fix(generator): emit valid default declarations

Author: chrisbbreuer

packages/dtsx/src/extractor/scanner.ts

@@ -1930,7 +1930,8 @@ function scanDeclarationsInternal(_source: string, _filename: string, _keepComme
     const exportPrefix = isExported
       ? (isDefault ? 'export default function ' : 'export declare function ')
       : 'declare function '
-    const text = `${exportPrefix}${name || 'default'}${generics}${dtsParams}: ${returnType};`
+    const declarationName = name || (isDefault ? '' : 'default')
+    const text = `${exportPrefix}${declarationName}${generics}${dtsParams}: ${returnType};`
     const comments = extractLeadingComments(declStart)
 
     // Record index if this function had a body (implementation signature for overloads)
@@ -3278,7 +3279,8 @@ function scanDeclarationsInternal(_source: string, _filename: string, _keepComme
     let helperName = '__dtsx_default_export__'
     let suffix = 1
     while (source.includes(helperName)) helperName = `__dtsx_default_export_${suffix++}__`
-    const inferredType = inferNarrowType(expression, false)
+    const assertedType = extractAssertion(expression)
+    const inferredType = assertedType || inferNarrowType(expression, false)
     const type = inferredType === 'unknown' && isEntityNameText(expression)
       ? `typeof ${expression}`
       : inferredType

packages/dtsx/test/generation-paths.test.ts

@@ -273,6 +273,29 @@ describe('declaration generation paths', () => {
     expect(result).not.toContain('export default {')
   })
 
+  it('uses the contract from asserted default-export expressions', () => {
+    const source = `
+      type ColorSpecifier = string | null
+      declare function hcl(): unknown
+      export default hcl() as (start: ColorSpecifier, end: ColorSpecifier) => (t: number) => string
+    `
+
+    const result = processSourceIsolated(source, 'asserted-default.ts', false)
+
+    expect(result).toContain('declare const __dtsx_default_export__: (start: ColorSpecifier, end: ColorSpecifier) => (t: number) => string;')
+    expect(result).toContain('export default __dtsx_default_export__;')
+    expect(result).not.toContain('hcl() as')
+  })
+
+  it('emits valid anonymous default function declarations', () => {
+    const source = 'export default function (name: string): string { return name }'
+
+    const result = processSourceIsolated(source, 'anonymous-default.ts', false)
+
+    expect(result).toContain('export default function (name: string): string;')
+    expect(result).not.toContain('function default')
+  })
+
   it('keeps result caches separate across import orders', () => {
     clearResultCache()
     const source = `