fix: crash in getWellKnownSymbolPropertyOfType for mapped typeof symb…

…ol (#698)

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## PR Checklist

- [x] Addresses an existing open issue: fixes
typescript-eslint/typescript-eslint#10747
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/ts-api-utils/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/ts-api-utils/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

As seen in
typescript-eslint/typescript-eslint#10747, we
can't assume that prop's declarations array can be found. If the type
being checked is a mapped type over the requested well-known symbol,
it'll come up as `undefined`.

💖

src/types/getters.test.ts

@@ -109,7 +109,7 @@ describe("getWellKnownSymbolPropertyOfType", () => {
 		const { sourceFile, typeChecker } = createSourceFileAndTypeChecker(`
 			declare const x: {
 				[Symbol.asyncIterator](): AsyncIterator<any>;
-			}>
+			};
 		`);
 
 		const node = (sourceFile.statements[0] as ts.VariableStatement)
@@ -143,4 +143,40 @@ describe("getWellKnownSymbolPropertyOfType", () => {
 			name: /^__@asyncIterator/,
 		});
 	});
+
+	it("returns undefined when the type maps over typeof the requested symbol", () => {
+		const { sourceFile, typeChecker } = createSourceFileAndTypeChecker(`
+			type MapsOverTypeofSymbolIterator = {
+				[K in typeof Symbol.iterator]: string;
+			};
+
+			declare const hoverStyles: MapsOverTypeofSymbolIterator;
+
+			const testObject = { ...hoverStyles };
+		`);
+
+		const node = (sourceFile.statements.at(-1) as ts.VariableStatement)
+			.declarationList.declarations[0];
+
+		const type = typeChecker.getTypeAtLocation(node);
+
+		expect(
+			getWellKnownSymbolPropertyOfType(type, "iterator", typeChecker),
+		).toBe(undefined);
+	});
+
+	it("returns undefined when given an error type", () => {
+		const { sourceFile, typeChecker } = createSourceFileAndTypeChecker(`
+			declare const x: invalid;
+		`);
+
+		const node = (sourceFile.statements[0] as ts.VariableStatement)
+			.declarationList.declarations[0].name;
+
+		const type = typeChecker.getTypeAtLocation(node);
+
+		expect(
+			getWellKnownSymbolPropertyOfType(type, "asyncIterator", typeChecker),
+		).toBe(undefined);
+	});
 });

src/types/getters.ts

@@ -97,8 +97,9 @@ export function getWellKnownSymbolPropertyOfType(
 			continue;
 		}
 
-		const declaration = prop.valueDeclaration ?? prop.getDeclarations()![0];
+		const declaration = prop.valueDeclaration ?? prop.getDeclarations()?.[0];
 		if (
+			!declaration ||
 			!isNamedDeclarationWithName(declaration) ||
 			declaration.name === undefined ||
 			!ts.isComputedPropertyName(declaration.name)