Sema: Fix logic error in associated type inference solver

If matching a candidate value witness against a protocol requirement produced
non-viable bindings, then don't consider the witness in the solver; it can
never lead to a valid solution.

Author: slavapestov

lib/Sema/AssociatedTypeInference.cpp

@@ -3116,6 +3116,11 @@ void AssociatedTypeInference::findSolutionsRec(
   // looking for solutions involving each one.
   const auto &inferredReq = inferred[reqDepth];
   for (const auto &witnessReq : inferredReq.second) {
+    // If this witness had invalid bindings, don't consider it since it can
+    // never lead to a valid solution.
+    if (!witnessReq.NonViable.empty())
+      continue;
+
     llvm::SaveAndRestore<unsigned> savedNumTypeWitnesses(numTypeWitnesses);
 
     // If we had at least one tautological witness, we must consider the

test/decl/protocol/req/associated_type_inference_tautology_2.swift

@@ -0,0 +1,20 @@
+// RUN: %target-typecheck-verify-swift -disable-experimental-associated-type-inference
+// RUN: %target-typecheck-verify-swift -enable-experimental-associated-type-inference
+
+protocol P {
+  associatedtype A: Q = Int
+
+  func f(_: A)
+}
+
+protocol Q {}
+
+extension Int: Q {}
+extension String: Q {}
+
+struct S: P {
+  // The presence of this overload where Float does not conform to Q would confuse
+  // the solver and it would find two ambiguous solutions, A := Int and A := String.
+  func f(_: Float) {}
+  func f(_: String) {}
+}