[Clang][Sema] Fix missing warning when comparing mismatched enums in C mode

Factored logic from `CheckImplicitConversion` into new methods
`Expr::getEnumConstantDecl` and `Expr::getEnumCoercedType` for use in
`checkEnumArithmeticConversions`.

Fix #29217

Author: martinkupa

clang/docs/ReleaseNotes.rst

@@ -161,6 +161,9 @@ Improvements to Clang's time-trace
 
 Bug Fixes in This Version
 -------------------------
+- Fixed missing warnings when comparing mismatched enumeration constants
+  in C (`#29217 <https://github.com/llvm/llvm-project/issues/29217>`).
+
 - Clang now accepts elaborated-type-specifiers that explicitly specialize
   a member class template for an implicit instantiation of a class template.
 

clang/include/clang/AST/Expr.h

@@ -153,6 +153,12 @@ class Expr : public ValueStmt {
     TR = t;
   }
 
+  /// If this expression is an enumeration constant, return the
+  /// enumeration type under which said constant was declared.
+  /// Otherwise return the expression's type.
+  /// Note this effectively circumvents the weak typing of C's enum constants
+  QualType getEnumCoercedType(const ASTContext &Ctx) const;
+
   ExprDependence getDependence() const {
     return static_cast<ExprDependence>(ExprBits.Dependent);
   }
@@ -471,6 +477,13 @@ class Expr : public ValueStmt {
   /// bit-fields, but it will return null for a conditional bit-field.
   FieldDecl *getSourceBitField();
 
+  /// If this expression refers to an enum constant, retrieve its declaration
+  EnumConstantDecl *getEnumConstantDecl();
+
+  const EnumConstantDecl *getEnumConstantDecl() const {
+    return const_cast<Expr *>(this)->getEnumConstantDecl();
+  }
+
   const FieldDecl *getSourceBitField() const {
     return const_cast<Expr*>(this)->getSourceBitField();
   }

clang/lib/AST/Expr.cpp

@@ -263,6 +263,14 @@ namespace {
   }
 }
 
+QualType Expr::getEnumCoercedType(const ASTContext &Ctx) const {
+  if (isa<EnumType>(this->getType()))
+    return this->getType();
+  else if (const EnumConstantDecl *ECD = this->getEnumConstantDecl())
+    return Ctx.getTypeDeclType(cast<EnumDecl>(ECD->getDeclContext()));
+  return this->getType();
+}
+
 SourceLocation Expr::getExprLoc() const {
   switch (getStmtClass()) {
   case Stmt::NoStmtClass: llvm_unreachable("statement without class");
@@ -4097,6 +4105,13 @@ FieldDecl *Expr::getSourceBitField() {
   return nullptr;
 }
 
+EnumConstantDecl *Expr::getEnumConstantDecl() {
+  Expr *E = this->IgnoreParenImpCasts();
+  if (DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(E))
+    return dyn_cast<EnumConstantDecl>(DRE->getDecl());
+  return nullptr;
+}
+
 bool Expr::refersToVectorElement() const {
   // FIXME: Why do we not just look at the ObjectKind here?
   const Expr *E = this->IgnoreParens();

clang/lib/Sema/SemaChecking.cpp

@@ -16050,15 +16050,8 @@ static void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
   // Diagnose conversions between different enumeration types.
   // In C, we pretend that the type of an EnumConstantDecl is its enumeration
   // type, to give us better diagnostics.
-  QualType SourceType = E->getType();
-  if (!S.getLangOpts().CPlusPlus) {
-    if (DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(E))
-      if (EnumConstantDecl *ECD = dyn_cast<EnumConstantDecl>(DRE->getDecl())) {
-        EnumDecl *Enum = cast<EnumDecl>(ECD->getDeclContext());
-        SourceType = S.Context.getTypeDeclType(Enum);
-        Source = S.Context.getCanonicalType(SourceType).getTypePtr();
-      }
-  }
+  QualType SourceType = E->getEnumCoercedType(S.Context);
+  Source = S.Context.getCanonicalType(SourceType).getTypePtr();
 
   if (const EnumType *SourceEnum = Source->getAs<EnumType>())
     if (const EnumType *TargetEnum = Target->getAs<EnumType>())

clang/lib/Sema/SemaExpr.cpp

@@ -1497,7 +1497,8 @@ static void checkEnumArithmeticConversions(Sema &S, Expr *LHS, Expr *RHS,
   //
   // Warn on this in all language modes. Produce a deprecation warning in C++20.
   // Eventually we will presumably reject these cases (in C++23 onwards?).
-  QualType L = LHS->getType(), R = RHS->getType();
+  QualType L = LHS->getEnumCoercedType(S.Context),
+           R = RHS->getEnumCoercedType(S.Context);
   bool LEnum = L->isUnscopedEnumerationType(),
        REnum = R->isUnscopedEnumerationType();
   bool IsCompAssign = ACK == Sema::ACK_CompAssign;

clang/test/Sema/builtins-elementwise-math.c

@@ -76,6 +76,7 @@ void test_builtin_elementwise_add_sat(int i, short s, double d, float4 v, int3 i
 
   enum f { three };
   enum f x = __builtin_elementwise_add_sat(one, three);
+  // expected-warning@-1 {{comparison of different enumeration types ('enum e' and 'enum f')}}
 
   _BitInt(32) ext; // expected-warning {{'_BitInt' in C17 and earlier is a Clang extension}}
   ext = __builtin_elementwise_add_sat(ext, ext);
@@ -134,6 +135,7 @@ void test_builtin_elementwise_sub_sat(int i, short s, double d, float4 v, int3 i
 
   enum f { three };
   enum f x = __builtin_elementwise_sub_sat(one, three);
+  // expected-warning@-1 {{comparison of different enumeration types ('enum e' and 'enum f')}}
 
   _BitInt(32) ext; // expected-warning {{'_BitInt' in C17 and earlier is a Clang extension}}
   ext = __builtin_elementwise_sub_sat(ext, ext);
@@ -189,6 +191,7 @@ void test_builtin_elementwise_max(int i, short s, double d, float4 v, int3 iv, u
 
   enum f { three };
   enum f x = __builtin_elementwise_max(one, three);
+  // expected-warning@-1 {{comparison of different enumeration types ('enum e' and 'enum f')}}
 
   _BitInt(32) ext; // expected-warning {{'_BitInt' in C17 and earlier is a Clang extension}}
   ext = __builtin_elementwise_max(ext, ext);
@@ -244,6 +247,7 @@ void test_builtin_elementwise_min(int i, short s, double d, float4 v, int3 iv, u
 
   enum f { three };
   enum f x = __builtin_elementwise_min(one, three);
+  // expected-warning@-1 {{comparison of different enumeration types ('enum e' and 'enum f')}}
 
   _BitInt(32) ext; // expected-warning {{'_BitInt' in C17 and earlier is a Clang extension}}
   ext = __builtin_elementwise_min(ext, ext);

clang/test/Sema/warn-compare-enum-types-mismatch.c

@@ -0,0 +1,42 @@
+// RUN: %clang_cc1 -x c -fsyntax-only -verify -Wenum-compare -Wno-unused-comparison %s
+// RUN: %clang_cc1 -x c++ -fsyntax-only -verify -Wenum-compare -Wno-unused-comparison %s
+
+typedef enum EnumA {
+  A
+} EnumA;
+
+enum EnumB {
+  B
+};
+
+enum {
+  C
+};
+
+void foo(void) {
+  enum EnumA a = A;
+  enum EnumB b = B;
+  A == B;
+  // expected-warning@-1 {{comparison of different enumeration types}}
+  a == (B);
+  // expected-warning@-1 {{comparison of different enumeration types}}
+  a == b;
+  // expected-warning@-1 {{comparison of different enumeration types}}
+  A > B;
+  // expected-warning@-1 {{comparison of different enumeration types}}
+  A >= b;
+  // expected-warning@-1 {{comparison of different enumeration types}}
+  a > b;
+  // expected-warning@-1 {{comparison of different enumeration types}}
+  (A) <= ((B));
+  // expected-warning@-1 {{comparison of different enumeration types}}
+  a < B;
+  // expected-warning@-1 {{comparison of different enumeration types}}
+  a < b;
+  // expected-warning@-1 {{comparison of different enumeration types}}
+
+  // In the following cases we purposefully differ from GCC and dont warn 
+  a == C; 
+  A < C;
+  b >= C; 
+}

clang/test/Sema/warn-conditional-emum-types-mismatch.c renamed to clang/test/Sema/warn-conditional-enum-types-mismatch.c

@@ -21,7 +21,7 @@ int get_flag(int cond) {
   #ifdef __cplusplus
   // expected-warning@-2 {{conditional expression between different enumeration types ('ro' and 'rw')}}
   #else 
-  // expected-no-diagnostics
+  // expected-warning@-4 {{conditional expression between different enumeration types ('enum ro' and 'enum rw')}}
   #endif
 }
 

clang/test/Sema/warn-overlap.c

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -Wtautological-overlap-compare %s
-// RUN: %clang_cc1 -fsyntax-only -verify -Wall -Wno-unused -Wno-loop-analysis %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wtautological-overlap-compare -Wno-enum-compare %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wall -Wno-unused -Wno-loop-analysis -Wno-enum-compare %s
 
 #define mydefine 2