Elide parens around assertions during emit (microsoft#441)

Author: rbuckton

internal/transformers/typeeraser.go

@@ -199,6 +199,15 @@ func (tx *TypeEraserTransformer) visit(node *ast.Node) *ast.Node {
 		// !!! Use PartiallyEmittedExpression to preserve comments
 		return tx.visitor.VisitNode(node.AsSatisfiesExpression().Expression)
 
+	case ast.KindParenthesizedExpression:
+		n := node.AsParenthesizedExpression()
+		expression := ast.SkipOuterExpressions(n.Expression, ^(ast.OEKTypeAssertions | ast.OEKExpressionsWithTypeArguments))
+		if ast.IsAssertionExpression(expression) || ast.IsSatisfiesExpression(expression) {
+			// !!! Use PartiallyEmittedExpression to preserve comments
+			return tx.visitor.VisitNode(n.Expression)
+		}
+		return tx.visitor.VisitEachChild(node)
+
 	case ast.KindJsxSelfClosingElement:
 		n := node.AsJsxSelfClosingElement()
 		return tx.factory.UpdateJsxSelfClosingElement(n, tx.visitor.VisitNode(n.TagName), nil, tx.visitor.VisitNode(n.Attributes))

internal/transformers/typeeraser_test.go

@@ -58,9 +58,12 @@ func TestTypeEraser(t *testing.T) {
 		{title: "NewExpression2", input: "new f<T>", output: "new f;"},
 		{title: "TaggedTemplateExpression", input: "f<T>``", output: "f ``;"},
 		{title: "NonNullExpression", input: "x!", output: "x;"},
-		{title: "TypeAssertionExpression", input: "<T>x", output: "x;"},
-		{title: "AsExpression", input: "x as T", output: "x;"},
-		{title: "SatisfiesExpression", input: "x satisfies T", output: "x;"},
+		{title: "TypeAssertionExpression#1", input: "<T>x", output: "x;"},
+		{title: "TypeAssertionExpression#2", input: "(<T>x).c", output: "x.c;"},
+		{title: "AsExpression#1", input: "x as T", output: "x;"},
+		{title: "AsExpression#2", input: "(x as T).c", output: "x.c;"},
+		{title: "SatisfiesExpression#1", input: "x satisfies T", output: "x;"},
+		{title: "SatisfiesExpression#2", input: "(x satisfies T).c", output: "x.c;"},
 		{title: "JsxSelfClosingElement", input: "<x<T> />", output: "<x />;", jsx: true},
 		{title: "JsxOpeningElement", input: "<x<T>></x>", output: "<x></x>;", jsx: true},
 		{title: "ImportEqualsDeclaration#1", input: "import x = require(\"m\");", output: "import x = require(\"m\");"},