Add note version

Author: MillePlateaux

clang/docs/ReleaseNotes.rst

@@ -327,7 +327,6 @@ Improvements to Clang's diagnostics
 
 - An error is now emitted when a ``musttail`` call is made to a function marked with the ``not_tail_called`` attribute. (#GH133509).
 
-
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3170,6 +3170,8 @@ def note_musttail_mismatch : Note<
     "|has type mismatch at %ordinal3 parameter"
     "%diff{ (expected $ but has $)|}1,2"
     "|has different return type%diff{ ($ expected but has $)|}1,2}0">;
+def note_musttail_disabled_by_not_tail_called : Note<
+  "'not_tail_called' attribute here prevents being called as a tail call">;
 def err_musttail_callconv_mismatch : Error<
   "cannot perform a tail call to function%select{| %1}0 because it uses an "
   "incompatible calling convention">;

clang/lib/Sema/SemaStmt.cpp

@@ -719,7 +719,8 @@ bool Sema::checkMustTailAttr(const Stmt *St, const Attr &MTA) {
 
   if (const FunctionDecl *CalleeDecl = CE->getDirectCallee();
       CalleeDecl && CalleeDecl->hasAttr<NotTailCalledAttr>()) {
-    Diag(St->getBeginLoc(), diag::err_musttail_mismatch) << true << CalleeDecl;
+    Diag(St->getBeginLoc(), diag::err_musttail_mismatch) << /*show-function-callee=*/true << CalleeDecl;
+    Diag(CalleeDecl->getLocation(), diag::note_musttail_disabled_by_not_tail_called) << CalleeDecl;
     return false;
   }
 

clang/test/Sema/attr-musttail.cpp

@@ -1,13 +1,13 @@
 // RUN: %clang_cc1 -verify -fsyntax-only %s
 
-int __attribute__((not_tail_called)) foo1(int a) {
+int __attribute__((not_tail_called)) foo1(int a) {// expected-note {{'not_tail_called' attribute here prevents being called as a tail call}}
     return a + 1;  
 }
 
 
 int foo2(int a) {
     [[clang::musttail]] 
-    return foo1(a);  // expected-error {{cannot perform a tail call to function 'foo1' because its signature is incompatible with the calling function}}
+    return foo1(a);  // expected-error {{cannot perform a tail call to function 'foo1' because its signature is incompatible with the calling function}}    `note: 'not_tail_called' attribute here prevents being called as a tail call'
 }
 
 int main() {