[C] Handle comma operator for implicit int->enum conversions (llvm#138752)

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)

Author: AaronBallman

clang/lib/Sema/SemaChecking.cpp

@@ -11647,6 +11647,15 @@ static void DiagnoseFloatingImpCast(Sema &S, Expr *E, QualType T,
   }
 }
 
+static void CheckCommaOperand(Sema &S, Expr *E, QualType T, SourceLocation CC,
+                              bool ExtraCheckForImplicitConversion) {
+  E = E->IgnoreParenImpCasts();
+  AnalyzeImplicitConversions(S, E, CC);
+
+  if (ExtraCheckForImplicitConversion && E->getType() != T)
+    S.CheckImplicitConversion(E, T, CC);
+}
+
 /// Analyze the given compound assignment for the possible losing of
 /// floating-point precision.
 static void AnalyzeCompoundAssignment(Sema &S, BinaryOperator *E) {
@@ -12464,7 +12473,7 @@ 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 +12499,19 @@ static void AnalyzeImplicitConversions(
                    (BO->getOpcode() == BO_And ? "&&" : "||"));
         S.Diag(BO->getBeginLoc(), diag::note_cast_operand_to_int);
       }
+    } else if (BO->isCommaOp() && !S.getLangOpts().CPlusPlus) {
+      /// 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.
+      CheckCommaOperand(S, BO->getLHS(), T, BO->getOperatorLoc(),
+                        /*ExtraCheckForImplicitConversion=*/false);
+      CheckCommaOperand(S, BO->getRHS(), T, BO->getOperatorLoc(),
+                        /*ExtraCheckForImplicitConversion=*/true);
+      return;
     }
 
   // For conditional operators, we analyze the arguments as if they

clang/test/Sema/implicit-cast.c

@@ -1,8 +1,13 @@
-// RUN: %clang_cc1 -fsyntax-only %s
+// RUN: %clang_cc1 -fsyntax-only -verify %s
 
 static char *test1(int cf) {
   return cf ? "abc" : 0;
 }
 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}}
+}

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'}}
+}