[clang] [clang] Don't evaluate the initializer of constexpr-unknown parameters. (PR #142498)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic created https://github.com/llvm/llvm-project/pull/142498

If we see a parameter of reference type that isn't part of the frame, don't try to evaluate its default argument. Just treat it as a constexpr-unknown value.

Fixes #141114.  Fixes #141858.

>From 08d64f594d60c8f488c1dfab70b7f8a5b8ae4157 Mon Sep 17 00:00:00 2001
From: Eli Friedman <efriedma at quicinc.com>
Date: Mon, 2 Jun 2025 15:44:35 -0700
Subject: [PATCH] [clang] Don't evaluate the initializer of constexpr-unknown
 parameters.

If we see a parameter of reference type that isn't part of the frame,
don't try to evaluate its defualt argument. Just treat it as a
constexpr-unknown value.

Fixes #141114.  Fixes #141858.
---
 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 b20e2690d0eee..ab964e592de80 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -3550,7 +3550,12 @@ 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 c14250a268f6c..50637917ba210 100644
--- a/clang/test/SemaCXX/constant-expression-p2280r4.cpp
+++ b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
@@ -250,3 +250,15 @@ namespace uninit_reference_used {
   // expected-note {{in call to 'g5()'}}
 
 }
+
+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);
+  }
+}