[DebugInfo] Fix infinite recursion when opaque return type is defined

inside function returning it

A stack overflow would happen when the compiler tried emitting debug
info for a function whose opaque return type was declared inside the
function itself. This fixes the issue by emitting a forward declaration
for the function before emitting it.

rdar://150313956

(cherry picked from commit c03831f)

Author: augusto2112

lib/IRGen/IRGenDebugInfo.cpp

@@ -979,7 +979,10 @@ class IRGenDebugInfoImpl : public IRGenDebugInfo {
       if (Scope)
         return MainFile;
     }
-    return cast<llvm::DIFile>(Scope);
+    if (Scope)
+      return cast<llvm::DIFile>(Scope);
+
+    return MainFile;
   }
 
   static unsigned getStorageSizeInBits(const llvm::DataLayout &DL,
@@ -3222,6 +3225,11 @@ IRGenDebugInfoImpl::emitFunction(const SILDebugScope *DS, llvm::Function *Fn,
     Name = LinkageName;
   }
 
+  llvm::DISubprogram *ReplaceableType = DBuilder.createTempFunctionFwdDecl(
+      Scope, Name, LinkageName, File, Line, /*Type=*/nullptr, ScopeLine);
+  auto FwdDecl = llvm::TempDISubprogram(ReplaceableType);
+  ScopeCache[DS] = llvm::TrackingMDNodeRef(FwdDecl.get());
+
   CanSILFunctionType FnTy = getFunctionType(SILTy);
   auto Params = Opts.DebugInfoLevel > IRGenDebugInfoLevel::LineTables
                     ? createParameterTypes(SILTy)
@@ -3321,6 +3329,7 @@ IRGenDebugInfoImpl::emitFunction(const SILDebugScope *DS, llvm::Function *Fn,
   if (!DS)
     return nullptr;
 
+  DBuilder.replaceTemporary(std::move(FwdDecl), SP);
   ScopeCache[DS] = llvm::TrackingMDNodeRef(SP);
   return SP;
 }

test/DebugInfo/nested_opaque.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-frontend -primary-file %s -emit-ir -g -o - | %FileCheck %s
+
+protocol TheProtocol {
+}
+
+// CHECK: ![[FUNC_ID:[0-9]+]] = distinct !DISubprogram(name: "theFunction", 
+// CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "TheType", scope: ![[FUNC_ID]]
+func theFunction() -> some TheProtocol {
+    struct TheType: TheProtocol {
+    }
+    return TheType()
+}
+
+theFunction()