[Clang] Tighten restrictions on enum out of range diagnostic to avoid constant initialization

The restrictions added in D131704 were not sufficient to avoid all non-constant
expression contexts. In particular constant initialization cases.

We need to check EvaluatingDecl to detect if the variable we are initializing is
constexpr or not.

At this point it looks like this is the remaining case affecting various projects
with this diagnostic.

Differential Revision: https://reviews.llvm.org/D131874

Author: shafik

clang/lib/AST/ExprConstant.cpp

@@ -13536,6 +13536,18 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {
     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();
+
       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.
@@ -13555,13 +13567,14 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {
         ED->getValueRange(Max, Min);
         --Max;
 
-        if (ED->getNumNegativeBits() &&
+        if (ED->getNumNegativeBits() && ConstexprVar &&
             (Max.slt(Result.getInt().getSExtValue()) ||
              Min.sgt(Result.getInt().getSExtValue())))
-          Info.Ctx.getDiagnostics().Report(E->getExprLoc(),
-                                       diag::warn_constexpr_unscoped_enum_out_of_range)
-	       << llvm::toString(Result.getInt(),10) << Min.getSExtValue() << Max.getSExtValue();
-        else if (!ED->getNumNegativeBits() &&
+          Info.Ctx.getDiagnostics().Report(
+              E->getExprLoc(), diag::warn_constexpr_unscoped_enum_out_of_range)
+              << llvm::toString(Result.getInt(), 10) << Min.getSExtValue()
+              << Max.getSExtValue();
+        else if (!ED->getNumNegativeBits() && ConstexprVar &&
                  Max.ult(Result.getInt().getZExtValue()))
           Info.Ctx.getDiagnostics().Report(E->getExprLoc(),
                                        diag::warn_constexpr_unscoped_enum_out_of_range)

clang/test/SemaCXX/constant-expression-cxx11.cpp

@@ -2419,6 +2419,13 @@ enum class EScoped {escoped1=-4, escoped2=4};
 
 enum EMaxInt {emaxint1=-1, emaxint2=__INT_MAX__};
 
+enum NumberType {};
+
+E2 testDefaultArgForParam(E2 e2Param = (E2)-1) { // ok, not a constant expression context
+  E2 e2LocalInit = e2Param; // ok, not a constant expression context
+  return e2LocalInit;
+}
+
 void testValueInRangeOfEnumerationValues() {
   constexpr E1 x1 = static_cast<E1>(-8);
   constexpr E1 x2 = static_cast<E1>(8);
@@ -2454,6 +2461,8 @@ void testValueInRangeOfEnumerationValues() {
   constexpr EMaxInt x19 = static_cast<EMaxInt>(__INT_MAX__-1);
   constexpr EMaxInt x20 = static_cast<EMaxInt>((long)__INT_MAX__+1);
   // expected-error@-1 {{integer value 2147483648 is outside the valid range of values [-2147483648, 2147483647] for this enumeration type}}
+
+  const NumberType neg_one = (NumberType) ((NumberType) 0 - (NumberType) 1); // ok, not a constant expression context
 }
 
 enum SortOrder {
@@ -2470,3 +2479,8 @@ void A::f(SortOrder order) {
     return;
 }
 }
+
+GH50055::E2 GlobalInitNotCE1 = (GH50055::E2)-1; // ok, not a constant expression context
+GH50055::E2 GlobalInitNotCE2 = GH50055::testDefaultArgForParam(); // ok, not a constant expression context
+constexpr GH50055::E2 GlobalInitCE = (GH50055::E2)-1;
+// expected-error@-1 {{integer value -1 is outside the valid range of values [0, 7] for this enumeration type}}

clang/test/SemaCXX/cxx2a-consteval.cpp

@@ -865,3 +865,15 @@ consteval int aConstevalFunction() { // expected-error {{consteval function neve
 }
 
 } // namespace multiple_default_constructors
+
+namespace GH50055 {
+enum E {e1=0, e2=1};
+consteval int testDefaultArgForParam(E eParam = (E)-1) {
+// expected-error@-1 {{integer value -1 is outside the valid range of values [0, 1] for this enumeration type}}
+  return (int)eParam;
+}
+
+int test() {
+  return testDefaultArgForParam() + testDefaultArgForParam((E)1);
+}
+}