Tagged templates can return any type, not just strings

Summary: Fixes #579
Closes #580

Reviewed By: @jeffmo

Differential Revision: D2198666

Pulled By: @mroch

src/typing/type_inference_js.ml

@@ -3289,13 +3289,14 @@ and expression_ ~is_cond cx loc e = Ast.Expression.(match e with
       let t = expression cx tag in
       let reason = mk_reason "encaps tag" loc in
       let reason_array = replace_reason "array" reason in
+      let ret = Flow_js.mk_tvar cx reason in
       let ft = Flow_js.mk_functiontype
         [ ArrT (reason_array, StrT.why reason, []);
           RestT (AnyT.why reason) ]
-        (StrT.why reason)
+        ret
       in
       Flow_js.flow cx (t, CallT (reason, ft));
-      StrT.at loc
+      ret
 
   | TemplateLiteral {
       TemplateLiteral.quasis;

tests/encaps/encaps.exp

@@ -3,16 +3,14 @@ encaps.js:3:14,16: property x
 Property not found in
 encaps.js:2:13,13: A
 
-encaps.js:9:10,22: encaps tag
-Error:
-encaps.js:7:12,12: number
-This type is incompatible with
-encaps.js:9:10,22: string
-
 encaps.js:9:10,22: encaps tag
 Error:
 encaps.js:9:10,22: string
 This type is incompatible with
 encaps.js:6:11,16: number
 
+encaps.js:16:2,7: string
+This type is incompatible with
+encaps.js:16:10,15: number
+
 Found 3 errors

tests/encaps/encaps.js

@@ -1,9 +1,16 @@
 class A { }
 var a = new A();
-var s1 = `l${a.x}r`;
+var s1 = `l${a.x}r`; // error: no prop x in A
 
 function tag(strings,...values) {
-    var x:number = strings[0];
+    var x:number = strings[0]; // error: string ~> number
     return x;
 }
 var s2 = tag `l${42}r`;
+
+function tag2(strings,...values) {
+  return { foo: "" }; // ok: tagged templates can return whatever
+}
+
+var s3 = tag2 `la la la`;
+(s3.foo: number); // error: string ~> number