Don't prevent going to next stage when a warning is elevated to an error (#2983)

fix #2972

.chronus/changes/fix-warn-as-error-keep-going-2024-2-5-16-55-37.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Warnings converted to error with `warn-as-error` do not prevent compilation from moving to the next stage like regular warnings

packages/compiler/src/core/program.ts

@@ -288,6 +288,7 @@ export async function compile(
   const loadedLibraries = new Map<string, TypeSpecLibraryReference>();
   const sourceFileLocationContexts = new WeakMap<SourceFile, LocationContext>();
   let error = false;
+  let continueToNextStage = true;
 
   const logger = createLogger({ sink: host.logSink });
   const tracer = createTracer(logger, { filter: options.trace });
@@ -383,7 +384,7 @@ export async function compile(
   program.checker = createChecker(program);
   program.checker.checkProgram();
 
-  if (program.hasError()) {
+  if (!continueToNextStage) {
     return program;
   }
   // onValidate stage
@@ -392,7 +393,7 @@ export async function compile(
   validateRequiredImports();
 
   await validateLoadedLibraries();
-  if (program.hasError()) {
+  if (!continueToNextStage) {
     return program;
   }
 
@@ -1049,6 +1050,10 @@ export async function compile(
       return;
     }
 
+    if (diagnostic.severity === "error") {
+      continueToNextStage = false;
+    }
+
     if (diagnostic.severity === "warning" && diagnostic.target !== NoTarget) {
       mutate(diagnostic).codefixes ??= [];
       mutate(diagnostic.codefixes).push(createSuppressCodeFix(diagnostic.target, diagnostic.code));