[clang] Do not diagnose conflicting types for cfi_unchecked_callee (#157762)

Clang would complain about conflicting types between a function
declaration and definition if the declaraion was marked with the
attribute but the definition wasn't. Do not treat this as an error. It
should only be necessary to mark the declaration with the attribute.

Author: PiJoules

clang/docs/ReleaseNotes.rst

@@ -250,6 +250,9 @@ Removed Compiler Flags
 
 Attribute Changes in Clang
 --------------------------
+- The definition of a function declaration with ``[[clang::cfi_unchecked_callee]]`` inherits this
+  attribute, allowing the attribute to only be attached to the declaration. Prior, this would be
+  treated as an error where the definition and declaration would have differing types.
 
 Improvements to Clang's diagnostics
 -----------------------------------

clang/lib/Sema/SemaDecl.cpp

@@ -3877,6 +3877,23 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD, Scope *S,
     RequiresAdjustment = true;
   }
 
+  // If the declaration is marked with cfi_unchecked_callee but the definition
+  // isn't, the definition is also cfi_unchecked_callee.
+  if (auto *FPT1 = OldType->getAs<FunctionProtoType>()) {
+    if (auto *FPT2 = NewType->getAs<FunctionProtoType>()) {
+      FunctionProtoType::ExtProtoInfo EPI1 = FPT1->getExtProtoInfo();
+      FunctionProtoType::ExtProtoInfo EPI2 = FPT2->getExtProtoInfo();
+
+      if (EPI1.CFIUncheckedCallee && !EPI2.CFIUncheckedCallee) {
+        EPI2.CFIUncheckedCallee = true;
+        NewQType = Context.getFunctionType(FPT2->getReturnType(),
+                                           FPT2->getParamTypes(), EPI2);
+        NewType = cast<FunctionType>(NewQType);
+        New->setType(NewQType);
+      }
+    }
+  }
+
   // Merge regparm attribute.
   if (OldTypeInfo.getHasRegParm() != NewTypeInfo.getHasRegParm() ||
       OldTypeInfo.getRegParm() != NewTypeInfo.getRegParm()) {

clang/test/AST/cfi-unchecked-callee.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -ast-dump %s | FileCheck %s
+
+
+// CHECK: FunctionDecl [[PTR:0x[a-z0-9]*]] {{.*}}func 'void () __attribute__((cfi_unchecked_callee))'
+__attribute__((cfi_unchecked_callee))
+void func(void);
+
+// CHECK-NEXT: FunctionDecl {{0x[a-z0-9]*}} prev [[PTR]] {{.*}}func 'void () __attribute__((cfi_unchecked_callee))'
+void func(void) {}

clang/test/Frontend/cfi-unchecked-callee-attribute.cpp

@@ -233,3 +233,7 @@ void lambdas() {
     checked_func = checked_lambda;
   };
 }
+
+CFI_UNCHECKED_CALLEE
+void func(void);
+void func(void) {}  // No warning expected.