Elide in/out variance modifiers from emitted JS (#4156)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Author: Copilot

internal/transformers/tstransforms/typeeraser.go

@@ -96,6 +96,16 @@ func (tx *TypeEraserTransformer) visit(node *ast.Node) *ast.Node {
 		ast.KindIndexSignature:
 		return nil
 
+	case ast.KindInKeyword, ast.KindOutKeyword:
+		// TypeScript `in`/`out` variance modifiers are elided. These keywords are only
+		// meaningful as modifiers on type parameters (which are themselves elided), but they may
+		// appear as a grammar error on other declarations and must not leak into the emitted JS.
+		// The `in` binary operator shares this token kind, so only elide when used as a modifier.
+		if tx.parentNode == nil || !ast.IsBinaryExpression(tx.parentNode) {
+			return nil
+		}
+		return tx.Visitor().VisitEachChild(node)
+
 	case ast.KindJSImportDeclaration:
 		// reparsed commonjs are elided
 		return nil

testdata/baselines/reference/compiler/varianceModifiersOnClassMembers.errors.txt

@@ -0,0 +1,21 @@
+varianceModifiersOnClassMembers.ts(4,3): error TS1274: 'in' modifier can only appear on a type parameter of a class, interface or type alias
+varianceModifiersOnClassMembers.ts(5,3): error TS1274: 'out' modifier can only appear on a type parameter of a class, interface or type alias
+
+
+==== varianceModifiersOnClassMembers.ts (2 errors) ====
+    // https://github.com/microsoft/typescript-go/issues/4123
+    
+    class C {
+      in x = 1;
+      ~~
+!!! error TS1274: 'in' modifier can only appear on a type parameter of a class, interface or type alias
+      out y = 2;
+      ~~~
+!!! error TS1274: 'out' modifier can only appear on a type parameter of a class, interface or type alias
+    }
+    
+    const isIn = "x" in { x: 1 };
+    for (const k in { x: 1 }) {
+      console.log(k);
+    }
+    

testdata/baselines/reference/compiler/varianceModifiersOnClassMembers.js

@@ -0,0 +1,27 @@
+//// [tests/cases/compiler/varianceModifiersOnClassMembers.ts] ////
+
+//// [varianceModifiersOnClassMembers.ts]
+// https://github.com/microsoft/typescript-go/issues/4123
+
+class C {
+  in x = 1;
+  out y = 2;
+}
+
+const isIn = "x" in { x: 1 };
+for (const k in { x: 1 }) {
+  console.log(k);
+}
+
+
+//// [varianceModifiersOnClassMembers.js]
+"use strict";
+// https://github.com/microsoft/typescript-go/issues/4123
+class C {
+    x = 1;
+    y = 2;
+}
+const isIn = "x" in { x: 1 };
+for (const k in { x: 1 }) {
+    console.log(k);
+}

testdata/baselines/reference/compiler/varianceModifiersOnClassMembers.symbols

@@ -0,0 +1,30 @@
+//// [tests/cases/compiler/varianceModifiersOnClassMembers.ts] ////
+
+=== varianceModifiersOnClassMembers.ts ===
+// https://github.com/microsoft/typescript-go/issues/4123
+
+class C {
+>C : Symbol(C, Decl(varianceModifiersOnClassMembers.ts, 0, 0))
+
+  in x = 1;
+>x : Symbol(C.x, Decl(varianceModifiersOnClassMembers.ts, 2, 9))
+
+  out y = 2;
+>y : Symbol(C.y, Decl(varianceModifiersOnClassMembers.ts, 3, 11))
+}
+
+const isIn = "x" in { x: 1 };
+>isIn : Symbol(isIn, Decl(varianceModifiersOnClassMembers.ts, 7, 5))
+>x : Symbol(x, Decl(varianceModifiersOnClassMembers.ts, 7, 21))
+
+for (const k in { x: 1 }) {
+>k : Symbol(k, Decl(varianceModifiersOnClassMembers.ts, 8, 10))
+>x : Symbol(x, Decl(varianceModifiersOnClassMembers.ts, 8, 17))
+
+  console.log(k);
+>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, --, --))
+>k : Symbol(k, Decl(varianceModifiersOnClassMembers.ts, 8, 10))
+}
+

testdata/baselines/reference/compiler/varianceModifiersOnClassMembers.types

@@ -0,0 +1,39 @@
+//// [tests/cases/compiler/varianceModifiersOnClassMembers.ts] ////
+
+=== varianceModifiersOnClassMembers.ts ===
+// https://github.com/microsoft/typescript-go/issues/4123
+
+class C {
+>C : C
+
+  in x = 1;
+>x : number
+>1 : 1
+
+  out y = 2;
+>y : number
+>2 : 2
+}
+
+const isIn = "x" in { x: 1 };
+>isIn : boolean
+>"x" in { x: 1 } : boolean
+>"x" : "x"
+>{ x: 1 } : { x: number; }
+>x : number
+>1 : 1
+
+for (const k in { x: 1 }) {
+>k : string
+>{ x: 1 } : { x: number; }
+>x : number
+>1 : 1
+
+  console.log(k);
+>console.log(k) : void
+>console.log : (...data: any[]) => void
+>console : Console
+>log : (...data: any[]) => void
+>k : string
+}
+

testdata/tests/cases/compiler/varianceModifiersOnClassMembers.ts

@@ -0,0 +1,13 @@
+// @target: esnext
+
+// https://github.com/microsoft/typescript-go/issues/4123
+
+class C {
+  in x = 1;
+  out y = 2;
+}
+
+const isIn = "x" in { x: 1 };
+for (const k in { x: 1 }) {
+  console.log(k);
+}