[clang] Don't evaluate the initializer of constexpr-unknown parameters.

[efriedma-quic]

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.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Eli Friedman (efriedma-quic)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/142498.diff

2 Files Affected:

• (modified) clang/lib/AST/ExprConstant.cpp (+6-1)
• (modified) clang/test/SemaCXX/constant-expression-p2280r4.cpp (+12)

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);
+  }
+}

[shafik]

I was literally just writing this PR and saw you just submitted this one, thank you!

LGTM

[efriedma-quic]

/cherry-pick 9788521

[llvmbot]

Failed to cherry-pick: 9788521

https://github.com/llvm/llvm-project/actions/runs/15423178980

Please manually backport the fix and push it to your github fork. Once this is done, please create a pull request

[efriedma-quic]

Backported manually: #142648