JS: Treat d.ts as a single extension in Folder.getJavaScriptFile

javascript/ql/lib/semmle/javascript/Files.qll

@@ -175,6 +175,15 @@ class Folder extends Container, @folder {
     result.getExtension() = extension
   }
 
+  /** Like `getFile` except `d.ts` is treated as a single extension. */
+  private File getFileLongExtension(string stem, string extension) {
+    not (stem.matches("%.d") and extension = "ts") and
+    result = this.getFile(stem, extension)
+    or
+    extension = "d.ts" and
+    result = this.getFile(stem + ".d", "ts")
+  }
+
   /**
    * Gets the file in this folder that has the given `stem` and any of the supported JavaScript extensions.
    *
@@ -188,7 +197,11 @@ class Folder extends Container, @folder {
    */
   File getJavaScriptFile(string stem) {
     result =
-      min(int p, string ext | p = getFileExtensionPriority(ext) | this.getFile(stem, ext) order by p)
+      min(int p, string ext |
+        p = getFileExtensionPriority(ext)
+      |
+        this.getFileLongExtension(stem, ext) order by p
+      )
   }
 
   /** Gets a subfolder contained in this folder. */

javascript/ql/lib/semmle/javascript/NodeModuleResolutionImpl.qll

@@ -33,6 +33,8 @@ int getFileExtensionPriority(string ext) {
   ext = "json" and result = 8
   or
   ext = "node" and result = 9
+  or
+  ext = "d.ts" and result = 10
 }
 
 int prioritiesPerCandidate() { result = 3 * (numberOfExtensions() + 1) }

javascript/ql/test/library-tests/TypeScript/RegressionTests/ImportDtsFile/test.expected

@@ -1 +1,2 @@
+| main.ts:1:1:1:52 | import  ... -file"; | ./only-declaration-file | only-declaration-file.d.ts:3:1:4:0 | <toplevel> |
 | main.ts:2:1:2:48 | import  ... -file"; | ./has-javascript-file | has-javascript-file.js:1:1:2:0 | <toplevel> |