[Runtime] Fix nested coding (#50)

[Runtime] Fix nested coding

### Motivation

Fixes apple/swift-openapi-generator#263.

### Modifications

Makes URIEncoder/URIDecoder able to handle custom Codable types in a single value container.

For more details check out the associated generator [PR](apple/swift-openapi-generator#271).

### Result

More Codable types can be handled.

### Test Plan

Updated unit tests.


Reviewed by: glbrntt

Builds:
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     ✔︎ pull request validation (5.9) - Build finished. 
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     ✔︎ pull request validation (docc test) - Build finished. 
     ✔︎ pull request validation (integration test) - Build finished. 
     ✔︎ pull request validation (nightly) - Build finished. 
     ✔︎ pull request validation (soundness) - Build finished. 

#50

Sources/OpenAPIRuntime/Conversion/CodableExtensions.swift

@@ -90,6 +90,16 @@ extension Decoder {
         return .init(uniqueKeysWithValues: keyValuePairs)
     }
 
+    /// Returns the decoded value by using a single value container.
+    /// - Parameter type: The type to decode.
+    /// - Returns: The decoded value.
+    public func decodeFromSingleValueContainer<T: Decodable>(
+        _ type: T.Type = T.self
+    ) throws -> T {
+        let container = try singleValueContainer()
+        return try container.decode(T.self)
+    }
+
     // MARK: - Private
 
     /// Returns the keys in the given decoder that are not present
@@ -146,6 +156,29 @@ extension Encoder {
             try container.encode(value, forKey: .init(key))
         }
     }
+
+    /// Encodes the value into the encoder using a single value container.
+    /// - Parameter value: The value to encode.
+    public func encodeToSingleValueContainer<T: Encodable>(
+        _ value: T
+    ) throws {
+        var container = singleValueContainer()
+        try container.encode(value)
+    }
+
+    /// Encodes the first non-nil value from the provided array into
+    /// the encoder using a single value container.
+    /// - Parameter values: An array of optional values.
+    public func encodeFirstNonNilValueToSingleValueContainer(
+        _ values: [(any Encodable)?]
+    ) throws {
+        for value in values {
+            if let value {
+                try encodeToSingleValueContainer(value)
+                return
+            }
+        }
+    }
 }
 
 /// A freeform String coding key for decoding undocumented values.

Sources/OpenAPIRuntime/URICoder/Decoding/URIValueFromNodeDecoder+Single.swift

@@ -17,26 +17,27 @@ import Foundation
 /// A single value container used by `URIValueFromNodeDecoder`.
 struct URISingleValueDecodingContainer {
 
-    /// The coder used to serialize Date values.
-    let dateTranscoder: any DateTranscoder
-
-    /// The coding path of the container.
-    let codingPath: [any CodingKey]
-
-    /// The underlying value.
-    let value: URIParsedValue
+    /// The associated decoder.
+    let decoder: URIValueFromNodeDecoder
 }
 
 extension URISingleValueDecodingContainer {
 
+    /// The underlying value as a single value.
+    var value: URIParsedValue {
+        get throws {
+            try decoder.currentElementAsSingleValue()
+        }
+    }
+
     /// Returns the value found in the underlying node converted to
     /// the provided type.
     /// - Returns: The converted value found.
     /// - Throws: An error if the conversion failed.
     private func _decodeBinaryFloatingPoint<T: BinaryFloatingPoint>(
         _: T.Type = T.self
     ) throws -> T {
-        guard let double = Double(value) else {
+        guard let double = try Double(value) else {
             throw DecodingError.typeMismatch(
                 T.self,
                 .init(
@@ -55,7 +56,7 @@ extension URISingleValueDecodingContainer {
     private func _decodeFixedWidthInteger<T: FixedWidthInteger>(
         _: T.Type = T.self
     ) throws -> T {
-        guard let parsedValue = T(value) else {
+        guard let parsedValue = try T(value) else {
             throw DecodingError.typeMismatch(
                 T.self,
                 .init(
@@ -74,7 +75,7 @@ extension URISingleValueDecodingContainer {
     private func _decodeLosslessStringConvertible<T: LosslessStringConvertible>(
         _: T.Type = T.self
     ) throws -> T {
-        guard let parsedValue = T(String(value)) else {
+        guard let parsedValue = try T(String(value)) else {
             throw DecodingError.typeMismatch(
                 T.self,
                 .init(
@@ -89,6 +90,10 @@ extension URISingleValueDecodingContainer {
 
 extension URISingleValueDecodingContainer: SingleValueDecodingContainer {
 
+    var codingPath: [any CodingKey] {
+        decoder.codingPath
+    }
+
     func decodeNil() -> Bool {
         false
     }
@@ -98,7 +103,7 @@ extension URISingleValueDecodingContainer: SingleValueDecodingContainer {
     }
 
     func decode(_ type: String.Type) throws -> String {
-        String(value)
+        try String(value)
     }
 
     func decode(_ type: Double.Type) throws -> Double {
@@ -180,9 +185,9 @@ extension URISingleValueDecodingContainer: SingleValueDecodingContainer {
         case is UInt64.Type:
             return try decode(UInt64.self) as! T
         case is Date.Type:
-            return try dateTranscoder.decode(String(value)) as! T
+            return try decoder.dateTranscoder.decode(String(value)) as! T
         default:
-            throw URIValueFromNodeDecoder.GeneralError.unsupportedType(T.self)
+            return try T.init(from: decoder)
         }
     }
 }

Sources/OpenAPIRuntime/URICoder/Decoding/URIValueFromNodeDecoder.swift

@@ -98,9 +98,6 @@ extension URIValueFromNodeDecoder {
     /// A decoder error.
     enum GeneralError: Swift.Error {
 
-        /// The decoder does not support the provided type.
-        case unsupportedType(Any.Type)
-
         /// The decoder was asked to create a nested container.
         case nestedContainersNotSupported
 
@@ -274,7 +271,7 @@ extension URIValueFromNodeDecoder {
     /// Extracts the node at the top of the coding stack and tries to treat it
     /// as a primitive value.
     /// - Returns: The value if it can be treated as a primitive value.
-    private func currentElementAsSingleValue() throws -> URIParsedValue {
+    func currentElementAsSingleValue() throws -> URIParsedValue {
         try nodeAsSingleValue(currentElement)
     }
 
@@ -368,11 +365,6 @@ extension URIValueFromNodeDecoder: Decoder {
     }
 
     func singleValueContainer() throws -> any SingleValueDecodingContainer {
-        let value = try currentElementAsSingleValue()
-        return URISingleValueDecodingContainer(
-            dateTranscoder: dateTranscoder,
-            codingPath: codingPath,
-            value: value
-        )
+        return URISingleValueDecodingContainer(decoder: self)
     }
 }

Sources/OpenAPIRuntime/URICoder/Encoding/URIValueToNodeEncoder+Single.swift

@@ -144,7 +144,7 @@ extension URISingleValueEncodingContainer: SingleValueEncodingContainer {
         case let value as Date:
             try _setValue(.date(value))
         default:
-            throw URIValueToNodeEncoder.GeneralError.nestedValueInSingleValueContainer
+            try value.encode(to: encoder)
         }
     }
 }

Sources/OpenAPIRuntime/URICoder/Encoding/URIValueToNodeEncoder.swift

@@ -40,9 +40,6 @@ final class URIValueToNodeEncoder {
 
         /// The encoder set a value for an index out of range of the container.
         case integerOutOfRange
-
-        /// The encoder tried to treat
-        case nestedValueInSingleValueContainer
     }
 
     /// The stack of nested values within the root node.

Tests/OpenAPIRuntimeTests/URICoder/Test_URICodingRoundtrip.swift

@@ -12,7 +12,10 @@
 //
 //===----------------------------------------------------------------------===//
 import XCTest
-@testable import OpenAPIRuntime
+@_spi(Generated)@testable import OpenAPIRuntime
+#if os(Linux)
+@preconcurrency import Foundation
+#endif
 
 final class Test_URICodingRoundtrip: Test_Runtime {
 
@@ -27,12 +30,60 @@ final class Test_URICodingRoundtrip: Test_Runtime {
             var maybeFoo: String?
         }
 
+        struct TrivialStruct: Codable, Equatable {
+            var foo: String
+        }
+
         enum SimpleEnum: String, Codable, Equatable {
             case red
             case green
             case blue
         }
 
+        struct AnyOf: Codable, Equatable, Sendable {
+            var value1: Foundation.Date?
+            var value2: SimpleEnum?
+            var value3: TrivialStruct?
+            init(value1: Foundation.Date? = nil, value2: SimpleEnum? = nil, value3: TrivialStruct? = nil) {
+                self.value1 = value1
+                self.value2 = value2
+                self.value3 = value3
+            }
+            init(from decoder: any Decoder) throws {
+                do {
+                    let container = try decoder.singleValueContainer()
+                    value1 = try? container.decode(Foundation.Date.self)
+                }
+                do {
+                    let container = try decoder.singleValueContainer()
+                    value2 = try? container.decode(SimpleEnum.self)
+                }
+                do {
+                    let container = try decoder.singleValueContainer()
+                    value3 = try? container.decode(TrivialStruct.self)
+                }
+                try DecodingError.verifyAtLeastOneSchemaIsNotNil(
+                    [value1, value2, value3],
+                    type: Self.self,
+                    codingPath: decoder.codingPath
+                )
+            }
+            func encode(to encoder: any Encoder) throws {
+                if let value1 {
+                    var container = encoder.singleValueContainer()
+                    try container.encode(value1)
+                }
+                if let value2 {
+                    var container = encoder.singleValueContainer()
+                    try container.encode(value2)
+                }
+                if let value3 {
+                    var container = encoder.singleValueContainer()
+                    try container.encode(value3)
+                }
+            }
+        }
+
         // An empty string.
         try _test(
             "",
@@ -210,6 +261,51 @@ final class Test_URICodingRoundtrip: Test_Runtime {
             )
         )
 
+        // A struct with a custom Codable implementation that forwards
+        // decoding to nested values.
+        try _test(
+            AnyOf(
+                value1: Date(timeIntervalSince1970: 1_674_036_251)
+            ),
+            key: "root",
+            .init(
+                formExplode: "root=2023-01-18T10%3A04%3A11Z",
+                formUnexplode: "root=2023-01-18T10%3A04%3A11Z",
+                simpleExplode: "2023-01-18T10%3A04%3A11Z",
+                simpleUnexplode: "2023-01-18T10%3A04%3A11Z",
+                formDataExplode: "root=2023-01-18T10%3A04%3A11Z",
+                formDataUnexplode: "root=2023-01-18T10%3A04%3A11Z"
+            )
+        )
+        try _test(
+            AnyOf(
+                value2: .green
+            ),
+            key: "root",
+            .init(
+                formExplode: "root=green",
+                formUnexplode: "root=green",
+                simpleExplode: "green",
+                simpleUnexplode: "green",
+                formDataExplode: "root=green",
+                formDataUnexplode: "root=green"
+            )
+        )
+        try _test(
+            AnyOf(
+                value3: .init(foo: "bar")
+            ),
+            key: "root",
+            .init(
+                formExplode: "foo=bar",
+                formUnexplode: "root=foo,bar",
+                simpleExplode: "foo=bar",
+                simpleUnexplode: "foo,bar",
+                formDataExplode: "foo=bar",
+                formDataUnexplode: "root=foo,bar"
+            )
+        )
+
         // An empty struct.
         struct EmptyStruct: Codable, Equatable {}
         try _test(