed : Note<
+  "assertion failed in constant evaluation: '%0'">;
 
 // Semantic analysis of constant literals.
 def ext_predef_outside_function : Warning<

clang/lib/AST/ExprConstant.cpp

@@ -8409,6 +8409,17 @@ class ExprEvaluatorBase
         return false;
     }
 
+    // If an assertion fails during constant evaluation, give a specific note explaining that
+    if (FD->getName() == "__assert_fail") {
+      const Expr *AssertionExpr = E->getArg(0);
+      const StringLiteral *AssertionText = dyn_cast<StringLiteral>(AssertionExpr->IgnoreParens()->IgnoreParenImpCasts());
+
+      Info.FFDiag(E->getBeginLoc(), diag::note_constexpr_assert_failed)
+          << (AssertionText ? AssertionText->getString() : "<unknown assertion>");
+
+      return false;
+    }
+
     SmallVector<QualType, 4> CovariantAdjustmentPath;
     if (This) {
       auto *NamedMember = dyn_cast<CXXMethodDecl>(FD);

clang/test/SemaCXX/consteval-assert.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -std=c++23 -verify=expected,cxx20_plus %s
+
+#ifdef __ASSERT_FUNCTION
+#undef __ASSERT_FUNCTION
+#endif
+extern "C" void __assert_fail(const char*, const char*, unsigned, const char*);
+
+#define assert(cond) \
+  ((cond) ? (void)0 : __assert_fail(#cond, __FILE__, __LINE__, __func__))
+
+consteval int square(int x) {
+  int result = x * x;
+  assert(result == 42); // expected-note {{assertion failed in constant evaluation: 'result == 42'}}
+  return result;
+}
+
+void test() {
+  auto val = square(2); // expected-note {{in call to 'square(2)'}} \
+  // expected-error {{call to consteval function 'square' is not a constant expression}}
+}