Support import completion for merged declaration exports (#24539) (#24572)

Author: Andy

src/services/completions.ts

@@ -628,7 +628,7 @@ namespace ts.Completions {
         }
 
         const { moduleSymbol } = symbolOriginInfo;
-        const exportedSymbol = skipAlias(symbol.exportSymbol || symbol, checker);
+        const exportedSymbol = checker.getMergedSymbol(skipAlias(symbol.exportSymbol || symbol, checker));
         const { moduleSpecifier, codeAction } = codefix.getImportCompletionAction(
             exportedSymbol,
             moduleSymbol,

tests/cases/fourslash/completionsImport_default_fromMergedDeclarations.ts

@@ -0,0 +1,31 @@
+/// <reference path="fourslash.ts" />
+
+// @module: esnext
+
+// @Filename: /a.ts
+////declare module "m" {
+////    export default class M {}
+////}
+
+// @Filename: /b.ts
+////declare module "m" {
+////    export default interface M {}
+////}
+
+// @Filename: /c.ts
+/////**/
+
+goTo.marker("");
+verify.completionListContains({ name: "M", source: "m" }, "class M", "", "class", /*spanIndex*/ undefined, /*hasAction*/ true, {
+    includeCompletionsForModuleExports: true,
+    sourceDisplay: "m",
+});
+
+verify.applyCodeActionFromCompletion("", {
+    name: "M",
+    source: "m",
+    description: `Import 'M' from module "m"`,
+    newFileContent: `import M from "m";
+
+`,
+});