Allow element accesses to be called

This commit adds a case to compileCallExpression(), allowing element
accesses to be used in function calls.

This commit fixes #2525.

Author: CountBleck

src/compiler.ts

@@ -6214,6 +6214,37 @@ export class Compiler extends DiagnosticEmitter {
         // fall-through
       }
 
+      case ElementKind.INDEXSIGNATURE: {
+        const functionExpr = expression.expression;
+        const type = this.resolver.getTypeOfElement(target);
+        assert(functionExpr.kind == NodeKind.ELEMENTACCESS);
+
+        if (!type) {
+          this.error(
+            DiagnosticCode.Expression_cannot_be_represented_by_a_type,
+            functionExpr.range
+          );
+          return module.unreachable();
+        }
+
+        if (!type.signatureReference) {
+          this.error(
+            DiagnosticCode.Type_0_has_no_call_signatures,
+            functionExpr.range,
+            type.toString()
+          );
+          return module.unreachable();
+        }
+
+        signature = type.signatureReference;
+        functionArg = this.compileElementAccessExpression(
+          <ElementAccessExpression>functionExpr,
+          contextualType,
+          Constraints.CONV_IMPLICIT
+        );
+        break;
+      }
+
       // not supported
       default: {
         let type = this.resolver.getTypeOfElement(target);