[Bailout] When Inputs Detected In DepGraph Cycle

Inputs occured in depGraph cycle is dangenrous and should be treated as an 
invariant since Forget _may_ generate broken code in this case, despite that 
technically this is a stricter then what we needed for the particular case of 
#633 and #634 and there   could be case that this is safe (like many `cfg-`   
tests that I have to mark as `bailout.`) 

I expect the next diff will fix them though. 

commit-id:0b13ed02

compiler/forget/src/Diagnostic.ts

@@ -126,6 +126,7 @@ export type ErrorCode =
   | ParseCode<"E0018">
   | ParseCode<"E0019">
   | ParseCode<"E0020">
+  | ParseCode<"E0021">
   ;
 
 // prettier-ignore
@@ -168,6 +169,7 @@ export type DiagnosticOpts =
   | { code: "E0018"; path: NodePath; context: null }
   | { code: "E0019"; path: NodePath; context: null }
   | { code: "E0020"; path: NodePath; context: null }
+  | { code: "E0021"; path: NodePath; context: { name: Name } }
   ;
 
 function getDiagnosticMessage(opts: DiagnosticOpts): DiagnosticMessage {
@@ -320,6 +322,12 @@ ${getSource(opts.path)}
         body: "Forget does not support the use of the arguments object.",
         suggestion: null,
       };
+    case "E0021":
+      return {
+        level: DiagnosticLevel.Error,
+        body: `Input "${opts.context.name}" is detected in a DepGraph cycle. This is dangenrous since Forget may (though not always) generate broken code like accessing "c_input" before it is defined.`,
+        suggestion: null,
+      };
     default:
       assertExhaustive(opts, `Unhandled error code ${opts}`);
   }

compiler/forget/src/MiddleEnd/DepGraphAnalysis.ts

@@ -47,6 +47,7 @@ function run(
   }
 
   const sccGraph = valGraph.condense();
+  checkSCCGraph(sccGraph, context);
 
   if (outputKinds.includes(OutputKind.SCCGraph)) {
     context.outputs[OutputKind.SCCGraph].push(sccGraph.snapshot());
@@ -184,6 +185,24 @@ function checkValGraph(valGraph: DepGraph.ValGraph) {
   }
 }
 
+function checkSCCGraph(sccGraph: DepGraph.SCCGraph, context: CompilerContext) {
+  for (const scc of sccGraph.vertices) {
+    if (scc.isInput()) continue;
+
+    for (const valVertex of scc.members) {
+      if (IR.isInputVal(valVertex.val)) {
+        context.bailout("InputInDepGraphCycle", {
+          code: "E0021",
+          path: valVertex.val.ast.path,
+          context: {
+            name: valVertex.val.binding.identifier.name,
+          },
+        });
+      }
+    }
+  }
+}
+
 export function computeCfgControlDeps(
   irFunc: IR.Func,
   cfg: CFG