[Diagnostics] Ignore warnings while diagnosing ambiguities

Warnings cannot lead to failures or ambiguity so let's remove
them from consideration when attempting to diagnose ambiguity
potentially caused by the same fix appearing in different
solutions.

Resolves: rdar://81228501

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -3989,8 +3989,14 @@ bool ConstraintSystem::diagnoseAmbiguityWithFixes(
 
   llvm::SmallSetVector<FixInContext, 4> fixes;
   for (auto &solution : solutions) {
-    for (auto *fix : solution.Fixes)
+    for (auto *fix : solution.Fixes) {
+      // Ignore warnings in favor of actual error fixes,
+      // because they are not the source of ambiguity/failures.
+      if (fix->isWarning())
+        continue;
+
       fixes.insert({&solution, fix});
+    }
   }
 
   llvm::MapVector<ConstraintLocator *, SmallVector<FixInContext, 4>>

test/Constraints/closures.swift

@@ -1163,3 +1163,13 @@ func rdar78917861() {
     }()
   }
 }
+
+// rdar://81228501 - type-checker crash due to applied invalid solution
+func test(arr: [[Int]]) {
+  struct A {
+    init(arg: [Int]) {}
+  }
+
+  arr.map { ($0 as? [Int]).map { A($0) } } // expected-error {{missing argument label 'arg:' in call}} {{36-36=arg: }}
+  // expected-warning@-1 {{conditional cast from '[Int]' to '[Int]' always succeeds}}
+}