[Serialization] Teach serialization to get a generic signature from opaque types

Previously, if we had an XREF to an opaque type, we would fail trying to
get the generic signature from that type.

Fixes rdar://53958358

Author: Harlan Haskins

include/swift/Serialization/ModuleFormat.h

@@ -52,7 +52,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 507; // exact_self_class sil attr
+const uint16_t SWIFTMODULE_VERSION_MINOR = 508; // generic opaque return type xref
 
 using DeclIDField = BCFixed<31>;
 

lib/Serialization/Deserialization.cpp

@@ -1719,6 +1719,8 @@ ModuleFile::resolveCrossReference(ModuleID MID, uint32_t pathLen) {
         currentSig = fn->getGenericSignature();
       } else if (auto subscript = dyn_cast<SubscriptDecl>(base)) {
         currentSig = subscript->getGenericSignature();
+      } else if (auto opaque = dyn_cast<OpaqueTypeDecl>(base)) {
+        currentSig = opaque->getGenericSignature();
       }
 
       if (!currentSig) {

test/Serialization/Inputs/xref-opaque-generic-type/best-protocol.swift

@@ -0,0 +1,16 @@
+public protocol BestProtocol {}
+public protocol GoodProtocol {
+  associatedtype A
+}
+
+public struct BestStruct: BestProtocol {
+  public init() {}
+}
+
+extension BestProtocol {
+  @available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
+  public func _bestValue<X: GoodProtocol>(_ x: X.Type, _ a: X.A) -> some BestProtocol {
+    return BestStruct()
+  }
+}
+

test/Serialization/Inputs/xref-opaque-generic-type/output.json

@@ -0,0 +1,8 @@
+{
+  "./best-protocol.swift": {
+    "swiftmodule": "./best-protocol.swiftmodule"
+  },
+  "./xref-opaque-generic-type.swift": {
+    "swiftmodule": "./xref-opaque-generic-type.swiftmodule"
+  }
+}

test/Serialization/xref-generic-opaque-type.swift

@@ -0,0 +1,19 @@
+// RUN: %empty-directory(%t)
+// RUN: cp -r %S/Inputs/xref-opaque-generic-type/* %t/.
+// RUN: cp %s %t/xref-generic-opaque-type.swift
+// RUN: cd %t
+// RUN: %target-swiftc_driver -emit-module -incremental %t/best-protocol.swift %t/xref-generic-opaque-type.swift -module-name A -output-file-map %t/output.json
+
+@usableFromInline
+struct GoodStruct: GoodProtocol {
+  @usableFromInline
+  typealias A = Int
+}
+
+extension BestProtocol {
+  @inlinable
+  @available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
+  public func bestValue(_ x: Int) -> some BestProtocol {
+    return _bestValue(GoodStruct.self, x)
+  }
+}