[CSFix] Make it possible to diagnose keypath inference failure in ambiguous contexts

Resolves: rdar://85458997

Author: xedin

include/swift/Sema/CSFix.h

@@ -2468,6 +2468,10 @@ class SpecifyKeyPathRootType final : public ConstraintFix {
 
   bool diagnose(const Solution &solution, bool asNote = false) const override;
 
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
+    return diagnose(*commonFixes.front().first);
+  }
+
   static SpecifyKeyPathRootType *create(ConstraintSystem &cs,
                                         ConstraintLocator *locator);
 

test/Constraints/keypath.swift

@@ -188,3 +188,21 @@ func testSR13442() {
   _ = SR13442(\String?.!.count) // OK
   let _: KeyPath<Int?, Int> = \Optional.!
 }
+
+// rdar://85458997 - failed to produce a diagnostic about key path root type
+func rdar85458997() {
+  struct S<R> {
+    init(_: KeyPath<R, String>) {
+    }
+
+    init(_: KeyPath<R, Int>) {
+    }
+  }
+
+  struct V {
+    var name: String
+  }
+
+  _ = S(\.name)
+  // expected-error@-1 {{cannot infer key path type from context; consider explicitly specifying a root type}} {{10-10=<#Root#>}}
+}