[5.9][CodeCompletion] Don’t crash when completing in a parameter position that only constraints one of two primary associated protocol types

lib/AST/GenericSignature.cpp

@@ -724,16 +724,16 @@ Type GenericSignatureImpl::getDependentUpperBounds(Type type) const {
           argTypes.push_back(reducedType);
       }
 
-      // We should have either constrained all primary associated types,
-      // or none of them.
-      if (!argTypes.empty()) {
-        if (argTypes.size() != primaryAssocTypes.size()) {
-          llvm::errs() << "Not all primary associated types constrained?\n";
-          llvm::errs() << "Interface type: " << type << "\n";
-          llvm::errs() << GenericSignature(this) << "\n";
-          abort();
-        }
-
+      // If we have constrained all primary associated types, create a
+      // parameterized protocol type. During code completion, we might call
+      // `getExistentialType` (which calls this method) on a generic parameter
+      // that doesn't have all parameters specified, e.g. to get a consise
+      // description of the parameter type to the following function.
+      //
+      // func foo<P: Publisher>(p: P) where P.Failure == Never
+      //
+      // In that case just add the base type in the default branch below.
+      if (argTypes.size() == primaryAssocTypes.size()) {
         types.push_back(ParameterizedProtocolType::get(ctx, baseType, argTypes));
         continue;
       }

validation-test/IDE/crashers_2_fixed/0039-not-all-primary-assoc-types-specified.swift

@@ -0,0 +1,15 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-ide-test -batch-code-completion -source-filename %s -filecheck %raw-FileCheck -completion-output-dir %t
+
+protocol Publisher<Output, Failure> {
+  associatedtype Output
+  associatedtype Failure
+}
+
+func foo<P: Publisher>(_ publisher: P) where P.Failure == Never
+
+func test() {
+  foo(#^COMPLETE^#)
+  // Make sure we don’t crash
+  // COMPLETE: Begin completions
+}