Sema: Don't be so eager to force type witnesses when resolving a value witness

resolveWitnessViaLookup() was only called from one place and didn't
need to force anything at all, and resolveSingleWitness() now only
forces the type witnesses actually referenced by the requirement.

An old crasher test case that was considered "fixed" when we started
to diagnose it now type checks correctly.

Author: slavapestov

lib/Sema/AssociatedTypeInference.cpp

@@ -4121,6 +4121,14 @@ TypeWitnessRequest::evaluate(Evaluator &eval,
       auto *better = getBetterConformanceForResolvingTypeWitnesses(
           conformance, requirement);
 
+      if (better == conformance) {
+        LLVM_DEBUG(llvm::dbgs() << "Conformance to " << conformance->getProtocol()
+                                << " is best\n";);
+      } else {
+        LLVM_DEBUG(llvm::dbgs() << "Conformance to " << better->getProtocol()
+                                << " is better than " << conformance->getProtocol()
+                                << "\n";);
+      }
       if (better != conformance &&
           !ctx.evaluator.hasActiveRequest(ResolveTypeWitnessesRequest{better})) {
         // Let's try to resolve type witnesses in the better conformance.

lib/Sema/TypeCheckProtocol.cpp

@@ -4018,22 +4018,6 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
   assert(!isa<AssociatedTypeDecl>(requirement) && "Use resolveTypeWitnessVia*");
   auto *nominal = DC->getSelfNominalTypeDecl();
 
-  // Resolve all associated types before trying to resolve this witness.
-  evaluateOrDefault(getASTContext().evaluator,
-                    ResolveTypeWitnessesRequest{Conformance},
-                    evaluator::SideEffect());
-
-  // If any of the type witnesses was erroneous, don't bother to check
-  // this value witness: it will fail.
-  auto referenced = evaluateOrDefault(getASTContext().evaluator,
-                                      ReferencedAssociatedTypesRequest{requirement},
-                                      TinyPtrVector<AssociatedTypeDecl *>());
-  for (auto assocType : referenced) {
-    if (Conformance->getTypeWitness(assocType)->hasError()) {
-      return ResolveWitnessResult::ExplicitFailed;
-    }
-  }
-
   // Determine whether we can derive a witness for this requirement.
   bool canDerive = false;
 
@@ -4537,18 +4521,13 @@ void ConformanceChecker::resolveSingleWitness(ValueDecl *requirement) {
   if (!requirement->isProtocolRequirement())
     return;
 
-  // Resolve all associated types before trying to resolve this witness.
-  evaluateOrDefault(getASTContext().evaluator,
-                    ResolveTypeWitnessesRequest{Conformance},
-                    evaluator::SideEffect());
-
-  // If any of the type witnesses was erroneous, don't bother to check
-  // this value witness: it will fail.
-  auto assocTypes = evaluateOrDefault(getASTContext().evaluator,
+  // Resolve the type witnesses for all associated types referenced by
+  // the requirement. If any are erroneous, don't bother resolving the
+  // witness.
+  auto referenced = evaluateOrDefault(getASTContext().evaluator,
                                       ReferencedAssociatedTypesRequest{requirement},
                                       TinyPtrVector<AssociatedTypeDecl *>());
-
-  for (auto assocType : assocTypes) {
+  for (auto assocType : referenced) {
     if (Conformance->getTypeWitness(assocType)->hasError()) {
       Conformance->setInvalid();
       return;

test/decl/protocol/req/associated_type_inference_stdlib_4.swift

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift -disable-experimental-associated-type-inference
+// RUN: %target-typecheck-verify-swift -enable-experimental-associated-type-inference
+
+// The 'for' loop has to come first, to force Sequence.makeIterator().
+for x in S() { _ = x }
+
+struct S: RandomAccessCollection {
+  public var startIndex: Int { 0 }
+  public var endIndex: Int { 0 }
+  public subscript(position: Int) -> Int { 0 }
+}
+

test/multifile/protocol-conformance-issue-53408.swift

@@ -1,8 +1,14 @@
-// RUN: %target-swift-frontend -emit-sil -module-name main -primary-file %s %S/Inputs/protocol-conformance-issue-53408-other.swift -verify
-// RUN: %target-swift-frontend -emit-sil -module-name main %s -primary-file %S/Inputs/protocol-conformance-issue-53408-other.swift
+// RUN: not %target-swift-frontend -emit-sil -module-name main -primary-file %s %S/Inputs/protocol-conformance-issue-53408-other.swift -disable-experimental-associated-type-inference
+// RUN: %target-swift-frontend -emit-sil -module-name main %s -primary-file %S/Inputs/protocol-conformance-issue-53408-other.swift -disable-experimental-associated-type-inference
+// RUN: not %target-swift-frontend -emit-sil -module-name main %s %S/Inputs/protocol-conformance-issue-53408-other.swift -disable-experimental-associated-type-inference
+// RUN: %target-swift-frontend -emit-sil -module-name main %S/Inputs/protocol-conformance-issue-53408-other.swift %s -disable-experimental-associated-type-inference
+
+// RUN: %target-swift-frontend -emit-sil -module-name main -primary-file %s %S/Inputs/protocol-conformance-issue-53408-other.swift -enable-experimental-associated-type-inference
+// RUN: %target-swift-frontend -emit-sil -module-name main %s -primary-file %S/Inputs/protocol-conformance-issue-53408-other.swift -enable-experimental-associated-type-inference
+// RUN: %target-swift-frontend -emit-sil -module-name main %s %S/Inputs/protocol-conformance-issue-53408-other.swift -enable-experimental-associated-type-inference
+// RUN: %target-swift-frontend -emit-sil -module-name main %S/Inputs/protocol-conformance-issue-53408-other.swift %s -enable-experimental-associated-type-inference
 
 // https://github.com/apple/swift/issues/53408
 
 func reproducer() -> Float { return Struct().func1(1.0) }
-// expected-error@-1 {{cannot convert value of type 'Double' to expected argument type 'Struct.Input'}}
-// expected-error@-2 {{type 'Struct' does not conform to protocol 'Proto1'}}
+