From 813fa390cebdc3c9da8826c8118985d695cc11eb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Adam=20Zieli=C5=84ski?= <adam@adamziel.com>
Date: Tue, 7 Jun 2022 16:54:29 +0200
Subject: [PATCH 1/3] Add SelectorWithCustomCurrySignature, a utility type to
 provide a custom curry signature for selectors when TypeScript inference
 falls short.

---
 packages/data/src/types.ts | 65 +++++++++++++++++++++++++++++++++++---
 1 file changed, 60 insertions(+), 5 deletions(-)

diff --git a/packages/data/src/types.ts b/packages/data/src/types.ts
index a43f8e7f27250..ebd0dfa5c1b0d 100644
--- a/packages/data/src/types.ts
+++ b/packages/data/src/types.ts
@@ -76,18 +76,73 @@ type Store<
 	: never;
 
 /**
- * Removes the first argument from a function
+ * Removes the first argument from a function.
  *
- * This is designed to remove the `state` parameter from
+ * By default, it removes the `state` parameter from
  * registered selectors since that argument is supplied
  * by the editor when calling `select(…)`.
  *
  * For functions with no arguments, which some selectors
  * are free to define, returns the original function.
+ *
+ * It is possible to manually provide a custom curried signature
+ * and avoid the automatic inference. When the
+ * F generic argument passed to this helper extends the
+ * SelectorWithCustomCurrySignature type, the F['CurriedSignature']
+ * property is used verbatim.
+ *
+ * This is useful because TypeScript does not correctly remove
+ * arguments from complex function signatures constrained by
+ * interdependent generic parameters.
+ */
+type CurriedState< F > =
+	F extends SelectorWithCustomCurrySignature
+		? F['CurriedSignature']
+		: F extends ( state: any, ...args: infer P ) => infer R
+			? ( ...args: P ) => R
+			: F;
+
+/**
+ * Utility to manually specify curried selector signatures.
+ *
+ * It comes handy when TypeScript can't automatically produce the
+ * correct curried function signature. For example:
+ *
+ * ```ts
+ * type BadlyInferredSignature = CurriedState<
+ *     <K extends string | number>(
+ *         state: any,
+ *         kind: K,
+ *         key: K extends string ? 'one value' : false
+ *     ) => K
+ * >
+ * // BadlyInferredSignature evaluates to:
+ * // (kind: string number, key: false "one value") => string number
+ * ```
+ *
+ * With SelectorWithCustomCurrySignature, we can provide a custom
+ * signature and avoid relying on TypeScript inference:
+ * ```ts
+ * interface MySelectorSignature extends SelectorWithCustomCurrySignature {
+ *     <K extends string | number>(
+ *         state: any,
+ *         kind: K,
+ *         key: K extends string ? 'one value' : false
+ *     ): K;
+ *
+ *     CurriedSignature: <K extends string | number>(
+ *         kind: K,
+ *         key: K extends string ? 'one value' : false
+ *     ): K;
+ * }
+ * type CorrectlyInferredSignature = CurriedState<MySelectorSignature>
+ * // <K extends string | number>(kind: K, key: K extends string ? 'one value' : false): K;
+ * ```
  */
-type CurriedState< F > = F extends ( state: any, ...args: infer P ) => infer R
-	? ( ...args: P ) => R
-	: F;
+export interface SelectorWithCustomCurrySignature {
+	__isCurryContainer: true;
+	CurriedSignature: Function;
+}
 
 /**
  * Returns a function whose return type is a Promise of the given return type.

From d1c09248c951e2f73fb133d408d97d27c0b571cd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Adam=20Zieli=C5=84ski?= <adam@adamziel.com>
Date: Tue, 7 Jun 2022 17:22:44 +0200
Subject: [PATCH 2/3] Add the PR link to the docstrings

---
 packages/data/src/types.ts | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/packages/data/src/types.ts b/packages/data/src/types.ts
index ebd0dfa5c1b0d..64dedeafbb192 100644
--- a/packages/data/src/types.ts
+++ b/packages/data/src/types.ts
@@ -94,6 +94,7 @@ type Store<
  * This is useful because TypeScript does not correctly remove
  * arguments from complex function signatures constrained by
  * interdependent generic parameters.
+ * For more context, see https://github.com/WordPress/gutenberg/pull/41578
  */
 type CurriedState< F > =
 	F extends SelectorWithCustomCurrySignature
@@ -137,6 +138,8 @@ type CurriedState< F > =
  * }
  * type CorrectlyInferredSignature = CurriedState<MySelectorSignature>
  * // <K extends string | number>(kind: K, key: K extends string ? 'one value' : false): K;
+ *
+ * For even more context, see https://github.com/WordPress/gutenberg/pull/41578
  * ```
  */
 export interface SelectorWithCustomCurrySignature {

From a37d2a2af9dd0de9f03dfe65eb7c4004b581815a Mon Sep 17 00:00:00 2001
From: Adam Zielinski <adam@adamziel.com>
Date: Thu, 7 Jul 2022 14:00:03 +0200
Subject: [PATCH 3/3] Update packages/data/src/types.ts

---
 packages/data/src/types.ts | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/packages/data/src/types.ts b/packages/data/src/types.ts
index 64dedeafbb192..c8392f42a1707 100644
--- a/packages/data/src/types.ts
+++ b/packages/data/src/types.ts
@@ -96,13 +96,11 @@ type Store<
  * interdependent generic parameters.
  * For more context, see https://github.com/WordPress/gutenberg/pull/41578
  */
-type CurriedState< F > =
-	F extends SelectorWithCustomCurrySignature
-		? F['CurriedSignature']
-		: F extends ( state: any, ...args: infer P ) => infer R
-			? ( ...args: P ) => R
-			: F;
-
+type CurriedState< F > = F extends SelectorWithCustomCurrySignature
+	? F[ 'CurriedSignature' ]
+	: F extends ( state: any, ...args: infer P ) => infer R
+	? ( ...args: P ) => R
+	: F;
 /**
  * Utility to manually specify curried selector signatures.
  *