AST: fix isWrittenWithConstraints

Surprisingly, there are some situations where an extension can end up
with _fewer_ constraints than the extended type. That was baked-in as an
assertion in this new-ish method.

I haven't figured out why that can happen in the reproducer only when
using `-interpret` mode. It didn't trigger the assertion for me when
compiling normally.

The fix is simple: check all the requirements, rather than using a
short-cut.

resolves rdar://125659789 / #72719

(cherry picked from commit 2735492)

Author: kavon

lib/AST/Decl.cpp

@@ -1809,14 +1809,20 @@ bool ExtensionDecl::isWrittenWithConstraints() const {
   typeSig->getRequirementsWithInverses(typeReqs, typeInverseReqs);
 
   // If the (non-inverse) requirements are different between the extension and
-  // the original type, it's written with constraints. Note that
-  // the extension can only add requirements, so we need only check the size
-  // (not the specific requirements).
-  if (extReqs.size() > typeReqs.size()) {
+  // the original type, it's written with constraints.
+  if (extReqs.size() != typeReqs.size()) {
     return true;
   }
 
-  assert(extReqs.size() == typeReqs.size());
+  // In case of equal number of constraints, we have to check the specific
+  // requirements. Extensions can end up with fewer requirements than the type
+  // extended, due to a same-type requirement in the extension.
+  //
+  // This mirrors the 'same' check in `ASTMangler::gatherGenericSignatureParts`
+  for (size_t i = 0; i < extReqs.size(); i++) {
+    if (extReqs[i] != typeReqs[i])
+      return true;
+  }
 
   // If the type has no inverse requirements, there are no extra constraints
   // to write.

validation-test/execution/issue-72719.swift

@@ -0,0 +1,17 @@
+// RUN: %target-swift-frontend -interpret %s
+// REQUIRES: executable_test
+
+// This only reproduced if you provide the -interpret flag!
+// https://github.com/apple/swift/issues/72719
+
+protocol D {}
+struct U: D, Equatable {}
+class Q<T> {}
+class R<V, E: D & Equatable> {}
+extension R where E == U {
+    struct S<X> {}
+    static func a<T>(_: T) -> R {
+        let x = Q<S<T>>()
+        fatalError()
+    }
+}