GSB: Workaround interator invalidation issue with getMinimalConformanceSource()

In a few places we call getMinimalConformanceSource() while iterating
over all equivalence classes, or the conformances of a specific
equivalence class.

In both cases, getMinimalConformanceSource() can modify the storage
of the collection being iterated over, because it can end up calling
maybeResolveEquivalenceClass(), which can introduce new constraints.

Work around this by iterating a copy of the list of equivalence
classes first, calling getMinimalConformanceSource() and discarding
the result on each conformance source.

This is a terrible workaround, but should address the issue in the
meantime until I finish removing getMinimalConformanceSource().

Fixes rdar://problem/46420886 / https://bugs.swift.org/browse/SR-9398,
rdar://problem/77807692.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -6379,6 +6379,37 @@ GenericSignatureBuilder::finalize(TypeArrayView<GenericTypeParamType> genericPar
   // Process any delayed requirements that we can handle now.
   processDelayedRequirements();
 
+  {
+    // In various places below, we iterate over the list of equivalence classes
+    // and call getMinimalConformanceSource(). Unfortunately, this function
+    // ends up calling maybeResolveEquivalenceClass(), which can delete equivalence
+    // classes. The workaround is to first iterate safely over a copy of the list,
+    // and pre-compute all minimal conformance sources, before proceeding with the
+    // rest of the function.
+    //
+    // FIXME: This is not even correct, because we may not have reached fixed point
+    // after one round of this. getMinimalConformanceSource() should be removed
+    // instead.
+    SmallVector<PotentialArchetype *, 8> equivalenceClassPAs;
+    for (auto &equivClass : Impl->EquivalenceClasses) {
+      equivalenceClassPAs.push_back(equivClass.members.front());
+    }
+
+    for (auto *pa : equivalenceClassPAs) {
+      auto &equivClass = *pa->getOrCreateEquivalenceClass(*this);
+
+      // Copy the vector and iterate over the copy to avoid iterator invalidation
+      // issues.
+      auto conformsTo = equivClass.conformsTo;
+      for (auto entry : conformsTo) {
+        for (const auto &constraint : entry.second) {
+          (void) constraint.source->getMinimalConformanceSource(
+              *this, constraint.getSubjectDependentType({ }), entry.first);
+        }
+      }
+    }
+  }
+
   computeRedundantRequirements(requirementSignatureSelfProto);
   diagnoseProtocolRefinement(requirementSignatureSelfProto);
   diagnoseRedundantRequirements();

test/Generics/rdar77807692.swift

@@ -0,0 +1,21 @@
+// RUN: %target-typecheck-verify-swift
+// RUN: %target-swift-frontend -typecheck -debug-generic-signatures %s 2>&1 | %FileCheck %s
+
+protocol P1 {
+  associatedtype T : Hashable
+}
+
+struct S1<Value> {}
+
+extension S1 : P1 where Value : P1 {
+  typealias T = Value.T
+}
+
+protocol P2 {
+  associatedtype Value: P1
+}
+
+struct S2<X, Y: P2> where Y.Value == X {
+  // CHECK-LABEL: Generic signature: <X, Y, T where X == S1<T>, Y : P2, T : P1, Y.Value == S1<T>>
+  init<T>(_: T) where X == S1<T> { }
+}