https://github.com/efriedma-quic created
https://github.com/llvm/llvm-project/pull/142648
Backport 97885213bd4507b204b050c3cd570e365d21cc7d
>From c40a997d92a6939e3fb75c6a8766c851c57f3c58 Mon Sep 17 00:00:00 2001
From: Eli Friedman <efrie...@quicinc.com>
Date: Tue, 3 Jun 2025 09:51:37 -0700
Subject: [PATCH] release/20.x: [clang] Don't evaluate the initializer of
constexpr-unknown parameters. (#142498)
Backport 97885213bd4507b204b050c3cd570e365d21cc7d
---
clang/lib/AST/ExprConstant.cpp | 7 ++++++-
clang/test/SemaCXX/constant-expression-p2280r4.cpp | 12 ++++++++++++
2 files changed, 18 insertions(+), 1 deletion(-)
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index e0746f4532245..209b269122a8e 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -3525,7 +3525,12 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const
Expr *E,
// should begin within the evaluation of E
// Used to be C++20 [expr.const]p5.12.2:
// ... its lifetime began within the evaluation of E;
- if (isa<ParmVarDecl>(VD) && !AllowConstexprUnknown) {
+ if (isa<ParmVarDecl>(VD)) {
+ if (AllowConstexprUnknown) {
+ Result = &Info.CurrentCall->createConstexprUnknownAPValues(VD, Base);
+ return true;
+ }
+
// Assume parameters of a potential constant expression are usable in
// constant expressions.
if (!Info.checkingPotentialConstantExpression() ||
diff --git a/clang/test/SemaCXX/constant-expression-p2280r4.cpp
b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
index 87beeb4d3dc84..dbaebb81b93e8 100644
--- a/clang/test/SemaCXX/constant-expression-p2280r4.cpp
+++ b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
@@ -200,3 +200,15 @@ int f() {
return !get_value<Dummy>(); // contextually convert the function call
result to bool
}
}
+
+namespace param_reference {
+ constexpr int arbitrary = -12345;
+ constexpr void f(const int &x = arbitrary) { // expected-note {{declared
here}}
+ constexpr const int &v1 = x; // expected-error {{must be initialized by a
constant expression}} \
+ // expected-note {{reference to 'x' is not a constant expression}}
+ constexpr const int &v2 = (x, arbitrary); // expected-warning {{left
operand of comma operator has no effect}}
+ constexpr int v3 = x; // expected-error {{must be initialized by a
constant expression}}
+ static_assert(x==arbitrary); // expected-error {{static assertion
expression is not an integral constant expression}}
+ static_assert(&x - &x == 0);
+ }
+}
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