[Diagnostics] Diagnose inability to infer type of a closure parameter

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -250,6 +250,9 @@ ERROR(no_candidates_match_result_type,none,
       "no '%0' candidates produce the expected contextual result type %1",
       (StringRef, Type))
 
+ERROR(cannot_infer_closure_parameter_type,none,
+      "unable to infer type of a closure parameter %0 in the current context",
+       (StringRef))
 ERROR(cannot_infer_closure_type,none,
       "unable to infer closure type in the current context", ())
 ERROR(cannot_infer_closure_result_type,none,

lib/Sema/CSDiagnostics.cpp

@@ -6125,6 +6125,28 @@ bool MissingContextualBaseInMemberRefFailure::diagnoseAsError() {
   return true;
 }
 
+bool UnableToInferClosureParameterType::diagnoseAsError() {
+  auto *closure = castToExpr<ClosureExpr>(getRawAnchor());
+  auto paramIdx = getLocator()
+                      ->castLastElementTo<LocatorPathElt::TupleElement>()
+                      .getIndex();
+
+  auto *PD = closure->getParameters()->get(paramIdx);
+
+  llvm::SmallString<16> id;
+  llvm::raw_svector_ostream OS(id);
+
+  if (PD->isAnonClosureParam()) {
+    OS << "$" << paramIdx;
+  } else {
+    OS << "'" << PD->getParameterName() << "'";
+  }
+
+  auto loc = PD->isAnonClosureParam() ? getLoc() : PD->getLoc();
+  emitDiagnosticAt(loc, diag::cannot_infer_closure_parameter_type, OS.str());
+  return true;
+}
+
 bool UnableToInferClosureReturnType::diagnoseAsError() {
   auto *closure = castToExpr<ClosureExpr>(getRawAnchor());
 

lib/Sema/CSDiagnostics.h

@@ -1971,6 +1971,15 @@ class MissingContextualBaseInMemberRefFailure final : public FailureDiagnostic {
   bool diagnoseAsError();
 };
 
+class UnableToInferClosureParameterType final : public FailureDiagnostic {
+public:
+  UnableToInferClosureParameterType(const Solution &solution,
+                                    ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator) {}
+
+  bool diagnoseAsError();
+};
+
 class UnableToInferClosureReturnType final : public FailureDiagnostic {
 public:
   UnableToInferClosureReturnType(const Solution &solution,