Explain which assertion failed during consteval

Author: jj-marr

clang/docs/ReleaseNotes.rst

@@ -419,6 +419,10 @@ Improvements to Clang's diagnostics
 - ``-Winitializer-overrides`` and ``-Wreorder-init-list`` are now grouped under
   the ``-Wc99-designator`` diagnostic group, as they also are about the
   behavior of the C99 feature as it was introduced into C++20. Fixes #GH47037
+  
+- Explanatory note is printed when ``assert`` fails during evaluation of a
+  constant expression. Prior to this, the error inaccurately implied that assert
+  could not be used at all in a constant expression (#GH130458)
 
 Improvements to Clang's time-trace
 ----------------------------------

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 during evaluation of constant expression">;
 
 // Semantic analysis of constant literals.
 def ext_predef_outside_function : Warning<

clang/lib/AST/ExprConstant.cpp

@@ -5975,9 +5975,22 @@ static bool CheckConstexprFunction(EvalInfo &Info, SourceLocation CallLoc,
                                         Definition->hasAttr<MSConstexprAttr>())))
     return true;
 
-  if (Info.getLangOpts().CPlusPlus11) {
-    const FunctionDecl *DiagDecl = Definition ? Definition : Declaration;
+  const FunctionDecl *DiagDecl = Definition ? Definition : Declaration;
+  // Special note for the assert() macro, as the normal error message falsely
+  // implies we cannot use an assertion during constant evaluation.
+  if (CallLoc.isMacroID() && DiagDecl->getIdentifier()) {
+    // FIXME: Instead of checking for an implementation-defined function,
+    // check and evaluate the assert() macro.
+    const bool assertFailed = DiagDecl->getName() == "__assert_rtn" ||
+                              DiagDecl->getName() == "__assert_fail" ||
+                              DiagDecl->getName() == "_wassert";
+    if(assertFailed) {
+      Info.FFDiag(CallLoc, diag::note_constexpr_assert_failed);
+      return false;
+    }
+  }
 
+  if (Info.getLangOpts().CPlusPlus11) {
     // If this function is not constexpr because it is an inherited
     // non-constexpr constructor, diagnose that directly.
     auto *CD = dyn_cast<CXXConstructorDecl>(DiagDecl);

clang/test/SemaCXX/consteval-assert.cpp

@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -std=c++23 -verify=expected,cxx20_plus -DTEST_LINUX %s
+// RUN: %clang_cc1 -std=c++23 -verify=expected,cxx20_plus -DTEST_WINDOWS %s
+// RUN: %clang_cc1 -std=c++23 -verify=expected,cxx20_plus -DTEST_DARWIN %s
+
+#ifdef __ASSERT_FUNCTION
+#undef __ASSERT_FUNCTION
+#endif
+
+#if defined(TEST_LINUX)
+  extern "C" void __assert_fail(const char*, const char*, unsigned, const char*);
+  #define assert(cond) \
+    ((cond) ? (void)0 : __assert_fail(#cond, __FILE__, __LINE__, __func__))
+#elif defined(TEST_DARWIN)
+  void __assert_rtn(const char *, const char *, int, const char *);
+  #define assert(cond) \
+  (__builtin_expect(!(cond), 0) ? __assert_rtn(__func__, __FILE__, __LINE__, #cond) : (void)0)
+#elif defined(TEST_WINDOWS)
+  void /*__cdecl*/ _wassert(const wchar_t*, const wchar_t*, unsigned);
+  #define _CRT_WIDE_(s) L ## s
+  #define _CRT_WIDE(s) _CRT_WIDE_(s)
+  #define assert(cond) \
+    (void)((!!(cond)) || (_wassert(_CRT_WIDE(#cond), _CRT_WIDE(__FILE__), (unsigned)(__LINE__)), 0))
+#endif
+
+consteval int square(int x) {
+  int result = x * x;
+  assert(result == 42); // expected-note {{assertion failed during evaluation of constant expression}}
+  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}}
+}