[clang] Do not diagnose conflicting types for cfi_unchecked_callee

clang/docs/ReleaseNotes.rst

@@ -233,6 +233,9 @@ 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.