From c53ff42dc1bfacb7d770d5bf5d0a26d1ba2d5143 Mon Sep 17 00:00:00 2001
From: Trevor Dixon <trevordixon@google.com>
Date: Thu, 30 Jun 2022 15:26:49 +0200
Subject: [PATCH] Include type predicate information for functions and use that
 information when comparing.

---
 src/is-assignable/is-assignable-to-simple-type.ts | 11 +++++++++++
 src/simple-type.ts                                |  8 ++++++++
 src/transform/to-simple-type.ts                   | 10 +++++++++-
 test/type-combinations.spec.ts                    |  9 ++++++++-
 4 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/src/is-assignable/is-assignable-to-simple-type.ts b/src/is-assignable/is-assignable-to-simple-type.ts
index 61f9bfb..717243c 100644
--- a/src/is-assignable/is-assignable-to-simple-type.ts
+++ b/src/is-assignable/is-assignable-to-simple-type.ts
@@ -567,6 +567,17 @@ function isAssignableToSimpleTypeInternal(typeA: SimpleType, typeB: SimpleType,
 				}
 			}
 
+			// If typeA has a type predicate, make sure typeB's matches
+			if (typeA.typePredicate) {
+				if (!typeB.typePredicate) return false;
+				if (!isAssignableToSimpleTypeCached(typeA.typePredicate.type, typeB.typePredicate.type, options)) {
+					return false;
+				}
+				if (typeA.typePredicate.parameterIndex !== typeB.typePredicate.parameterIndex) {
+					return false;
+				}
+			}
+
 			// Test "this" types
 			const typeAThisParam = typeA.parameters.find(arg => arg.name === "this");
 			const typeBThisParam = typeB.parameters.find(arg => arg.name === "this");
diff --git a/src/simple-type.ts b/src/simple-type.ts
index 435aeb9..dc07aeb 100644
--- a/src/simple-type.ts
+++ b/src/simple-type.ts
@@ -206,11 +206,18 @@ export interface SimpleTypeFunctionParameter {
 	readonly initializer: boolean;
 }
 
+export interface SimpleTypeTypePredicate {
+	readonly paramaterName: string;
+	readonly paramaterIndex: number;
+	readonly type: SimpleType;
+}
+
 export interface SimpleTypeFunction extends SimpleTypeBase {
 	readonly kind: "FUNCTION";
 	readonly parameters?: SimpleTypeFunctionParameter[];
 	readonly typeParameters?: SimpleTypeGenericParameter[];
 	readonly returnType?: SimpleType;
+	readonly typePredicate?: SimpleTypeTypePredicate;
 }
 
 export interface SimpleTypeMethod extends SimpleTypeBase {
@@ -218,6 +225,7 @@ export interface SimpleTypeMethod extends SimpleTypeBase {
 	readonly parameters: SimpleTypeFunctionParameter[];
 	readonly typeParameters?: SimpleTypeGenericParameter[];
 	readonly returnType: SimpleType;
+	readonly typePredicate?: SimpleTypeTypePredicate;
 }
 
 // ##############################
diff --git a/src/transform/to-simple-type.ts b/src/transform/to-simple-type.ts
index 7a7d02e..e5f42bd 100644
--- a/src/transform/to-simple-type.ts
+++ b/src/transform/to-simple-type.ts
@@ -614,7 +614,15 @@ function getSimpleFunctionFromSignatureDeclaration(
 
 	const typeParameters = getTypeParameters(signatureDeclaration, options);
 
-	return { name, kind, returnType, parameters, typeParameters } as SimpleTypeFunction | SimpleTypeMethod;
+	const tsTypePredicate = checker.getTypePredicateOfSignature(signature);
+	const typePredicateType = tsTypePredicate && toSimpleTypeCached(tsTypePredicate.type, options);
+	const typePredicate = typePredicateType && {
+		parameterName: tsTypePredicate.parameterName,
+		parameterIndex: tsTypePredicate.parameterIndex,
+		type: typePredicateType
+	};
+
+	return { name, kind, returnType, parameters, typeParameters, typePredicate } as SimpleTypeFunction | SimpleTypeMethod;
 }
 
 function getRealSymbolName(symbol: ESSymbol, ts: typeof tsModule): string | undefined {
diff --git a/test/type-combinations.spec.ts b/test/type-combinations.spec.ts
index bc133aa..b712487 100644
--- a/test/type-combinations.spec.ts
+++ b/test/type-combinations.spec.ts
@@ -150,7 +150,14 @@ export const FUNCTION_TYPES: TypescriptType[] = [
 	`((a: number, b?: string) => string)`,
 	`((a: number, b: string) => string)`,
 	`(): string | null`,
-	`(): string | boolean | null | {a: string}`
+	`(): string | boolean | null | {a: string}`,
+	`(a: unknown) => boolean`,
+	`(a: string|number) => a is string`,
+	`(b: string|number) => b is number`,
+	`(a: unknown) => a is string|number`
+	// TODO: the following line exercises type predicate parameterIndex compatability,
+	// but fails the test for assignability with one of FUNCTION_REST_TYPES.
+	// `(a: string|number, b: string|number) => b is number`
 ];
 
 export const FUNCTION_THIS_TYPES: TypescriptType[] = [`(this: string, a: number) => any`, `(this: number, a: number) => any`, `(this: number) => any`];