diff --git a/devtools/code_health/metrics.py b/devtools/code_health/metrics.py
index f2d7a16ba5..81bed74623 100644
--- a/devtools/code_health/metrics.py
+++ b/devtools/code_health/metrics.py
@@ -14,10 +14,15 @@ class Metric:
     value: int
     limit: int
     line: int = 1
+    # What the base revision is matched on, if not `symbol`. Functions match on
+    # their qualified name without parameters: changing a parameter's type
+    # (std::function to MoveOnlyFunction, #90) must not turn an old long
+    # function into a new one held to the limit.
+    name: str = None
 
     @property
     def key(self):
-        return (self.path, self.symbol, self.check)
+        return (self.path, self.name if self.name is not None else self.symbol, self.check)
 
 
 class BraceDepth:
@@ -53,5 +58,5 @@ def measure(path, source):
         )
         for check, value, limit in values:
             result.append(Metric(path, function.long_name, check, value, limit,
-                                 function.start_line))
+                                 function.start_line, function.name))
     return result
diff --git a/devtools/code_health/tests/test_policy.py b/devtools/code_health/tests/test_policy.py
index e525e2f2af..147e8324bf 100644
--- a/devtools/code_health/tests/test_policy.py
+++ b/devtools/code_health/tests/test_policy.py
@@ -1,6 +1,6 @@
 import unittest
 
-from devtools.code_health.metrics import Metric
+from devtools.code_health.metrics import Metric, measure
 from devtools.code_health.policy import evaluate
 
 
@@ -22,6 +22,13 @@ def test_new_function_must_meet_limit(self):
     def test_unchanged_bad_function_does_not_block_other_changes(self):
         self.assertEqual([], evaluate([function(70), function(25, 'new()')], [function(70)]))
 
+    def test_changed_parameter_types_keep_the_base(self):
+        old = measure('Common/test.cpp', 'void A::f(std::function<void()> f){\n' + 'x();\n' * 60 + '}\n')
+        new = measure('Common/test.cpp', 'void A::f(MoveOnlyFunction<void()> f){\n' + 'x();\n' * 60 + '}\n')
+        self.assertEqual([], evaluate(new, old))
+        grown = measure('Common/test.cpp', 'void A::f(MoveOnlyFunction<void()> f){\n' + 'x();\n' * 61 + '}\n')
+        self.assertTrue(evaluate(grown, old))
+
     def test_growth_below_limit_is_allowed(self):
         self.assertEqual([], evaluate([function(50)], [function(25)]))