[js-api] Reject a mutable global import before allocating

document/js-api/index.bs

@@ -520,10 +520,12 @@ To <dfn>instantiate imported strings</dfn> with module |module| and |importedStr
                 1. Let |index| be the number of external functions in |imports|. This value |index| is known as the <dfn>index of the host function</dfn> |funcaddr|.
             1. Let |externfunc| be the [=external value=] [=external value/func=] |funcaddr|.
             1. [=list/Append=] |externfunc| to |imports|.
-        1. If |externtype| is of the form [=external-type/global=] <var ignore>mut</var> |valtype|,
+        1. If |externtype| is of the form [=external-type/global=] |mut| |valtype|,
             1. If |v| [=implements=] {{Global}},
                 1. Let |globaladdr| be |v|.\[[Global]].
             1. Otherwise,
+                1. If |mut| is [=mut=],
+                    1. Throw a {{LinkError}} exception.
                 1. If |valtype| is [=i64=] and |v| [=is not a BigInt=],
                     1. Throw a {{LinkError}} exception.
                 1. If |valtype| is one of [=i32=], [=f32=] or [=f64=] and |v| [=is not a Number=],
@@ -552,8 +554,9 @@ To <dfn>instantiate imported strings</dfn> with module |module| and |importedStr
             1. [=list/Append=] |externtag| to |imports|.
     1. Return |imports|.
 
-Note: This algorithm only verifies the right kind of JavaScript values are passed.
-The verification of WebAssembly type requirements is deferred to the
+Note: This algorithm verifies the right kind of JavaScript values are passed.
+A value that does not [=implement=] {{Global}} can only initialize an immutable global.
+The remaining WebAssembly type requirements are deferred to the
 "[=instantiate the core of a WebAssembly module=]" algorithm.
 </div>
 

test/js-api/bad-imports.js

@@ -123,6 +123,18 @@ function test_bad_imports(t) {
           "global": global,
         },
       });
+
+    const mutableGlobal = new WebAssembly.Global({ "value": type, "mutable": true }, value);
+    t(`Importing an ${type} global with a mutable Global object`,
+      WebAssembly.LinkError,
+      builder => {
+        builder.addImportedGlobal("module", "global", value_type(type));
+      },
+      {
+        "module": {
+          "global": mutableGlobal,
+        },
+      });
   }
 
   const nonMemories = [