Process attributes 'ifunc' and 'alias' when checking for redefinition

[sepavloff]

Summary

These attributes effectively turns a non-defining declaration into a
definition, so the case when the declaration already has a body must
be diagnosed properly.

[aaron.ballman]

LGTM

lib/Sema/SemaDecl.cpp

if (!D)
     return D;
   FunctionDecl *FD = nullptr;
 
   if (FunctionTemplateDecl *FunTmpl = dyn_cast<FunctionTemplateDecl>(D))
     FD = FunTmpl->getTemplatedDecl();
   else
     FD = cast<FunctionDecl>(D);
 
+  // Check for defining attributes before the check for redefinition.
+  if (const auto *Attr = FD->getAttr<AliasAttr>()) {
+    Diag(Attr->getLocation(), diag::err_alias_is_definition) << FD << 0;
+    FD->dropAttr<AliasAttr>();
+    FD->setInvalidDecl();
+  }
+  if (const auto *Attr = FD->getAttr<IFuncAttr>()) {
+    Diag(Attr->getLocation(), diag::err_alias_is_definition) << FD << 1;
+    FD->dropAttr<IFuncAttr>();
+    FD->setInvalidDecl();
+  }
+
   // See if this is a redefinition.
   if (!FD->isLateTemplateParsed()) {
     CheckForFunctionRedefinition(FD, nullptr, SkipBody);
 
     // If we're skipping the body, we're done. Don't enter the scope.
     if (SkipBody && SkipBody->ShouldSkip)
       return D;
   }

test/Sema/alias-redefinition.c

 void f3() {}
 void fun3(void) __attribute((alias("f3"))); // expected-note {{previous definition}}
 void fun3(void) __attribute((alias("f3"))); // expected-error {{redefinition of 'fun3'}}
 
 void f4() {}
 void fun4(void) __attribute((alias("f4")));
 void fun4(void);
 
-// FIXME: We should produce a special case error for this.
 void f5() {}
-void __attribute((alias("f5"))) fun5(void) {} // expected-error {{redefinition of 'fun5'}} // expected-note {{previous definition}}
+void __attribute((alias("f5"))) fun5(void) {} // expected-error {{definition 'fun5' cannot also be an alias}}
 
 int var1 __attribute((alias("v1"))); // expected-error {{definition 'var1' cannot also be an alias}}
 static int var2 __attribute((alias("v2"))) = 2; // expected-error {{definition 'var2' cannot also be an alias}}
 
 extern int var3 __attribute__((alias("C"))); // expected-note{{previous definition is here}}
 int var3 = 3; // expected-error{{redefinition of 'var3'}}
 
 int var4; // expected-note{{previous definition is here}}
 extern int var4 __attribute__((alias("v4"))); // expected-error{{alias definition of 'var4' after tentative definition}}

test/Sema/attr-ifunc.c

 #else
 void f1a() __asm("f1");
 void f1a() {}
 //expected-note@-1 {{previous definition is here}}
 void f1() __attribute__((ifunc("f1_ifunc")));
 //expected-error@-1 {{definition with same mangled name as another definition}}
 void* f1_ifunc() { return 0; }
 
+void* f6_ifunc(int i);
+void __attribute__((ifunc("f6_ifunc"))) f6() {}
+//expected-error@-1 {{definition 'f6' cannot also be an ifunc}}
+
 #endif
 #endif