Just change the type

Author: weswigham

src/services/signatureHelp.ts

@@ -154,7 +154,7 @@ namespace ts.SignatureHelp {
             const tagExpression = <TaggedTemplateExpression>templateExpression.parent;
             Debug.assert(templateExpression.kind === SyntaxKind.TemplateExpression);
 
-            const argumentIndex = 1;
+            const argumentIndex = isInsideTemplateLiteral(node, position) ? 0 : 1;
 
             return getArgumentListInfoForTemplate(tagExpression, argumentIndex, sourceFile);
         }
@@ -165,7 +165,7 @@ namespace ts.SignatureHelp {
             Debug.assert(templateExpression.kind === SyntaxKind.TemplateExpression);
 
             // If we're just after a template tail, don't show signature help.
-            if (node.kind === SyntaxKind.TemplateTail) {
+            if (node.kind === SyntaxKind.TemplateTail && !isInsideTemplateLiteral(node, position)) {
                 return undefined;
             }
 

src/services/utilities.ts

@@ -955,7 +955,7 @@ namespace ts {
         return SyntaxKind.FirstPunctuation <= kind && kind <= SyntaxKind.LastPunctuation;
     }
 
-    export function isInsideTemplateLiteral(node: LiteralExpression, position: number) {
+    export function isInsideTemplateLiteral(node: LiteralLikeNode, position: number) {
         return isTemplateLiteralKind(node.kind)
             && (node.getStart() < position && position < node.getEnd()) || (!!node.isUnterminated && position === node.getEnd());
     }