[CSDiagnostics] Add ephemeralness ambiguity notes

Author: hamishknight

include/swift/AST/DiagnosticsSema.def

@@ -426,6 +426,10 @@ NOTE(ephemeral_use_array_with_unsafe_buffer,none,
      "use the 'withUnsafe%select{Bytes|MutableBytes|BufferPointer|"
      "MutableBufferPointer}0' method on Array in order to explicitly convert "
      "argument to buffer pointer valid for a defined scope", (unsigned))
+NOTE(candidate_performs_illegal_ephemeral_conv,none,
+     "candidate expects pointer that outlives the call for parameter #%0",
+     (unsigned))
+
 ERROR(cannot_convert_argument_value_protocol,none,
       "argument type %0 does not conform to expected type %1", (Type, Type))
 ERROR(cannot_convert_partial_argument_value_protocol,none,

lib/Sema/CSDiagnostics.cpp

@@ -5535,6 +5535,16 @@ void NonEphemeralConversionFailure::emitSuggestionNotes() const {
   }
 }
 
+bool NonEphemeralConversionFailure::diagnoseAsNote() {
+  // We can only emit a useful note if we have a callee.
+  if (auto *callee = getCallee()) {
+    emitDiagnostic(callee, diag::candidate_performs_illegal_ephemeral_conv,
+                   getParamPosition());
+    return true;
+  }
+  return false;
+}
+
 bool NonEphemeralConversionFailure::diagnoseAsError() {
   auto *argExpr = getArgExpr();
   if (isa<InOutExpr>(argExpr)) {

lib/Sema/CSDiagnostics.h

@@ -1933,6 +1933,7 @@ class NonEphemeralConversionFailure final : public ArgumentMismatchFailure {
   }
 
   bool diagnoseAsError() override;
+  bool diagnoseAsNote() override;
 
 private:
   /// Emits a note explaining to the user that an ephemeral conversion is only

test/Sema/diag_non_ephemeral.swift

@@ -446,3 +446,12 @@ let f8: (UnsafeMutablePointer<Int8>) -> Void = takesMutable
 func higherOrder(_ fn: (UnsafeMutableRawPointer, Int) -> Void) {}
 higherOrder(takesMutableRaw)
 
+
+// @_nonEphemeral ambiguities
+func takesPointerOverload(x: Int = 0, @_nonEphemeral _ ptr: UnsafePointer<Int>) {} // expected-note {{candidate expects pointer that outlives the call for parameter #2}}
+func takesPointerOverload(x: Int = 0, @_nonEphemeral _ ptr: UnsafeMutablePointer<Int>) {} // expected-note {{candidate expects pointer that outlives the call for parameter #2}}
+
+func testAmbiguity() {
+  var arr = [1, 2, 3]
+  takesPointerOverload(&arr) // expected-error {{no exact matches in call to global function 'takesPointerOverload(x:_:)'}}
+}