[Clang][CodeGen] fix failed assertion

[nickdesaulniers]

Summary

Ensure we can emit symbol aliases via function attribute
even when function signatures contain incomplete types.

Via bugreport:
https://reviews.llvm.org/D66492#2350947

[erichkeane]

clang/lib/CodeGen/CodeGenModule.cpp

4643

I'd suggest making this just 'else', that way you don't have to do the assert below. The first line of getFunctionLinkage does the cast to FunctionDecl, so that part is already taken care of.

[aaron.ballman]

clang/lib/CodeGen/CodeGenModule.cpp

4640–4643

Rather than isa<> followed by cast<>, this should be using dyn_cast<>. e.g.,

if (const auto *VD = dyn_cast<VarDecl>(GD.getDecl()))
  ...
else
  ...

[nickdesaulniers]

• dyn_cast, else

[erichkeane]

Please make sure the commit message is significantly more descriptive here than what is on the review.

clang/lib/CodeGen/CodeGenModule.cpp

void CodeGenModule::EmitAliasDefinition(GlobalDecl GD) {
   if (isa<llvm::FunctionType>(DeclTy)) {
     Aliasee = GetOrCreateLLVMFunction(AA->getAliasee(), DeclTy, GD,
                                       /*ForVTable=*/false);
     LT = getFunctionLinkage(GD);
   } else {
     Aliasee = GetOrCreateLLVMGlobal(AA->getAliasee(),
                                     llvm::PointerType::getUnqual(DeclTy),
                                     /*D=*/nullptr);
-    LT = getLLVMLinkageVarDefinition(cast<VarDecl>(GD.getDecl()),
-                                     D->getType().isConstQualified());
+    if (const auto *VD = dyn_cast<VarDecl>(GD.getDecl()))
+      LT = getLLVMLinkageVarDefinition(VD, D->getType().isConstQualified());
+    else
+      LT = getFunctionLinkage(GD);
   }
 
   // Create the new alias itself, but don't set a name yet.
   unsigned AS = Aliasee->getType()->getPointerAddressSpace();
   auto *GA =
       llvm::GlobalAlias::create(DeclTy, AS, LT, "", Aliasee, &getModule());
 
   if (Entry) {

clang/test/CodeGen/alias.c

 // CHECKGLOBALS-NOT: @test11_foo = dso_local
 void test11(void) {}
 static void test11_foo(void) __attribute__((alias("test11")));
 
 // Test that gnu_inline+alias work.
 // CHECKGLOBALS: @test12_alias = alias void (), void ()* @test12
 void test12(void) {}
 inline void test12_alias(void) __attribute__((gnu_inline, alias("test12")));
+
+// Test that a non visible (-Wvisibility) type doesn't assert.
+// CHECKGLOBALS: @test13_alias = alias {}, bitcast (void (i32)* @test13 to {}*)
+enum a_type { test13_a };
+void test13(enum a_type y) {}
+void test13_alias(enum undeclared_type y) __attribute__((alias ("test13")));