From bfc1cbe34d9a345d042d474e73861a112621b57e Mon Sep 17 00:00:00 2001
From: Titian Cernicova-Dragomir <dragomirtitian@yahoo.com>
Date: Sun, 3 Mar 2019 00:02:38 +0200
Subject: [PATCH] Modified type checking to correctly infer literal types for
 method name and parameter index.

---
 src/compiler/checker.ts | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 7e6cb97a9302e..42c917234ebff 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -20284,11 +20284,20 @@ namespace ts {
                     // A parameter declaration decorator will have three arguments (see
                     // `ParameterDecorator` in core.d.ts).
                     const func = <FunctionLikeDeclaration>parent.parent;
-                    return [
-                        createSyntheticExpression(expr, parent.parent.kind === SyntaxKind.Constructor ? getTypeOfSymbol(getSymbolOfNode(func)) : errorType),
-                        createSyntheticExpression(expr, anyType),
-                        createSyntheticExpression(expr, numberType)
-                    ];
+                    if (func.kind === SyntaxKind.Constructor) {
+                        return [
+                            createSyntheticExpression(expr, getTypeOfSymbol(getSymbolOfNode(func.parent))),
+                            createSyntheticExpression(expr, undefinedType),
+                            createSyntheticExpression(expr, getLiteralType(func.parameters.indexOf(<ParameterDeclaration>parent)))
+                        ];
+                    }
+                    else {
+                        return [
+                            createSyntheticExpression(expr, getParentTypeOfClassElement(<ClassElement>func)),
+                            createSyntheticExpression(expr, getClassElementPropertyKeyType(<ClassElement>func)),
+                            createSyntheticExpression(expr, getLiteralType(func.parameters.indexOf(<ParameterDeclaration>parent)))
+                        ];
+                    }
                 case SyntaxKind.PropertyDeclaration:
                 case SyntaxKind.MethodDeclaration:
                 case SyntaxKind.GetAccessor: