https://github.com/jj-marr created
https://github.com/llvm/llvm-project/pull/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.```
>From 8a09c78d3fdb151c86785822da6cb451025b4654 Mon Sep 17 00:00:00 2001
From: JJ Marr <jjm...@gmail.com>
Date: Sat, 8 Mar 2025 22:00:45 -0500
Subject: [PATCH 1/2] Explain which assertion failed during consteval
---
clang/include/clang/Basic/DiagnosticSemaKinds.td | 2 ++
clang/lib/AST/ExprConstant.cpp | 11 +++++++++++
2 files changed, 13 insertions(+)
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td
b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 21be7c358a61d..ff5f88d6ac572 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -107,6 +107,8 @@ 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 consteval context: '%0'">;
// 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 d9a1e5bb42343..7013f0b143a0a 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -8361,6 +8361,17 @@ 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);
>From c8fe23e62a6a0090ef050a3ca3ac9abe549b0aa6 Mon Sep 17 00:00:00 2001
From: JJ Marr <jjm...@gmail.com>
Date: Sat, 8 Mar 2025 22:36:26 -0500
Subject: [PATCH 2/2] Add unit test
---
clang/test/SemaCXX/consteval_assert.cpp | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
create mode 100644 clang/test/SemaCXX/consteval_assert.cpp
diff --git a/clang/test/SemaCXX/consteval_assert.cpp
b/clang/test/SemaCXX/consteval_assert.cpp
new file mode 100644
index 0000000000000..94ca9ffd4888d
--- /dev/null
+++ b/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 consteval
context: '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}}
+}
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