C++: Add class representing PCH files

cpp/downgrades/c16b29b27f71247023321cc0d0360998b318837c/upgrade.properties

@@ -0,0 +1,4 @@
+description: Link PCH creations and uses
+compatibility: full
+pch_uses.rel: delete
+pch_creations.rel: delete

cpp/ql/lib/change-notes/2025-08-27-pch.md

@@ -0,0 +1,5 @@
+---
+category: feature
+---
+* Added a new class `PchFile` representing precompiled header (PCH) files used during project compilation.
+

cpp/ql/lib/cpp.qll

@@ -15,6 +15,7 @@
 
 import Customizations
 import semmle.code.cpp.File
+import semmle.code.cpp.PchFile
 import semmle.code.cpp.Linkage
 import semmle.code.cpp.Location
 import semmle.code.cpp.Compilation

cpp/ql/lib/semmle/code/cpp/PchFile.qll

@@ -0,0 +1,26 @@
+/**
+ * Provides the `PchFile` class representing precompiled header (PCH) files created and
+ * used during the build process.
+ */
+
+import semmle.code.cpp.File
+
+/**
+ * A precompiled header (PCH) file created during the build process.
+ */
+class PchFile extends @pch {
+  /**
+   *  Gets a textual representation of this element.
+   */
+  string toString() { result = "PCH for " + this.getHeaderFile() }
+
+  /**
+   * Gets the header file from which the PCH file was created.
+   */
+  File getHeaderFile() { pch_creations(this, _, result) }
+
+  /**
+   * Gets a source file that includes the PCH.
+   */
+  File getAUse() { pch_uses(this, _, result) }
+}

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -222,6 +222,19 @@ extractor_version(
     string frontend_version: string ref
 )
 
+pch_uses(
+    int pch: @pch ref,
+    int compilation: @compilation ref,
+    int id: @file ref
+)
+
+#keyset[pch, compilation]
+pch_creations(
+    int pch: @pch,
+    int compilation: @compilation ref,
+    int from: @file ref
+)
+
 /** An element for which line-count information is available. */
 @sourceline = @file | @function | @variable | @enumconstant | @xmllocatable;