[clang] b77109f - Better diagnostics when assertion fails in `consteval` (#130458)
via cfe-commits
cfe-commits at lists.llvm.org
Tue May 13 23:47:42 PDT 2025
Author: JJ Marr
Date: 2025-05-14T08:47:38+02:00
New Revision: b77109ff8c17cd20ac2c791761028a358c9e3447
URL: https://github.com/llvm/llvm-project/commit/b77109ff8c17cd20ac2c791761028a358c9e3447
DIFF: https://github.com/llvm/llvm-project/commit/b77109ff8c17cd20ac2c791761028a358c9e3447.diff
LOG: Better diagnostics when assertion fails in `consteval` (#130458)
Take this piece of code:
```cpp
#include <cassert>
consteval int square(int x) {
int result = x * x;
assert(result == 42);
return result;
}
void test() {
auto val = square(2);
}
```
The assertion will fail, and `clang++` will output
(https://godbolt.org/z/hjz3KbTTv):
```cpp
<source>:10:14: error: call to consteval function 'square' is not a constant expression
10 | auto val = square(2);
| ^
<source>:5:3: note: non-constexpr function '__assert_fail' cannot be used in a constant expression
5 | assert(result == 42);
| ^
/usr/include/assert.h:95:9: note: expanded from macro 'assert'
95 | : __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION))
| ^
<source>:10:14: note: in call to 'square(2)'
10 | auto val = square(2);
| ^~~~~~~~~
/usr/include/assert.h:69:13: note: declared here
69 | extern void __assert_fail (const char *__assertion, const char *__file,
| ^
1 error generated.
Compiler returned: 1
```
This is confusing because it implies that the issue was using an
assertion in a constant-evaluted context, and not that the assertion
failed (`assert()` is OK in constant evaluation). This PR changes the
error message to:
```cpp
test.cpp:10:14: error: call to consteval function 'square' is not a constant expression
10 | auto val = square(2);
| ^
test.cpp:5:3: note: assertion failed in consteval context: 'result == 42'
5 | assert(result == 42);
| ^
/nix/store/lw21wr626v5sdcaxxkv2k4zf1121hfc9-glibc-2.40-36-dev/include/assert.h:102:9: note: expanded from macro 'assert'
102 | : __assert_fail (#expr, __ASSERT_FILE, __ASSERT_LINE, \
| ^
test.cpp:10:14: note: in call to 'square(2)'
10 | auto val = square(2);
| ^~~~~~~~~
1 error generated.```
Added:
clang/test/SemaCXX/consteval-assert.cpp
Modified:
clang/docs/ReleaseNotes.rst
clang/include/clang/Basic/DiagnosticSemaKinds.td
clang/lib/AST/ExprConstant.cpp
Removed:
################################################################################
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 611758dc145ca..da8bc6ea755a5 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -525,6 +525,10 @@ Improvements to Clang's diagnostics
- An error is now emitted when OpenMP ``collapse`` and ``ordered`` clauses have an
argument larger than what can fit within a 64-bit integer.
+
+- 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)
- A new off-by-default warning ``-Wms-bitfield-padding`` has been added to alert to cases where bit-field
packing may
diff er under the MS struct ABI (#GH117428).
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 87c2f57837ddd..6c74336addddd 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -164,6 +164,8 @@ 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<
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 500d43accb082..c09afef9c4bf9 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/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.
+ StringRef Name = DiagDecl->getName();
+ bool AssertFailed =
+ Name == "__assert_rtn" || Name == "__assert_fail" || Name == "_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);
diff --git a/clang/test/SemaCXX/consteval-assert.cpp b/clang/test/SemaCXX/consteval-assert.cpp
new file mode 100644
index 0000000000000..b54a38ff26105
--- /dev/null
+++ b/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}}
+}
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