Index: clang-tidy/readability/ElseAfterReturnCheck.cpp
===================================================================
--- clang-tidy/readability/ElseAfterReturnCheck.cpp
+++ clang-tidy/readability/ElseAfterReturnCheck.cpp
@@ -25,7 +25,8 @@
                  expr(ignoringImplicit(cxxThrowExpr().bind("throw")))));
   Finder->addMatcher(
       compoundStmt(forEach(
-          ifStmt(hasThen(stmt(
+          ifStmt(unless(isConstexpr()),
+                 hasThen(stmt(
                      anyOf(ControlFlowInterruptorMatcher,
                            compoundStmt(has(ControlFlowInterruptorMatcher))))),
                  hasElse(stmt().bind("else")))
Index: test/clang-tidy/readability-else-after-return-if-constexpr.cpp
===================================================================
--- /dev/null
+++ test/clang-tidy/readability-else-after-return-if-constexpr.cpp
@@ -0,0 +1,22 @@
+// RUN: %check_clang_tidy %s readability-else-after-return %t -- -- -std=c++17
+
+// Constexpr if is an exception to the rule, we cannot remove the else.
+void f() {
+  if (sizeof(int) > 4)
+    return;
+  else
+    return;
+  // CHECK-MESSAGES: [[@LINE-2]]:3: warning: do not use 'else' after 'return'
+
+  if constexpr (sizeof(int) > 4)
+    return;
+  else
+    return;
+
+  if constexpr (sizeof(int) > 4)
+    return;
+  else if constexpr (sizeof(long) > 4)
+    return;
+  else
+    return;
+}