[clang] [clang] Check constexpr int->enum conversions consistently. (PR #143034)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic created https://github.com/llvm/llvm-project/pull/143034

In 8de51375f12d91675a18d17f262276e65f43fbe0 and related patches, we added some code to avoid triggering -Wenum-constexpr-conversion in some cases.  This isn't necessary anymore because -Wenum-constexpr-conversion doesn't exist anymore.  And the checks are subtly wrong: they exclude cases where we actually do need to check the conversion. This patch gets rid of the unnecessary checks.

>From 416a1671246edd683da68449c711b89117a4375a Mon Sep 17 00:00:00 2001
From: Eli Friedman <efriedma at quicinc.com>
Date: Wed, 4 Jun 2025 19:40:37 -0700
Subject: [PATCH] [clang] Check constexpr int->enum conversions consistently.

In 8de51375f12d91675a18d17f262276e65f43fbe0 and related patches, we
added some code to avoid triggering -Wenum-constexpr-conversion in some
cases.  This isn't necessary anymore because -Wenum-constexpr-conversion
doesn't exist anymore.  And the checks are subtly wrong: they exclude
cases where we actually do need to check the conversion. This patch gets
rid of the unnecessary checks.
---
 clang/lib/AST/ExprConstant.cpp                | 20 +++----------------
 .../SemaCXX/constant-expression-cxx11.cpp     |  7 +++++++
 2 files changed, 10 insertions(+), 17 deletions(-)

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index ab964e592de80..3417e47e33db6 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -15223,21 +15223,7 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {
       return Info.Ctx.getTypeSize(DestType) == Info.Ctx.getTypeSize(SrcType);
     }
 
-    if (Info.Ctx.getLangOpts().CPlusPlus && Info.InConstantContext &&
-        Info.EvalMode == EvalInfo::EM_ConstantExpression &&
-        DestType->isEnumeralType()) {
-
-      bool ConstexprVar = true;
-
-      // We know if we are here that we are in a context that we might require
-      // a constant expression or a context that requires a constant
-      // value. But if we are initializing a value we don't know if it is a
-      // constexpr variable or not. We can check the EvaluatingDecl to determine
-      // if it constexpr or not. If not then we don't want to emit a diagnostic.
-      if (const auto *VD = dyn_cast_or_null<VarDecl>(
-              Info.EvaluatingDecl.dyn_cast<const ValueDecl *>()))
-        ConstexprVar = VD->isConstexpr();
-
+    if (Info.Ctx.getLangOpts().CPlusPlus && DestType->isEnumeralType()) {
       const EnumType *ET = dyn_cast<EnumType>(DestType.getCanonicalType());
       const EnumDecl *ED = ET->getDecl();
       // Check that the value is within the range of the enumeration values.
@@ -15257,13 +15243,13 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {
         ED->getValueRange(Max, Min);
         --Max;
 
-        if (ED->getNumNegativeBits() && ConstexprVar &&
+        if (ED->getNumNegativeBits() &&
             (Max.slt(Result.getInt().getSExtValue()) ||
              Min.sgt(Result.getInt().getSExtValue())))
           Info.CCEDiag(E, diag::note_constexpr_unscoped_enum_out_of_range)
               << llvm::toString(Result.getInt(), 10) << Min.getSExtValue()
               << Max.getSExtValue() << ED;
-        else if (!ED->getNumNegativeBits() && ConstexprVar &&
+        else if (!ED->getNumNegativeBits() &&
                  Max.ult(Result.getInt().getZExtValue()))
           Info.CCEDiag(E, diag::note_constexpr_unscoped_enum_out_of_range)
               << llvm::toString(Result.getInt(), 10) << Min.getZExtValue()
diff --git a/clang/test/SemaCXX/constant-expression-cxx11.cpp b/clang/test/SemaCXX/constant-expression-cxx11.cpp
index 0a135654fab18..10c514be5025f 100644
--- a/clang/test/SemaCXX/constant-expression-cxx11.cpp
+++ b/clang/test/SemaCXX/constant-expression-cxx11.cpp
@@ -2513,6 +2513,9 @@ void testValueInRangeOfEnumerationValues() {
   // expected-error at -1 {{constexpr variable 'x2' must be initialized by a constant expression}}
   // expected-note at -2 {{integer value 8 is outside the valid range of values [-8, 7] for the enumeration type 'E1'}}
   E1 x2b = static_cast<E1>(8); // ok, not a constant expression context
+  static_assert(static_cast<E1>(8));
+  // expected-error at -1 {{static assertion expression is not an integral constant expression}}
+  // expected-note at -2 {{integer value 8 is outside the valid range of values [-8, 7] for the enumeration type 'E1'}}
 
   constexpr E2 x3 = static_cast<E2>(-8);
   // expected-error at -1 {{constexpr variable 'x3' must be initialized by a constant expression}}
@@ -2551,6 +2554,10 @@ void testValueInRangeOfEnumerationValues() {
   // expected-note at -2 {{integer value 2147483648 is outside the valid range of values [-2147483648, 2147483647] for the enumeration type 'EMaxInt'}}
 
   const NumberType neg_one = (NumberType) ((NumberType) 0 - (NumberType) 1); // ok, not a constant expression context
+  constexpr NumberType neg_one_constexpr = neg_one;
+  // expected-error at -1 {{constexpr variable 'neg_one_constexpr' must be initialized by a constant expression}}
+  // expected-note at -2 {{initializer of 'neg_one' is not a constant expression}}
+  // expected-note at -4 {{declared here}}
 
   CONSTEXPR_CAST_TO_SYSTEM_ENUM_OUTSIDE_OF_RANGE;
   // expected-error at -1 {{constexpr variable 'system_enum' must be initialized by a constant expression}}