Add regression test

Author: ahejlsberg

tests/baselines/reference/voidUndefinedReduction.js

@@ -0,0 +1,23 @@
+//// [voidUndefinedReduction.ts]
+// Repro from #42786
+
+function isDefined<T>(value: T | undefined | null | void): value is T {
+  return value !== undefined && value !== null;
+}
+
+declare const foo: string | undefined;
+
+if (isDefined(foo)) {
+  console.log(foo.toUpperCase()); 
+}
+
+
+//// [voidUndefinedReduction.js]
+"use strict";
+// Repro from #42786
+function isDefined(value) {
+    return value !== undefined && value !== null;
+}
+if (isDefined(foo)) {
+    console.log(foo.toUpperCase());
+}

tests/baselines/reference/voidUndefinedReduction.symbols

@@ -0,0 +1,33 @@
+=== tests/cases/compiler/voidUndefinedReduction.ts ===
+// Repro from #42786
+
+function isDefined<T>(value: T | undefined | null | void): value is T {
+>isDefined : Symbol(isDefined, Decl(voidUndefinedReduction.ts, 0, 0))
+>T : Symbol(T, Decl(voidUndefinedReduction.ts, 2, 19))
+>value : Symbol(value, Decl(voidUndefinedReduction.ts, 2, 22))
+>T : Symbol(T, Decl(voidUndefinedReduction.ts, 2, 19))
+>value : Symbol(value, Decl(voidUndefinedReduction.ts, 2, 22))
+>T : Symbol(T, Decl(voidUndefinedReduction.ts, 2, 19))
+
+  return value !== undefined && value !== null;
+>value : Symbol(value, Decl(voidUndefinedReduction.ts, 2, 22))
+>undefined : Symbol(undefined)
+>value : Symbol(value, Decl(voidUndefinedReduction.ts, 2, 22))
+}
+
+declare const foo: string | undefined;
+>foo : Symbol(foo, Decl(voidUndefinedReduction.ts, 6, 13))
+
+if (isDefined(foo)) {
+>isDefined : Symbol(isDefined, Decl(voidUndefinedReduction.ts, 0, 0))
+>foo : Symbol(foo, Decl(voidUndefinedReduction.ts, 6, 13))
+
+  console.log(foo.toUpperCase()); 
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>foo.toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+>foo : Symbol(foo, Decl(voidUndefinedReduction.ts, 6, 13))
+>toUpperCase : Symbol(String.toUpperCase, Decl(lib.es5.d.ts, --, --))
+}
+

tests/baselines/reference/voidUndefinedReduction.types

@@ -0,0 +1,37 @@
+=== tests/cases/compiler/voidUndefinedReduction.ts ===
+// Repro from #42786
+
+function isDefined<T>(value: T | undefined | null | void): value is T {
+>isDefined : <T>(value: T | undefined | null | void) => value is T
+>value : void | T | null | undefined
+>null : null
+
+  return value !== undefined && value !== null;
+>value !== undefined && value !== null : boolean
+>value !== undefined : boolean
+>value : void | T | null | undefined
+>undefined : undefined
+>value !== null : boolean
+>value : T | null
+>null : null
+}
+
+declare const foo: string | undefined;
+>foo : string | undefined
+
+if (isDefined(foo)) {
+>isDefined(foo) : boolean
+>isDefined : <T>(value: void | T | null | undefined) => value is T
+>foo : string | undefined
+
+  console.log(foo.toUpperCase()); 
+>console.log(foo.toUpperCase()) : void
+>console.log : (...data: any[]) => void
+>console : Console
+>log : (...data: any[]) => void
+>foo.toUpperCase() : string
+>foo.toUpperCase : () => string
+>foo : string
+>toUpperCase : () => string
+}
+

tests/cases/compiler/voidUndefinedReduction.ts

@@ -0,0 +1,13 @@
+// @strict: true
+
+// Repro from #42786
+
+function isDefined<T>(value: T | undefined | null | void): value is T {
+  return value !== undefined && value !== null;
+}
+
+declare const foo: string | undefined;
+
+if (isDefined(foo)) {
+  console.log(foo.toUpperCase()); 
+}