[Clang][Sema] Implement GCC -Wcast-function-type

```
Warn when a function pointer is cast to an incompatible function
pointer. In a cast involving function types with a variable argument
list only the types of initial arguments that are provided are
considered. Any parameter of pointer-type matches any other
pointer-type. Any benign differences in integral types are ignored, like
int vs. long on ILP32 targets. Likewise type qualifiers are ignored. The
function type void (*) (void) is special and matches everything, which
can be used to suppress this warning. In a cast involving pointer to
member types this warning warns whenever the type cast is changing the
pointer to member type. This warning is enabled by -Wextra.
```

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D97831

clang/docs/DiagnosticsReference.rst

@@ -851,6 +851,13 @@ This diagnostic is enabled by default.
 |:warning:`warning:` |nbsp| :diagtext:`cast from function call of type` |nbsp| :placeholder:`A` |nbsp| :diagtext:`to non-matching type` |nbsp| :placeholder:`B`|
 +--------------------------------------------------------------------------------------------------------------------------------------------------------------+
 
+-Wcast-function-type
+-------------------
+**Diagnostic text:**
+
++--------------------------------------------------------------------------------------------------------------------------------------------------------------+
+|:warning:`warning:` |nbsp| :diagtext:`cast from` |nbsp| :placeholder:`A` |nbsp| :diagtext:`to` |nbsp| :placeholder:`B` |nbsp| :diagtext:`converts to incompatible function types`|
++--------------------------------------------------------------------------------------------------------------------------------------------------------------+
 
 -Wbinary-literal
 ----------------

clang/include/clang/Basic/DiagnosticGroups.td

@@ -499,6 +499,7 @@ def PrivateExtern : DiagGroup<"private-extern">;
 def SelTypeCast : DiagGroup<"cast-of-sel-type">;
 def FunctionDefInObjCContainer : DiagGroup<"function-def-in-objc-container">;
 def BadFunctionCast : DiagGroup<"bad-function-cast">;
+def CastFunctionType : DiagGroup<"cast-function-type">;
 def ObjCPropertyImpl : DiagGroup<"objc-property-implementation">;
 def ObjCPropertyNoAttribute : DiagGroup<"objc-property-no-attribute">;
 def ObjCPropertyAssignOnObjectType : DiagGroup<"objc-property-assign-on-object-type">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -8386,6 +8386,9 @@ def note_change_calling_conv_fixit : Note<
 def warn_bad_function_cast : Warning<
   "cast from function call of type %0 to non-matching type %1">,
   InGroup<BadFunctionCast>, DefaultIgnore;
+def warn_cast_function_type : Warning<
+  "cast from %0 to %1 converts to incompatible function types">,
+  InGroup<CastFunctionType>, DefaultIgnore;
 def err_cast_pointer_to_non_pointer_int : Error<
   "pointer cannot be cast to type %0">;
 def err_cast_to_bfloat16 : Error<"cannot type-cast to __bf16">;

clang/lib/Sema/SemaCast.cpp

@@ -13,8 +13,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "clang/Sema/SemaInternal.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTStructuralEquivalence.h"
 #include "clang/AST/CXXInheritance.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
@@ -23,6 +23,7 @@
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/Initialization.h"
+#include "clang/Sema/SemaInternal.h"
 #include "llvm/ADT/SmallVector.h"
 #include <set>
 using namespace clang;
@@ -1035,6 +1036,90 @@ static void DiagnoseReinterpretUpDownCast(Sema &Self, const Expr *SrcExpr,
     << FixItHint::CreateReplacement(BeginLoc, "static_cast");
 }
 
+static bool argTypeIsABIEquivalent(QualType SrcType, QualType DestType,
+                                   ASTContext &Context) {
+  if (SrcType->isPointerType() && DestType->isPointerType())
+    return true;
+
+  // Allow integral type mismatch if their size are equal.
+  if (SrcType->isIntegralType(Context) && DestType->isIntegralType(Context))
+    if (Context.getTypeInfoInChars(SrcType).Width ==
+        Context.getTypeInfoInChars(DestType).Width)
+      return true;
+
+  return Context.hasSameUnqualifiedType(SrcType, DestType);
+}
+
+static bool checkCastFunctionType(Sema &Self, const ExprResult &SrcExpr,
+                                  QualType DestType) {
+  if (Self.Diags.isIgnored(diag::warn_cast_function_type,
+                           SrcExpr.get()->getExprLoc()))
+    return true;
+
+  QualType SrcType = SrcExpr.get()->getType();
+  const FunctionType *SrcFTy = nullptr;
+  const FunctionType *DstFTy = nullptr;
+  if (((SrcType->isBlockPointerType() || SrcType->isFunctionPointerType()) &&
+       DestType->isFunctionPointerType()) ||
+      (SrcType->isMemberFunctionPointerType() &&
+       DestType->isMemberFunctionPointerType())) {
+    SrcFTy = SrcType->getPointeeType()->castAs<FunctionType>();
+    DstFTy = DestType->getPointeeType()->castAs<FunctionType>();
+  } else if (SrcType->isFunctionType() && DestType->isFunctionReferenceType()) {
+    SrcFTy = SrcType->castAs<FunctionType>();
+    DstFTy = DestType.getNonReferenceType()->castAs<FunctionType>();
+  } else {
+    return true;
+  }
+  assert(SrcFTy && DstFTy);
+
+  auto IsVoidVoid = [](const FunctionType *T) {
+    if (!T->getReturnType()->isVoidType())
+      return false;
+    if (const auto *PT = T->getAs<FunctionProtoType>())
+      return !PT->isVariadic() && PT->getNumParams() == 0;
+    return false;
+  };
+
+  // Skip if either function type is void(*)(void)
+  if (IsVoidVoid(SrcFTy) || IsVoidVoid(DstFTy))
+    return true;
+
+  // Check return type.
+  if (!argTypeIsABIEquivalent(SrcFTy->getReturnType(), DstFTy->getReturnType(),
+                              Self.Context))
+    return false;
+
+  // Check if either has unspecified number of parameters
+  if (SrcFTy->isFunctionNoProtoType() || DstFTy->isFunctionNoProtoType())
+    return true;
+
+  // Check parameter types.
+
+  const auto *SrcFPTy = cast<FunctionProtoType>(SrcFTy);
+  const auto *DstFPTy = cast<FunctionProtoType>(DstFTy);
+
+  // In a cast involving function types with a variable argument list only the
+  // types of initial arguments that are provided are considered.
+  unsigned NumParams = SrcFPTy->getNumParams();
+  unsigned DstNumParams = DstFPTy->getNumParams();
+  if (NumParams > DstNumParams) {
+    if (!DstFPTy->isVariadic())
+      return false;
+    NumParams = DstNumParams;
+  } else if (NumParams < DstNumParams) {
+    if (!SrcFPTy->isVariadic())
+      return false;
+  }
+
+  for (unsigned i = 0; i < NumParams; ++i)
+    if (!argTypeIsABIEquivalent(SrcFPTy->getParamType(i),
+                                DstFPTy->getParamType(i), Self.Context))
+      return false;
+
+  return true;
+}
+
 /// CheckReinterpretCast - Check that a reinterpret_cast\<DestType\>(SrcExpr) is
 /// valid.
 /// Refer to C++ 5.2.10 for details. reinterpret_cast is typically used in code
@@ -1072,6 +1157,10 @@ void CastOperation::CheckReinterpretCast() {
     if (Self.getLangOpts().allowsNonTrivialObjCLifetimeQualifiers())
       checkObjCConversion(Sema::CCK_OtherCast);
     DiagnoseReinterpretUpDownCast(Self, SrcExpr.get(), DestType, OpRange);
+
+    if (!checkCastFunctionType(Self, SrcExpr, DestType))
+      Self.Diag(OpRange.getBegin(), diag::warn_cast_function_type)
+          << SrcExpr.get()->getType() << DestType << OpRange;
   } else {
     SrcExpr = ExprError();
   }
@@ -2645,6 +2734,11 @@ void CastOperation::CheckCXXCStyleCast(bool FunctionalStyle,
   if (isValidCast(tcr)) {
     if (Kind == CK_BitCast)
       checkCastAlign();
+
+    if (!checkCastFunctionType(Self, SrcExpr, DestType))
+      Self.Diag(OpRange.getBegin(), diag::warn_cast_function_type)
+          << SrcExpr.get()->getType() << DestType << OpRange;
+
   } else {
     SrcExpr = ExprError();
   }
@@ -2957,6 +3051,10 @@ void CastOperation::CheckCStyleCast() {
     }
   }
 
+  if (!checkCastFunctionType(Self, SrcExpr, DestType))
+    Self.Diag(OpRange.getBegin(), diag::warn_cast_function_type)
+        << SrcType << DestType << OpRange;
+
   DiagnoseCastOfObjCSEL(Self, SrcExpr, DestType);
   DiagnoseCallingConvCast(Self, SrcExpr, DestType, OpRange);
   DiagnoseBadFunctionCast(Self, SrcExpr, DestType);

clang/test/Sema/warn-cast-function-type.c

@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -x c %s -fsyntax-only -Wcast-function-type -triple x86_64-- -verify
+
+int x(long);
+
+typedef int (f1)(long);
+typedef int (f2)(void*);
+typedef int (f3)();
+typedef void (f4)();
+typedef void (f5)(void);
+typedef int (f6)(long, int);
+typedef int (f7)(long,...);
+
+f1 *a;
+f2 *b;
+f3 *c;
+f4 *d;
+f5 *e;
+f6 *f;
+f7 *g;
+
+void foo(void) {
+  a = (f1 *)x;
+  b = (f2 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f2 *' (aka 'int (*)(void *)') converts to incompatible function types}} */
+  c = (f3 *)x;
+  d = (f4 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f4 *' (aka 'void (*)()') converts to incompatible function types}} */
+  e = (f5 *)x;
+  f = (f6 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f6 *' (aka 'int (*)(long, int)') converts to incompatible function types}} */
+  g = (f7 *)x;
+}

clang/test/Sema/warn-cast-function-type.cpp

@@ -0,0 +1,47 @@
+// RUN: %clang_cc1 -x c++ %s -fblocks -fsyntax-only -Wcast-function-type -triple x86_64-- -verify
+
+int x(long);
+
+typedef int (f1)(long);
+typedef int (f2)(void*);
+typedef int (f3)(...);
+typedef void (f4)(...);
+typedef void (f5)(void);
+typedef int (f6)(long, int);
+typedef int (f7)(long,...);
+typedef int (&f8)(long, int);
+
+f1 *a;
+f2 *b;
+f3 *c;
+f4 *d;
+f5 *e;
+f6 *f;
+f7 *g;
+
+struct S
+{
+  void foo (int*);
+  void bar (int);
+};
+
+typedef void (S::*mf)(int);
+
+void foo() {
+  a = (f1 *)x;
+  b = (f2 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f2 *' (aka 'int (*)(void *)') converts to incompatible function types}} */
+  b = reinterpret_cast<f2 *>(x); /* expected-warning {{cast from 'int (*)(long)' to 'f2 *' (aka 'int (*)(void *)') converts to incompatible function types}} */
+  c = (f3 *)x;
+  d = (f4 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f4 *' (aka 'void (*)(...)') converts to incompatible function types}} */
+  e = (f5 *)x;
+  f = (f6 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f6 *' (aka 'int (*)(long, int)') converts to incompatible function types}} */
+  g = (f7 *)x;
+
+  mf p1 = (mf)&S::foo; /* expected-warning {{cast from 'void (S::*)(int *)' to 'mf' (aka 'void (S::*)(int)') converts to incompatible function types}} */
+
+  f8 f2 = (f8)x; /* expected-warning {{cast from 'int (long)' to 'f8' (aka 'int (&)(long, int)') converts to incompatible function types}} */
+  (void)f2;
+
+  int (^y)(long);
+  f = (f6 *)y; /* expected-warning {{cast from 'int (^)(long)' to 'f6 *' (aka 'int (*)(long, int)') converts to incompatible function types}} */
+}