fix(42166): Assertion signature not working on private identifier

src/compiler/checker.ts

@@ -21655,10 +21655,15 @@ namespace ts {
                         return getExplicitThisType(node);
                     case SyntaxKind.SuperKeyword:
                         return checkSuperExpression(node);
-                    case SyntaxKind.PropertyAccessExpression:
+                    case SyntaxKind.PropertyAccessExpression: {
                         const type = getTypeOfDottedName((<PropertyAccessExpression>node).expression, diagnostic);
-                        const prop = type && getPropertyOfType(type, (<PropertyAccessExpression>node).name.escapedText);
-                        return prop && getExplicitTypeOfSymbol(prop, diagnostic);
+                        if (type) {
+                            const name = (<PropertyAccessExpression>node).name;
+                            const prop = getPropertyOfType(type, isPrivateIdentifier(name) ? getSymbolNameForPrivateIdentifier(type.symbol, name.escapedText) : name.escapedText);
+                            return prop && getExplicitTypeOfSymbol(prop, diagnostic);
+                        }
+                        return undefined;
+                    }
                     case SyntaxKind.ParenthesizedExpression:
                         return getTypeOfDottedName((<ParenthesizedExpression>node).expression, diagnostic);
                 }

tests/baselines/reference/privateNamesAssertion.js

@@ -0,0 +1,30 @@
+//// [privateNamesAssertion.ts]
+class Foo {
+    #p1: (v: any) => asserts v is string = (v) => {
+        if (typeof v !== "string") {
+            throw new Error();
+        }
+    }
+    m1(v: unknown) {
+        this.#p1(v);
+        v;
+    }
+}
+
+
+//// [privateNamesAssertion.js]
+"use strict";
+class Foo {
+    constructor() {
+        this.#p1 = (v) => {
+            if (typeof v !== "string") {
+                throw new Error();
+            }
+        };
+    }
+    #p1;
+    m1(v) {
+        this.#p1(v);
+        v;
+    }
+}

tests/baselines/reference/privateNamesAssertion.symbols

@@ -0,0 +1,31 @@
+=== tests/cases/conformance/classes/members/privateNames/privateNamesAssertion.ts ===
+class Foo {
+>Foo : Symbol(Foo, Decl(privateNamesAssertion.ts, 0, 0))
+
+    #p1: (v: any) => asserts v is string = (v) => {
+>#p1 : Symbol(Foo.#p1, Decl(privateNamesAssertion.ts, 0, 11))
+>v : Symbol(v, Decl(privateNamesAssertion.ts, 1, 10))
+>v : Symbol(v, Decl(privateNamesAssertion.ts, 1, 10))
+>v : Symbol(v, Decl(privateNamesAssertion.ts, 1, 44))
+
+        if (typeof v !== "string") {
+>v : Symbol(v, Decl(privateNamesAssertion.ts, 1, 44))
+
+            throw new Error();
+>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+        }
+    }
+    m1(v: unknown) {
+>m1 : Symbol(Foo.m1, Decl(privateNamesAssertion.ts, 5, 5))
+>v : Symbol(v, Decl(privateNamesAssertion.ts, 6, 7))
+
+        this.#p1(v);
+>this.#p1 : Symbol(Foo.#p1, Decl(privateNamesAssertion.ts, 0, 11))
+>this : Symbol(Foo, Decl(privateNamesAssertion.ts, 0, 0))
+>v : Symbol(v, Decl(privateNamesAssertion.ts, 6, 7))
+
+        v;
+>v : Symbol(v, Decl(privateNamesAssertion.ts, 6, 7))
+    }
+}
+

tests/baselines/reference/privateNamesAssertion.types

@@ -0,0 +1,36 @@
+=== tests/cases/conformance/classes/members/privateNames/privateNamesAssertion.ts ===
+class Foo {
+>Foo : Foo
+
+    #p1: (v: any) => asserts v is string = (v) => {
+>#p1 : (v: any) => asserts v is string
+>v : any
+>(v) => {        if (typeof v !== "string") {            throw new Error();        }    } : (v: any) => void
+>v : any
+
+        if (typeof v !== "string") {
+>typeof v !== "string" : boolean
+>typeof v : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>v : any
+>"string" : "string"
+
+            throw new Error();
+>new Error() : Error
+>Error : ErrorConstructor
+        }
+    }
+    m1(v: unknown) {
+>m1 : (v: unknown) => void
+>v : unknown
+
+        this.#p1(v);
+>this.#p1(v) : void
+>this.#p1 : (v: any) => asserts v is string
+>this : this
+>v : unknown
+
+        v;
+>v : string
+    }
+}
+

tests/cases/conformance/classes/members/privateNames/privateNamesAssertion.ts

@@ -0,0 +1,14 @@
+// @strict: true
+// @target: esnext
+
+class Foo {
+    #p1: (v: any) => asserts v is string = (v) => {
+        if (typeof v !== "string") {
+            throw new Error();
+        }
+    }
+    m1(v: unknown) {
+        this.#p1(v);
+        v;
+    }
+}