Assert no bad cycles, instead of throwing

spec.html

@@ -21076,8 +21076,13 @@ <h1>Abstract Module Records</h1>
       <!-- es6num="15.2.1.16" -->
       <emu-clause id="sec-source-text-module-records">
         <h1>Source Text Module Records</h1>
+
         <p>A <dfn>Source Text Module Record</dfn> is used to represent information about a module that was defined from ECMAScript source text (10) that was parsed using the goal symbol |Module|. Its fields contain digested information about the names that are imported by the module and its concrete methods use this digest to link, instantiate, and evaluate the module.</p>
+
+        <p>A Source Text Module Record can exist in a module graph with other subclasses of the abstract Module Record type. However, non-source text Module Records must not participate in dependency cycles with Source Text Module Records. Concretely, this is enforced below by assertions that non-source text Module Record dependencies do not have a [[Status]] of `"instantiating"` or `"evaluating"` during a call to ModuleDeclarationInstantiation() or ModuleEvaluation() on a Source Text Module Record.</p>
+
         <p>In addition to the fields, defined in <emu-xref href="#table-36"></emu-xref>, Source Text Module Records have the additional fields listed in <emu-xref href="#table-38"></emu-xref>. Each of these fields initially has the value *undefined*.</p>
+
         <emu-table id="table-38" caption="Additional Fields of Source Text Module Records">
           <table>
             <tbody>
@@ -21701,8 +21706,8 @@ <h1>Runtime Semantics: InnerModuleDeclarationInstantiation( _module_, _stack_, _
             <p>The InnerModuleDeclarationInstantiation abstract operation performs the following steps:</p>
             <emu-alg>
               1. If _module_ is not a Source Text Module Record,
-                1. If _module_.[[Status]] is `"instantiating"`, throw a *TypeError* exception.
-                1. Otherwise, return _module_.ModuleDeclarationInstantiation().
+                1. Assert: _module_.[[Status]] is not `"instantiating"`.
+                1. Return _module_.ModuleDeclarationInstantiation().
               1. If _module_.[[Status]] is `"instantiating"`, `"instantiated"`, or `"evaluated"`,
                 1. Return *undefined*.
               1. If _module_.[[Status]] is `"errored"`,
@@ -21803,8 +21808,8 @@ <h1>Runtime Semantics: InnerModuleEvaluation( _module_, _stack_, _index_ )</h1>
             <p>The InnerModuleEvaluation abstract operation performs the following steps:</p>
             <emu-alg>
               1. If _module_ is not a Source Text Module Record,
-                1. If _module_.[[Status]] is `"evaluating"`, throw a *TypeError* exception.
-                1. Otherwise, return _module_.ModuleEvaluation().
+                1. Assert: _module_.[[Status]] is not `"evaluating"`.
+                1. Return _module_.ModuleEvaluation().
               1. If _module_.[[Status]] is `"evaluating"` or `"evaluated"`,
                 1. Return *undefined*.
               1. If _module_.[[Status]] is `"errored"`,