[Sema] Properly handle generics in enum Codable synthesis

rdar://79916494

Author: drexin

lib/Sema/DerivedConformanceCodable.cpp

@@ -1146,8 +1146,9 @@ deriveBodyEncodable_enum_encode(AbstractFunctionDecl *encodeDecl, void *) {
     }
 
     // generate: case .<Case>:
+    auto enumType = funcDC->mapTypeIntoContext(enumDecl->getDeclaredInterfaceType());
     auto pat = new (C) EnumElementPattern(
-        TypeExpr::createImplicit(enumDecl->getDeclaredType(), C), SourceLoc(),
+        TypeExpr::createImplicit(enumType, C), SourceLoc(),
         DeclNameLoc(), DeclNameRef(), elt, subpattern);
     pat->setImplicit();
 

test/decl/protocol/special/coding/enum_codable_nested_generics.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift
+
+struct Container<Element> {
+    let options: [Option]
+}
+
+extension Container {
+    enum Option {
+        case first (Element)
+        case second (Element)
+    }
+}
+
+extension Container: Codable where Element: Codable {}
+extension Container.Option: Codable where Element: Codable {}