diff --git a/lib/SymbolGraphGen/SymbolGraphASTWalker.cpp b/lib/SymbolGraphGen/SymbolGraphASTWalker.cpp
index a48d7ddf715d7..98b2fad193240 100644
--- a/lib/SymbolGraphGen/SymbolGraphASTWalker.cpp
+++ b/lib/SymbolGraphGen/SymbolGraphASTWalker.cpp
@@ -13,6 +13,7 @@
 #include "llvm/ADT/StringSwitch.h"
 #include "swift/AST/Decl.h"
 #include "swift/AST/Module.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "swift/Serialization/SerializedModuleLoader.h"
 #include "swift/SymbolGraphGen/SymbolGraphGen.h"
 
@@ -181,79 +182,33 @@ bool SymbolGraphASTWalker::walkToDeclPre(Decl *D, CharSourceRange Range) {
       // We want to add conformsTo relationships for all protocols implicitly
       // implied by those explicitly stated on the extension.
       //
-      // Thus, we have to expand two syntactic constructs:
-      //  * `protocol A: B, C { ... }` declarations, where those that still have
-      //    to be expanded are stored in `UnexpandedProtocols`
-      //    that still have to be expanded
-      //  * `typealias A = B & C` declarations, which are directly expanded to
-      //    unexpanded protocols in `HandleProtocolOrComposition`
-      //
-      // The expansion adds the base protocol to `Protocols` and calls
-      // `HandleProtocolOrComposition` for the implied protocols. This process
-      // continues until there is nothing left to expand (`UnexpandedProtocols`
-      // is empty), because `HandleProtocolOrComposition` didn't add any new
-      // unexpanded protocols. At that point, all direct and indirect
-      // conformances are stored in `Protocols`.
-
-      SmallVector<const ProtocolDecl *, 4> Protocols;
+      // We start by collecting the conformances declared on the extension with
+      // `getLocalConformances`. From there, we inspect each protocol for any
+      // other protocols it inherits (whether stated explicitly or via a
+      // composed protocol type alias) with `getInheritedProtocols`. Each new
+      // protocol is added to `UnexpandedProtocols` until there are no new
+      // protocols to add. At that point, all direct and indirect conformances
+      // are stored in `Protocols`.
+
+      SmallPtrSet<const ProtocolDecl *, 4> Protocols;
       SmallVector<const ProtocolDecl *, 4> UnexpandedProtocols;
 
-      // Unwrap `UnexpandedCompositions` and add all unexpanded protocols to the
-      // `UnexpandedProtocols` list for expansion.
-      auto HandleProtocolOrComposition = [&](Type Ty) {
-        if (const auto *Proto =
-                dyn_cast_or_null<ProtocolDecl>(Ty->getAnyNominal())) {
-          UnexpandedProtocols.push_back(Proto);
-          return;
-        }
-
-        SmallVector<const ProtocolCompositionType *, 4> UnexpandedCompositions;
-
-        if (const auto *Comp = Ty->getAs<ProtocolCompositionType>()) {
-          UnexpandedCompositions.push_back(Comp);
-        } else {
-          llvm_unreachable("Expected ProtocolDecl or ProtocolCompositionType");
-        }
-
-        while (!UnexpandedCompositions.empty()) {
-          const auto *Comp = UnexpandedCompositions.pop_back_val();
-          for (const auto &Member : Comp->getMembers()) {
-            if (const auto *Proto =
-                    dyn_cast_or_null<ProtocolDecl>(Member->getAnyNominal())) {
-              Protocols.push_back(Proto);
-              UnexpandedProtocols.push_back(Proto);
-            } else if (const auto *Comp =
-                           Member->getAs<ProtocolCompositionType>()) {
-              UnexpandedCompositions.push_back(Comp);
-            } else {
-              abort();
-            }
-          }
-        }
-      };
-
       // Start the process with the conformances stated
       // explicitly on the extension.
-      for (const auto &InheritedLoc : Extension->getInherited()) {
-        auto InheritedTy = InheritedLoc.getType();
-        if (!InheritedTy) {
-          continue;
-        }
-        HandleProtocolOrComposition(InheritedTy);
+      for (const auto *Conformance : Extension->getLocalConformances()) {
+        UnexpandedProtocols.push_back(Conformance->getProtocol());
       }
 
       // "Recursively" expand the unexpanded list and populate
       // the expanded `Protocols` list (in an iterative manner).
       while (!UnexpandedProtocols.empty()) {
         const auto *Proto = UnexpandedProtocols.pop_back_val();
-        for (const auto &InheritedEntry : Proto->getInherited()) {
-          auto InheritedTy = InheritedEntry.getType();
-          if (!InheritedTy) {
-            continue;
+        if (!Protocols.contains(Proto)) {
+          for (const auto *InheritedProtocol : Proto->getInheritedProtocols()) {
+            UnexpandedProtocols.push_back(InheritedProtocol);
           }
-          HandleProtocolOrComposition(InheritedTy);
+          Protocols.insert(Proto);
         }
-        Protocols.push_back(Proto);
       }
 
       // Record the expanded list of protocols.
diff --git a/test/SymbolGraph/Symbols/ProtocolClassInheritance.swift b/test/SymbolGraph/Symbols/ProtocolClassInheritance.swift
new file mode 100644
index 0000000000000..2fbd0398b83da
--- /dev/null
+++ b/test/SymbolGraph/Symbols/ProtocolClassInheritance.swift
@@ -0,0 +1,42 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -module-name ProtocolClassInheritance -emit-module -emit-module-path %t/
+// RUN: %target-swift-symbolgraph-extract -module-name ProtocolClassInheritance -I %t -output-dir %t
+// RUN: %FileCheck %s --input-file %t/ProtocolClassInheritance.symbols.json
+
+// When a protocol that declares a class inheritance requirement is added by an extension, make sure
+// that SymbolGraphGen does not crash (rdar://109418762)
+
+public class ClassOne {}
+public protocol ProtoOne: ClassOne {}
+
+public class ClassTwo: ClassOne {}
+extension ClassTwo: ProtoOne {}
+
+// Same for a generic class inheritance requirement
+
+public class ClassThree<T> {}
+public protocol ProtoTwo: ClassThree<Int> {}
+
+public class ClassFour: ClassThree<Int> {}
+extension ClassFour: ProtoTwo {}
+
+// Same for a protocol with a primary associated type
+
+public protocol ProtoThree<T> {
+    associatedtype T
+}
+public protocol ProtoFour: ProtoThree<Int> {}
+
+public class ClassFive: ProtoThree {
+    public typealias T = Int
+}
+extension ClassFive: ProtoFour {}
+
+// ClassTwo conforms to ProtoOne
+// CHECK-DAG: {"kind":"conformsTo","source":"s:24ProtocolClassInheritance0B3TwoC","target":"s:24ProtocolClassInheritance8ProtoOneP"}
+
+// ClassFour conforms to ProtoTwo
+// CHECK-DAG: {"kind":"conformsTo","source":"s:24ProtocolClassInheritance0B4FourC","target":"s:24ProtocolClassInheritance8ProtoTwoP"}
+
+// ClassFive conforms to ProtoFour
+// CHECK-DAG: {"kind":"conformsTo","source":"s:24ProtocolClassInheritance0B4FiveC","target":"s:24ProtocolClassInheritance9ProtoFourP"}