Move JS file diagnostic check to common file loading path

Moved the TS6504 diagnostic check from addRootTask to parseTask.load() so it catches all file inclusion paths (root files, triple-slash refs, type refs, etc.), not just root files. This matches TypeScript's behavior where getSourceFileFromReferenceWorker is called for all file types.

Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Author: Copilot

internal/compiler/fileloader.go

@@ -10,7 +10,6 @@ import (
 	"github.com/microsoft/typescript-go/internal/ast"
 	"github.com/microsoft/typescript-go/internal/collections"
 	"github.com/microsoft/typescript-go/internal/core"
-	"github.com/microsoft/typescript-go/internal/diagnostics"
 	"github.com/microsoft/typescript-go/internal/module"
 	"github.com/microsoft/typescript-go/internal/tsoptions"
 	"github.com/microsoft/typescript-go/internal/tspath"
@@ -260,6 +259,8 @@ func (p *fileLoader) toPath(file string) tspath.Path {
 
 func (p *fileLoader) addRootTask(fileName string, libFile *LibFile, includeReason *FileIncludeReason) {
 	absPath := tspath.GetNormalizedAbsolutePath(fileName, p.opts.Host.GetCurrentDirectory())
+	// Check if file should be included based on extension
+	// This mirrors the logic from getSourceFileFromReferenceWorker in TypeScript
 	if core.Tristate.IsTrue(p.opts.Config.CompilerOptions().AllowNonTsExtensions) || slices.Contains(p.supportedExtensions, tspath.TryGetExtensionFromPath(absPath)) {
 		p.rootTasks = append(p.rootTasks, &parseTask{
 			normalizedFilePath: absPath,
@@ -269,17 +270,14 @@ func (p *fileLoader) addRootTask(fileName string, libFile *LibFile, includeReaso
 	} else if tspath.HasExtension(absPath) {
 		// File has an extension but it's not in the supported extensions list
 		// Check if it's a JavaScript file and report the appropriate diagnostic
-		canonicalFileName := tspath.GetCanonicalFileName(absPath, p.opts.Host.FS().UseCaseSensitiveFileNames())
-		if tspath.HasJSFileExtension(canonicalFileName) {
-			p.includeProcessor.addProcessingDiagnostic(&processingDiagnostic{
-				kind: processingDiagnosticKindExplainingFileInclude,
-				data: &includeExplainingDiagnostic{
-					diagnosticReason: includeReason,
-					message:          diagnostics.File_0_is_a_JavaScript_file_Did_you_mean_to_enable_the_allowJs_option,
-					args:             []any{fileName},
-				},
-			})
-		}
+		// This will be caught later in parseTask.load() for consistency with other file inclusion paths
+		// But for root files, we need to add them to rootTasks so they can be processed
+		// The diagnostic will be reported in load() before attempting to parse
+		p.rootTasks = append(p.rootTasks, &parseTask{
+			normalizedFilePath: absPath,
+			libFile:            libFile,
+			includeReason:      includeReason,
+		})
 	}
 }
 

internal/compiler/filesparser.go

@@ -7,6 +7,7 @@ import (
 	"github.com/microsoft/typescript-go/internal/ast"
 	"github.com/microsoft/typescript-go/internal/collections"
 	"github.com/microsoft/typescript-go/internal/core"
+	"github.com/microsoft/typescript-go/internal/diagnostics"
 	"github.com/microsoft/typescript-go/internal/module"
 	"github.com/microsoft/typescript-go/internal/tsoptions"
 	"github.com/microsoft/typescript-go/internal/tspath"
@@ -68,6 +69,38 @@ func (t *parseTask) load(loader *fileLoader) {
 		return
 	}
 
+	// Check if file has unsupported extension (similar to getSourceFileFromReferenceWorker in TypeScript)
+	// Do this check before incrementing file counts
+	if tspath.HasExtension(t.normalizedFilePath) {
+		compilerOptions := loader.opts.Config.CompilerOptions()
+		allowNonTsExtensions := core.Tristate.IsTrue(compilerOptions.AllowNonTsExtensions)
+		if !allowNonTsExtensions {
+			canonicalFileName := tspath.GetCanonicalFileName(t.normalizedFilePath, loader.opts.Host.FS().UseCaseSensitiveFileNames())
+			supported := false
+			for _, ext := range loader.supportedExtensions {
+				if tspath.FileExtensionIs(canonicalFileName, ext) {
+					supported = true
+					break
+				}
+			}
+			if !supported {
+				// Report appropriate diagnostic for unsupported extension
+				if tspath.HasJSFileExtension(canonicalFileName) {
+					loader.includeProcessor.addProcessingDiagnostic(&processingDiagnostic{
+						kind: processingDiagnosticKindExplainingFileInclude,
+						data: &includeExplainingDiagnostic{
+							diagnosticReason: t.includeReason,
+							message:          diagnostics.File_0_is_a_JavaScript_file_Did_you_mean_to_enable_the_allowJs_option,
+							args:             []any{t.normalizedFilePath},
+						},
+					})
+				}
+				// File has unsupported extension, don't try to parse it
+				return
+			}
+		}
+	}
+
 	loader.totalFileCount.Add(1)
 	if t.libFile != nil {
 		loader.libFileCount.Add(1)