[Diagnostics] Correctly diagnose situations when immutable value is passed to inout parameter

include/swift/AST/DiagnosticsSema.def

@@ -5177,6 +5177,9 @@ ERROR(assignment_bang_has_immutable_subcomponent,none,
 NOTE(candidate_is_not_assignable,none,
      "candidate is not assignable: %kind0",
      (const ValueDecl *))
+NOTE(candidate_expects_inout_argument,none,
+     "candidate expects in-out value for parameter #%0 but argument is immutable",
+     (unsigned))
 
 NOTE(change_to_mutating,none,
      "mark %select{method|%1}0 'mutating' to make 'self' mutable",

lib/Sema/CSDiagnostics.cpp

@@ -2049,11 +2049,24 @@ bool RValueTreatedAsLValueFailure::diagnoseAsError() {
 }
 
 bool RValueTreatedAsLValueFailure::diagnoseAsNote() {
-  auto overload = getCalleeOverloadChoiceIfAvailable(getLocator());
+  auto *locator = getLocator();
+
+  auto overload = getCalleeOverloadChoiceIfAvailable(locator);
   if (!(overload && overload->choice.isDecl()))
     return false;
 
   auto *decl = overload->choice.getDecl();
+
+  if (locator->isLastElement<LocatorPathElt::ArgumentAttribute>()) {
+    auto argConv = locator->findLast<LocatorPathElt::ApplyArgToParam>();
+    if (!argConv)
+      return false;
+
+    emitDiagnosticAt(decl, diag::candidate_expects_inout_argument,
+                     argConv->getParamIdx() + 1);
+    return true;
+  }
+
   emitDiagnosticAt(decl, diag::candidate_is_not_assignable, decl);
   return true;
 }

lib/Sema/ConstraintSystem.cpp

@@ -730,7 +730,9 @@ ConstraintLocator *ConstraintSystem::getCalleeLocator(
   }
 
   if (locator->isLastElement<LocatorPathElt::ArgumentAttribute>()) {
-    return getConstraintLocator(anchor, path.drop_back());
+    auto argLoc = getConstraintLocator(anchor, path.drop_back());
+    return getCalleeLocator(argLoc, lookThroughApply, getType, simplifyType,
+                            getOverloadFor);
   }
 
   // If we have a locator that starts with a key path component element, we

test/Constraints/overload.swift

@@ -349,3 +349,31 @@ do {
     }
   }
 }
+
+do {
+  struct S {
+    let x: Int
+    var y: Int
+  }
+
+  func overloaded(_: String) {}
+  // expected-note@-1 3 {{candidate expects value of type 'String' for parameter #1 (got 'Int')}}
+  func overloaded(_: inout Int) {}
+  // expected-note@-1 3 {{candidate expects in-out value for parameter #1 but argument is immutable}}
+
+  func testImmutable(s: S) {
+    overloaded(s.x) // expected-error {{no exact matches in call to local function 'overloaded'}}
+  }
+
+  func testMutable(s: inout S) {
+    overloaded(s.x) // expected-error {{no exact matches in call to local function 'overloaded'}}
+  }
+
+  func testImmutableBase(s: S) {
+    overloaded(s.y) // expected-error {{no exact matches in call to local function 'overloaded'}}
+  }
+
+  func testMissingAddressOf(s: inout S) {
+    overloaded(s.y) // expected-error {{passing value of type 'Int' to an inout parameter requires explicit '&'}}
+  }
+}