[C] Handle comma operator for implicit int->enum conversions #138752
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In C++, the type of an enumerator is the type of the enumeration,
whereas in C, the type of the enumerator is 'int'. The type of a comma
operator is the type of the right-hand operand, which means you can get
an implicit conversion with this code in C but not in C++:
enum E { Zero };
enum E foo() {
return ((void)0, Zero);
}
We were previously incorrectly diagnosing this code as being
incompatible with C++ because the type of the paren expression would be
'int' there, whereas in C++ the type is 'E'.
So now we handle the comma operator with special logic when analyzing
implicit conversions in C. When analyzing the left-hand operand of a
comma operator, we do not need to check for that operand causing an
implicit conversion for the entire comma expression. So we only check
for that case with the right-hand operand.
This addresses a concern brought up post-commit:
llvm#137658 (comment)
|
@llvm/pr-subscribers-clang Author: Aaron Ballman (AaronBallman) ChangesIn C++, the type of an enumerator is the type of the enumeration, whereas in C, the type of the enumerator is 'int'. The type of a comma operator is the type of the right-hand operand, which means you can get an implicit conversion with this code in C but not in C++: We were previously incorrectly diagnosing this code as being incompatible with C++ because the type of the paren expression would be 'int' there, whereas in C++ the type is 'E'. So now we handle the comma operator with special logic when analyzing implicit conversions in C. When analyzing the left-hand operand of a comma operator, we do not need to check for that operand causing an implicit conversion for the entire comma expression. So we only check for that case with the right-hand operand. This addresses a concern brought up post-commit: Full diff: https://github.com/llvm/llvm-project/pull/138752.diff 3 Files Affected:
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 7f45533713bae..5f7769f9758a6 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -11647,6 +11647,29 @@ static void DiagnoseFloatingImpCast(Sema &S, Expr *E, QualType T,
}
}
+static void CheckCommaOperand(Sema &S, Expr *E, QualType T, SourceLocation CC,
+ bool Check) {
+ E = E->IgnoreParenImpCasts();
+ AnalyzeImplicitConversions(S, E, CC);
+
+ if (Check && E->getType() != T)
+ S.CheckImplicitConversion(E, T, CC);
+}
+
+/// Analyze the given comma operator. The basic idea behind the analysis is to
+/// analyze the left and right operands slightly differently. The left operand
+/// needs to check whether the operand itself has an implicit conversion, but
+/// not whether the left operand induces an implicit conversion for the entire
+/// comma expression itself. This is similar to how CheckConditionalOperand
+/// behaves; it's as-if the correct operand were directly used for the implicit
+/// conversion check.
+static void AnalyzeCommaOperator(Sema &S, BinaryOperator *E, QualType T) {
+ assert(E->isCommaOp() && "Must be a comma operator");
+
+ CheckCommaOperand(S, E->getLHS(), T, E->getOperatorLoc(), false);
+ CheckCommaOperand(S, E->getRHS(), T, E->getOperatorLoc(), true);
+}
+
/// Analyze the given compound assignment for the possible losing of
/// floating-point precision.
static void AnalyzeCompoundAssignment(Sema &S, BinaryOperator *E) {
@@ -12464,7 +12487,7 @@ static void AnalyzeImplicitConversions(
<< OrigE->getSourceRange() << T->isBooleanType()
<< FixItHint::CreateReplacement(UO->getBeginLoc(), "!");
- if (const auto *BO = dyn_cast<BinaryOperator>(SourceExpr))
+ if (auto *BO = dyn_cast<BinaryOperator>(SourceExpr))
if ((BO->getOpcode() == BO_And || BO->getOpcode() == BO_Or) &&
BO->getLHS()->isKnownToHaveBooleanValue() &&
BO->getRHS()->isKnownToHaveBooleanValue() &&
@@ -12490,6 +12513,9 @@ static void AnalyzeImplicitConversions(
(BO->getOpcode() == BO_And ? "&&" : "||"));
S.Diag(BO->getBeginLoc(), diag::note_cast_operand_to_int);
}
+ } else if (BO->isCommaOp() && !S.getLangOpts().CPlusPlus) {
+ AnalyzeCommaOperator(S, BO, T);
+ return;
}
// For conditional operators, we analyze the arguments as if they
diff --git a/clang/test/Sema/implicit-cast.c b/clang/test/Sema/implicit-cast.c
index 088b1958d9b85..4700b7d37a855 100644
--- a/clang/test/Sema/implicit-cast.c
+++ b/clang/test/Sema/implicit-cast.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only %s
+// RUN: %clang_cc1 -fsyntax-only -verify %s
static char *test1(int cf) {
return cf ? "abc" : 0;
@@ -6,3 +6,8 @@ static char *test1(int cf) {
static char *test2(int cf) {
return cf ? 0 : "abc";
}
+
+int baz(void) {
+ int f;
+ return ((void)0, f = 1.4f); // expected-warning {{implicit conversion from 'float' to 'int' changes value from 1.4 to 1}}
+}
diff --git a/clang/test/Sema/implicit-int-enum-conversion.c b/clang/test/Sema/implicit-int-enum-conversion.c
index 13afb5d297aba..36717f36dd083 100644
--- a/clang/test/Sema/implicit-int-enum-conversion.c
+++ b/clang/test/Sema/implicit-int-enum-conversion.c
@@ -50,3 +50,25 @@ enum E1 quux(void) {
return E2_Zero; // expected-warning {{implicit conversion from enumeration type 'enum E2' to different enumeration type 'enum E1'}} \
cxx-error {{cannot initialize return object of type 'enum E1' with an rvalue of type 'E2'}}
}
+
+enum E1 comma1(void) {
+ return ((void)0, E1_One);
+}
+
+enum E1 comma2(void) {
+ enum E1 x;
+ return
+ (x = 12, // expected-warning {{implicit conversion from 'int' to enumeration type 'enum E1' is invalid in C++}} \
+ cxx-error {{assigning to 'enum E1' from incompatible type 'int'}}
+ E1_One);
+}
+
+enum E1 comma3(void) {
+ enum E1 x;
+ return ((void)0, foo()); // Okay, no conversion in C++
+}
+
+enum E1 comma4(void) {
+ return ((void)1, 2); // expected-warning {{implicit conversion from 'int' to enumeration type 'enum E1' is invalid in C++}} \
+ cxx-error {{cannot initialize return object of type 'enum E1' with an rvalue of type 'int'}}
+}
|
erichkeane
reviewed
May 6, 2025
* Renamed a parameter to be more clear * Inlined a simple function body into its sole call site
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In C++, the type of an enumerator is the type of the enumeration, whereas in C, the type of the enumerator is 'int'. The type of a comma operator is the type of the right-hand operand, which means you can get an implicit conversion with this code in C but not in C++:
We were previously incorrectly diagnosing this code as being incompatible with C++ because the type of the paren expression would be 'int' there, whereas in C++ the type is 'E'.
So now we handle the comma operator with special logic when analyzing implicit conversions in C. When analyzing the left-hand operand of a comma operator, we do not need to check for that operand causing an implicit conversion for the entire comma expression. So we only check for that case with the right-hand operand.
This addresses a concern brought up post-commit:
#137658 (comment)