Add a regression test related to narrowing in plain JS files (#59611)

Author: Andarist

tests/baselines/reference/narrowingPlainJsNoCrash1.js

@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/narrowingPlainJsNoCrash1.ts] ////
+
+//// [index.js]
+// https://github.com/microsoft/TypeScript/issues/59594
+
+var a$b = {};
+var c, d;
+d = a$b;
+while (d !== a$b);
+while ((c = a$b != a$b)) c.e;
+
+
+//// [index.js]
+// https://github.com/microsoft/TypeScript/issues/59594
+var a$b = {};
+var c, d;
+d = a$b;
+while (d !== a$b)
+    ;
+while ((c = a$b != a$b))
+    c.e;

tests/baselines/reference/narrowingPlainJsNoCrash1.symbols

@@ -0,0 +1,26 @@
+//// [tests/cases/compiler/narrowingPlainJsNoCrash1.ts] ////
+
+=== index.js ===
+// https://github.com/microsoft/TypeScript/issues/59594
+
+var a$b = {};
+>a$b : Symbol(a$b, Decl(index.js, 2, 3))
+
+var c, d;
+>c : Symbol(c, Decl(index.js, 3, 3))
+>d : Symbol(d, Decl(index.js, 3, 6))
+
+d = a$b;
+>d : Symbol(d, Decl(index.js, 3, 6))
+>a$b : Symbol(a$b, Decl(index.js, 2, 3))
+
+while (d !== a$b);
+>d : Symbol(d, Decl(index.js, 3, 6))
+>a$b : Symbol(a$b, Decl(index.js, 2, 3))
+
+while ((c = a$b != a$b)) c.e;
+>c : Symbol(c, Decl(index.js, 3, 3))
+>a$b : Symbol(a$b, Decl(index.js, 2, 3))
+>a$b : Symbol(a$b, Decl(index.js, 2, 3))
+>c : Symbol(c, Decl(index.js, 3, 3))
+

tests/baselines/reference/narrowingPlainJsNoCrash1.types

@@ -0,0 +1,48 @@
+//// [tests/cases/compiler/narrowingPlainJsNoCrash1.ts] ////
+
+=== index.js ===
+// https://github.com/microsoft/TypeScript/issues/59594
+
+var a$b = {};
+>a$b : {}
+>    : ^^
+>{} : {}
+>   : ^^
+
+var c, d;
+>c : any
+>d : any
+
+d = a$b;
+>d = a$b : {}
+>        : ^^
+>d : any
+>a$b : {}
+>    : ^^
+
+while (d !== a$b);
+>d !== a$b : boolean
+>          : ^^^^^^^
+>d : {}
+>  : ^^
+>a$b : {}
+>    : ^^
+
+while ((c = a$b != a$b)) c.e;
+>(c = a$b != a$b) : boolean
+>                 : ^^^^^^^
+>c = a$b != a$b : boolean
+>               : ^^^^^^^
+>c : any
+>a$b != a$b : boolean
+>           : ^^^^^^^
+>a$b : {}
+>    : ^^
+>a$b : {}
+>    : ^^
+>c.e : error
+>c : true
+>  : ^^^^
+>e : any
+>  : ^^^
+

tests/cases/compiler/narrowingPlainJsNoCrash1.ts

@@ -0,0 +1,13 @@
+// @allowJs: true
+// @checkJs: false
+// @outDir: true
+
+// @filename: index.js
+
+// https://github.com/microsoft/TypeScript/issues/59594
+
+var a$b = {};
+var c, d;
+d = a$b;
+while (d !== a$b);
+while ((c = a$b != a$b)) c.e;