AST: Fix crash in getContextSubstitutions() when a class has a malformed superclass type

It is possible for getSuperclassDecl() to return a non-null type, while
getSuperclass() returns an ErrorType. In this case, getContextSubstitutions()
could crash because the walk of the superclass chain via getSuperclass()
might not find the context class.

Instead of crashing in asserts builds, let getContextSubstitutions()
silently build an invalid substitution map here, just as it does in no-asserts
builds.

Author: slavapestov

lib/AST/Type.cpp

@@ -3572,8 +3572,6 @@ TypeBase::getContextSubstitutions(const DeclContext *dc,
   if (auto *ownerClass = dyn_cast<ClassDecl>(ownerNominal))
     baseTy = baseTy->getSuperclassForDecl(ownerClass);
 
-  assert(ownerNominal == baseTy->getAnyNominal());
-
   // Gather all of the substitutions for all levels of generic arguments.
   auto genericSig = dc->getGenericSignatureOfContext();
   if (!genericSig)
@@ -3583,6 +3581,9 @@ TypeBase::getContextSubstitutions(const DeclContext *dc,
   unsigned n = params.size();
 
   while (baseTy && n > 0) {
+    if (baseTy->is<ErrorType>())
+      break;
+
     // For a bound generic type, gather the generic parameter -> generic
     // argument substitutions.
     if (auto boundGeneric = baseTy->getAs<BoundGenericType>()) {

test/decl/circularity.swift

@@ -59,6 +59,7 @@ extension SIMD3 where SIMD3.Scalar == Float {
 // Test case with circular overrides
 protocol P {
     associatedtype A
+    // expected-note@-1 {{protocol requires nested type 'A'; do you want to add it?}}
     func run(a: A)
 }
 
@@ -71,3 +72,15 @@ class C2: C1, P {
     // expected-error@-1 {{circular reference}}
     // expected-note@-2 2{{through reference here}}
 }
+
+// Another crash to the above
+open class G1<A> {
+    open func run(a: A) {}
+}
+
+class C3: G1<A>, P {
+    // expected-error@-1 {{type 'C3' does not conform to protocol 'P'}}
+    // expected-error@-2 {{use of undeclared type 'A'}}
+    override func run(a: A) {}
+    // expected-error@-1 {{method does not override any method from its superclass}}
+}