Prevent Infinite Loop in OverlappingFieldsCanBeMergedRule

src/validation/__tests__/OverlappingFieldsCanBeMergedRule-test.ts

@@ -1011,18 +1011,64 @@ describe('Validate: Overlapping fields can be merged', () => {
 
   it('does not infinite loop on recursive fragment', () => {
     expectValid(`
+      {
+        ...fragA
+      }
+
       fragment fragA on Human { name, relatives { name, ...fragA } }
     `);
   });
 
   it('does not infinite loop on immediately recursive fragment', () => {
     expectValid(`
+      {
+        ...fragA
+      }
+
       fragment fragA on Human { name, ...fragA }
     `);
   });
 
+  it('does not infinite loop on recursive fragment with a field named after fragment', () => {
+    expectValid(`
+      {
+        ...fragA
+        fragA
+      }
+
+      fragment fragA on Query { ...fragA }
+    `);
+  });
+
+  it('finds invalid cases even with field named after fragment', () => {
+    expectErrors(`
+      {
+        fragA
+        ...fragA
+      }
+
+      fragment fragA on Type {
+        fragA: b
+      }
+    `).toDeepEqual([
+      {
+        message:
+          'Fields "fragA" conflict because "fragA" and "b" are different fields. Use different aliases on the fields to fetch both if this was intentional.',
+        locations: [
+          { line: 3, column: 9 },
+          { line: 8, column: 9 },
+        ],
+      },
+    ]);
+  });
+
   it('does not infinite loop on transitively recursive fragment', () => {
     expectValid(`
+      {
+        ...fragA
+        fragB
+      }
+
       fragment fragA on Human { name, ...fragB }
       fragment fragB on Human { name, ...fragC }
       fragment fragC on Human { name, ...fragA }

src/validation/rules/OverlappingFieldsCanBeMergedRule.ts

@@ -267,6 +267,22 @@ function collectConflictsBetweenFieldsAndFragment(
   // (E) Then collect any conflicts between the provided collection of fields
   // and any fragment names found in the given fragment.
   for (const referencedFragmentName of referencedFragmentNames) {
+    // Memoize so two fragments are not compared for conflicts more than once.
+    if (
+      comparedFragmentPairs.has(
+        referencedFragmentName,
+        fragmentName,
+        areMutuallyExclusive,
+      )
+    ) {
+      continue;
+    }
+    comparedFragmentPairs.add(
+      referencedFragmentName,
+      fragmentName,
+      areMutuallyExclusive,
+    );
+
     collectConflictsBetweenFieldsAndFragment(
       context,
       conflicts,