[Sema] Be more resilient about unavailable conditional requirements in error cases.

When we're trying to diagnose something, it's a bad look if we crash
instead. This changes the bad-associated-type recovery path to not require that
the conditional requirements have been computed, and instead detects that as a
possible error.

Fixes https://bugs.swift.org/browse/SR-8033.

Author: huonw

include/swift/AST/DiagnosticsSema.def

@@ -647,6 +647,9 @@ ERROR(ambiguous_module_type,none,
 ERROR(use_nonmatching_operator,none,
       "%0 is not a %select{binary|prefix unary|postfix unary}1 operator",
       (DeclName, unsigned))
+ERROR(unsupported_recursion_in_associated_type_reference,none,
+      "unsupported recursion for reference to associated type %0 of type %1",
+      (DeclName, Type))
 ERROR(broken_associated_type_witness,none,
       "reference to invalid associated type %0 of type %1", (DeclName, Type))
 

lib/Sema/TypeCheckProtocol.cpp

@@ -3829,11 +3829,18 @@ Optional<ProtocolConformanceRef> TypeChecker::conformsToProtocol(
     markConformanceUsed(*lookupResult, DC);
   }
 
-  // If we have a concrete conformance with conditional requirements that
+  auto condReqs = lookupResult->getConditionalRequirementsIfAvailable();
+  // If we have a conditional requirements that
   // we need to check, do so now.
-  if (lookupResult->isConcrete() &&
-      !lookupResult->getConditionalRequirements().empty() &&
-      !options.contains(ConformanceCheckFlags::SkipConditionalRequirements)) {
+  if (!condReqs) {
+    assert(
+        options.contains(
+            ConformanceCheckFlags::AllowUnavailableConditionalRequirements) &&
+        "unhandled recursion: missing conditional requirements when they're "
+        "required");
+  } else if (!condReqs->empty() &&
+             !options.contains(
+                 ConformanceCheckFlags::SkipConditionalRequirements)) {
     // Figure out the location of the conditional conformance.
     auto conformanceDC = lookupResult->getConcrete()->getDeclContext();
     SourceLoc noteLoc;
@@ -3846,7 +3853,7 @@ Optional<ProtocolConformanceRef> TypeChecker::conformsToProtocol(
       checkGenericArguments(DC, ComplainLoc, noteLoc, T,
                             { Type(lookupResult->getRequirement()
                                 ->getProtocolSelfType()) },
-                            lookupResult->getConditionalRequirements(),
+                            *condReqs,
                             [](SubstitutableType *dependentType) {
                               return Type(dependentType);
                             },

lib/Sema/TypeCheckType.cpp

@@ -665,11 +665,11 @@ static void maybeDiagnoseBadConformanceRef(TypeChecker &tc,
                                            AssociatedTypeDecl *assocType) {
   // If we weren't given a conformance, go look it up.
   ProtocolConformance *conformance = nullptr;
-  if (auto conformanceRef =
-        tc.conformsToProtocol(
+  if (auto conformanceRef = tc.conformsToProtocol(
           parentTy, assocType->getProtocol(), dc,
-          (ConformanceCheckFlags::InExpression|
-           ConformanceCheckFlags::SuppressDependencyTracking))) {
+          (ConformanceCheckFlags::InExpression |
+           ConformanceCheckFlags::SuppressDependencyTracking |
+           ConformanceCheckFlags::AllowUnavailableConditionalRequirements))) {
     if (conformanceRef->isConcrete())
       conformance = conformanceRef->getConcrete();
   }
@@ -679,9 +679,14 @@ static void maybeDiagnoseBadConformanceRef(TypeChecker &tc,
   if (tc.Context.Diags.hadAnyError())
     return;
 
-  tc.diagnose(loc, diag::broken_associated_type_witness,
-              assocType->getFullName(), parentTy);
-};
+  auto diagCode =
+      (conformance && !conformance->getConditionalRequirementsIfAvailable())
+          ? diag::unsupported_recursion_in_associated_type_reference
+          : diag::broken_associated_type_witness;
+
+  tc.diagnose(loc, diagCode, assocType->getFullName(), parentTy);
+}
+
 /// \brief Returns a valid type or ErrorType in case of an error.
 static Type resolveTypeDecl(TypeChecker &TC, TypeDecl *typeDecl, SourceLoc loc,
                             DeclContext *foundDC,

lib/Sema/TypeChecker.h

@@ -702,6 +702,13 @@ enum class ConformanceCheckFlags {
   /// correctly used. Otherwise (the default), all of the conditional
   /// requirements will be checked.
   SkipConditionalRequirements = 0x08,
+
+  /// Whether to require that the conditional requirements have been computed.
+  ///
+  /// When set, if the conditional requirements aren't available, they are just
+  /// skipped, and the caller is responsible for detecting and handling this
+  /// case (likely via another call to getConditionalRequirementsIfAvailable).
+  AllowUnavailableConditionalRequirements = 0x10,
 };
 
 /// Options that control protocol conformance checking.

validation-test/compiler_crashers_2_fixed/0163-sr8033.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift
+
+struct Foo<T> {}
+
+protocol P1 {
+    associatedtype A // expected-note {{protocol requires nested type 'A'; do you want to add it?}}
+}
+extension Foo: P1 where A : P1 {} // expected-error {{unsupported recursion for reference to associated type 'A' of type 'Foo<T>'}}
+// expected-error@-1 {{type 'Foo<T>' does not conform to protocol 'P1'}}