Improve symlink resolution in module specifier generation

Extends the symlink support in GetEachFileNameOfModule to properly
resolve module specifiers across symlinked packages and workspaces.

Key changes:
- Move knownsymlinks from compiler to dedicated symlinks package
- Implement active resolution via ResolveModuleName to populate cache
- Add dependency resolution from package.json to detect symlinks early
- Improve ignored path handling (node_modules/., .git, .# emacs locks)
- Add comprehensive test coverage for symlink resolution
- Fix declaration emit to prefer original paths over symlink paths

This aligns with upstream TypeScript's symlink resolution behavior,
ensuring correct module specifiers in declaration files for monorepos
and symlinked dependencies.

Fixes baseline mismatches in:
- declarationEmitReexportedSymlinkReference2/3
- symlinkedWorkspaceDependencies* tests
- nodeModuleReexportFromDottedPath

Author: shinichy

internal/checker/nodebuilderimpl.go

@@ -1215,6 +1215,10 @@ func (b *nodeBuilderImpl) getSpecifierForModuleSymbol(symbol *ast.Symbol, overri
 		},
 		false, /*forAutoImports*/
 	)
+	if len(allSpecifiers) == 0 {
+		links.specifierCache[cacheKey] = ""
+		return ""
+	}
 	specifier := allSpecifiers[0]
 	links.specifierCache[cacheKey] = specifier
 	return specifier

internal/compiler/emitHost.go

@@ -9,6 +9,7 @@ import (
 	"github.com/microsoft/typescript-go/internal/modulespecifiers"
 	"github.com/microsoft/typescript-go/internal/outputpaths"
 	"github.com/microsoft/typescript-go/internal/printer"
+	"github.com/microsoft/typescript-go/internal/symlinks"
 	"github.com/microsoft/typescript-go/internal/transformers/declarations"
 	"github.com/microsoft/typescript-go/internal/tsoptions"
 	"github.com/microsoft/typescript-go/internal/tspath"
@@ -126,3 +127,12 @@ func (host *emitHost) GetEmitResolver() printer.EmitResolver {
 func (host *emitHost) IsSourceFileFromExternalLibrary(file *ast.SourceFile) bool {
 	return host.program.IsSourceFileFromExternalLibrary(file)
 }
+
+func (host *emitHost) GetSymlinkCache() *symlinks.KnownSymlinks {
+	return host.program.GetSymlinkCache()
+}
+
+func (host *emitHost) ResolveModuleName(moduleName string, containingFile string, resolutionMode core.ResolutionMode) *module.ResolvedModule {
+	resolved, _ := host.program.resolver.ResolveModuleName(moduleName, containingFile, resolutionMode, nil)
+	return resolved
+}

internal/compiler/knownsymlinks.go

@@ -23,6 +23,7 @@ import (
 	"github.com/microsoft/typescript-go/internal/printer"
 	"github.com/microsoft/typescript-go/internal/scanner"
 	"github.com/microsoft/typescript-go/internal/sourcemap"
+	"github.com/microsoft/typescript-go/internal/symlinks"
 	"github.com/microsoft/typescript-go/internal/tsoptions"
 	"github.com/microsoft/typescript-go/internal/tspath"
 )
@@ -66,6 +67,7 @@ type Program struct {
 	// Cached unresolved imports for ATA
 	unresolvedImportsOnce sync.Once
 	unresolvedImports     *collections.Set[string]
+	knownSymlinks         *symlinks.KnownSymlinks
 }
 
 // FileExists implements checker.Program.
@@ -210,6 +212,10 @@ func NewProgram(opts ProgramOptions) *Program {
 	p.initCheckerPool()
 	p.processedFiles = processAllProgramFiles(p.opts, p.SingleThreaded())
 	p.verifyCompilerOptions()
+	p.knownSymlinks = symlinks.NewKnownSymlink(p.GetCurrentDirectory(), p.UseCaseSensitiveFileNames())
+	if len(p.resolvedModules) > 0 || len(p.typeResolutionsInFile) > 0 {
+		p.knownSymlinks.SetSymlinksFromResolutions(p.ForEachResolvedModule, p.ForEachResolvedTypeReferenceDirective)
+	}
 	return p
 }
 
@@ -240,6 +246,10 @@ func (p *Program) UpdateProgram(changedFilePath tspath.Path, newHost CompilerHos
 	result.filesByPath = maps.Clone(result.filesByPath)
 	result.filesByPath[newFile.Path()] = newFile
 	updateFileIncludeProcessor(result)
+	result.knownSymlinks = symlinks.NewKnownSymlink(result.GetCurrentDirectory(), result.UseCaseSensitiveFileNames())
+	if len(result.resolvedModules) > 0 || len(result.typeResolutionsInFile) > 0 {
+		result.knownSymlinks.SetSymlinksFromResolutions(result.ForEachResolvedModule, result.ForEachResolvedTypeReferenceDirective)
+	}
 	return result, true
 }
 
@@ -1630,6 +1640,45 @@ func (p *Program) SourceFileMayBeEmitted(sourceFile *ast.SourceFile, forceDtsEmi
 	return sourceFileMayBeEmitted(sourceFile, p, forceDtsEmit)
 }
 
+func (p *Program) GetSymlinkCache() *symlinks.KnownSymlinks {
+	// if p.Host().GetSymlinkCache() != nil {
+	// 	return p.Host().GetSymlinkCache()
+	// }
+	if p.knownSymlinks == nil {
+		p.knownSymlinks = symlinks.NewKnownSymlink(p.GetCurrentDirectory(), p.UseCaseSensitiveFileNames())
+	}
+	return p.knownSymlinks
+}
+
+func (p *Program) ResolveModuleName(moduleName string, containingFile string, resolutionMode core.ResolutionMode) *module.ResolvedModule {
+	resolved, _ := p.resolver.ResolveModuleName(moduleName, containingFile, resolutionMode, nil)
+	return resolved
+}
+
+func (p *Program) ForEachResolvedModule(callback func(resolution *module.ResolvedModule, moduleName string, mode core.ResolutionMode, filePath tspath.Path), file *ast.SourceFile) {
+	forEachResolution(p.resolvedModules, callback, file)
+}
+
+func (p *Program) ForEachResolvedTypeReferenceDirective(callback func(resolution *module.ResolvedTypeReferenceDirective, moduleName string, mode core.ResolutionMode, filePath tspath.Path), file *ast.SourceFile) {
+	forEachResolution(p.typeResolutionsInFile, callback, file)
+}
+
+func forEachResolution[T any](resolutionCache map[tspath.Path]module.ModeAwareCache[T], callback func(resolution T, moduleName string, mode core.ResolutionMode, filePath tspath.Path), file *ast.SourceFile) {
+	if file != nil {
+		if resolutions, ok := resolutionCache[file.Path()]; ok {
+			for key, resolution := range resolutions {
+				callback(resolution, key.Name, key.Mode, file.Path())
+			}
+		}
+	} else {
+		for filePath, resolutions := range resolutionCache {
+			for key, resolution := range resolutions {
+				callback(resolution, key.Name, key.Mode, filePath)
+			}
+		}
+	}
+}
+
 var plainJSErrors = collections.NewSetFromItems(
 	// binder errors
 	diagnostics.Cannot_redeclare_block_scoped_variable_0.Code(),

internal/compiler/program.go

@@ -7,6 +7,7 @@ import (
 	"github.com/microsoft/typescript-go/internal/collections"
 	"github.com/microsoft/typescript-go/internal/core"
 	"github.com/microsoft/typescript-go/internal/module"
+	"github.com/microsoft/typescript-go/internal/symlinks"
 	"github.com/microsoft/typescript-go/internal/tspath"
 	"github.com/microsoft/typescript-go/internal/vfs"
 	"github.com/microsoft/typescript-go/internal/vfs/cachedvfs"
@@ -26,7 +27,7 @@ func newProjectReferenceDtsFakingHost(loader *fileLoader) module.ResolutionHost
 		fs: cachedvfs.From(&projectReferenceDtsFakingVfs{
 			projectReferenceFileMapper: loader.projectReferenceFileMapper,
 			dtsDirectories:             loader.dtsDirectories,
-			knownSymlinks:              knownSymlinks{},
+			knownSymlinks:              symlinks.KnownSymlinks{},
 		}),
 	}
 	return host
@@ -45,7 +46,7 @@ func (h *projectReferenceDtsFakingHost) GetCurrentDirectory() string {
 type projectReferenceDtsFakingVfs struct {
 	projectReferenceFileMapper *projectReferenceFileMapper
 	dtsDirectories             collections.Set[tspath.Path]
-	knownSymlinks              knownSymlinks
+	knownSymlinks              symlinks.KnownSymlinks
 }
 
 var _ vfs.FS = (*projectReferenceDtsFakingVfs)(nil)
@@ -150,7 +151,7 @@ func (fs *projectReferenceDtsFakingVfs) handleDirectoryCouldBeSymlink(directory
 		// not symlinked
 		return
 	}
-	fs.knownSymlinks.SetDirectory(directory, directoryPath, &knownDirectoryLink{
+	fs.knownSymlinks.SetDirectory(directory, directoryPath, &symlinks.KnownDirectoryLink{
 		Real:     tspath.EnsureTrailingDirectorySeparator(realDirectory),
 		RealPath: realPath,
 	})
@@ -181,7 +182,7 @@ func (fs *projectReferenceDtsFakingVfs) fileOrDirectoryExistsUsingSource(fileOrD
 
 	// If it contains node_modules check if its one of the symlinked path we know of
 	var exists bool
-	knownDirectoryLinks.Range(func(directoryPath tspath.Path, knownDirectoryLink *knownDirectoryLink) bool {
+	knownDirectoryLinks.Range(func(directoryPath tspath.Path, knownDirectoryLink *symlinks.KnownDirectoryLink) bool {
 		relative, hasPrefix := strings.CutPrefix(string(fileOrDirectoryPath), string(directoryPath))
 		if !hasPrefix {
 			return true

internal/compiler/projectreferencedtsfakinghost.go

@@ -164,7 +164,8 @@ func (e *exportInfoMap) add(
 	}
 
 	moduleName := stringutil.StripQuotes(moduleSymbol.Name)
-	id := e.exportInfoId + 1
+	id := e.exportInfoId
+	e.exportInfoId += 1
 	target := ch.SkipAlias(symbol)
 
 	if flagMatch != nil && !flagMatch(target.Flags) {