[clang][Sema][FMV] Add a note to the 'cannot become multiversioned' diagnostic (#124364)

... pointing out the previous declaration.

Author: jroelofs

clang/lib/Sema/SemaDecl.cpp

@@ -11380,8 +11380,11 @@ static bool CheckMultiVersionAdditionalRules(Sema &S, const FunctionDecl *OldFD,
     return true;
 
   // Only allow transition to MultiVersion if it hasn't been used.
-  if (OldFD && CausesMV && OldFD->isUsed(false))
-    return S.Diag(NewFD->getLocation(), diag::err_multiversion_after_used);
+  if (OldFD && CausesMV && OldFD->isUsed(false)) {
+    S.Diag(NewFD->getLocation(), diag::err_multiversion_after_used);
+    S.Diag(OldFD->getLocation(), diag::note_previous_declaration);
+    return true;
+  }
 
   return S.areMultiversionVariantFunctionsCompatible(
       OldFD, NewFD, S.PDiag(diag::err_multiversion_noproto),

clang/test/Sema/attr-cpuspecific.c

@@ -44,7 +44,8 @@ int allow_fwd_decl2(void);
 void use_fwd_decl(void) {
   allow_fwd_decl2();
 }
-// expected-error@+1 {{function declaration cannot become a multiversioned function after first usage}}
+// expected-error@+2 {{function declaration cannot become a multiversioned function after first usage}}
+// expected-note@-5 {{previous declaration is here}}
 int __attribute__((cpu_dispatch(atom))) allow_fwd_decl2(void) {}
 
 

clang/test/Sema/attr-target-mv.c

@@ -66,7 +66,8 @@ int use3(void) {
   return mv_after_use();
 }
 
-// expected-error@+1 {{function declaration cannot become a multiversioned function after first usage}}
+// expected-error@+2 {{function declaration cannot become a multiversioned function after first usage}}
+// expected-note@-6 {{previous declaration is here}}
 int __attribute__((target("arch=sandybridge")))  mv_after_use(void) { return 2; }
 
 int __attribute__((target("sse4.2,arch=sandybridge"))) mangle(void) { return 1; }

clang/test/Sema/attr-target-version.c

@@ -88,7 +88,8 @@ int bar() {
   nodef();
   return def();
 }
-// expected-error@+1 {{function declaration cannot become a multiversioned function after first usage}}
+// expected-error@+2 {{function declaration cannot become a multiversioned function after first usage}}
+// expected-note@-13 {{previous declaration is here}}
 int __attribute__((target_version("sha2"))) def(void) { return 1; }
 
 int __attribute__((target_version("sve"))) prot();