[CSDiagnostics] Use expr's start loc for 'self.' fix-it

Resolves SR-11787.

Author: hamishknight

lib/Sema/CSDiagnostics.cpp

@@ -1647,9 +1647,10 @@ bool AssignmentFailure::diagnoseAsError() {
         });
 
         if (foundProperty != results.end()) {
-          emitDiagnostic(Loc, diag::masked_instance_variable,
+          auto startLoc = immutableExpr->getStartLoc();
+          emitDiagnostic(startLoc, diag::masked_instance_variable,
                          typeContext->getSelfTypeInContext())
-              .fixItInsert(Loc, "self.");
+              .fixItInsert(startLoc, "self.");
         }
       }
 

test/Sema/immutability.swift

@@ -712,4 +712,17 @@ struct S {
       x += 1 // expected-error {{left side of mutating operator isn't mutable: 'x' is a 'let' constant}}
     }
   }
+
+  func bar() {
+    // SR-11787: Make sure we insert "self." in the right location.
+    let x = 0 // expected-note 3{{change 'let' to 'var' to make it mutable}}
+    x += 1 // expected-error {{left side of mutating operator isn't mutable: 'x' is a 'let' constant}}
+    // expected-note@-1 {{add explicit 'self.' to refer to mutable property of 'S'}} {{5-5=self.}}
+
+    (try x) += 1 // expected-error {{left side of mutating operator isn't mutable: 'x' is a 'let' constant}}
+    // expected-note@-1 {{add explicit 'self.' to refer to mutable property of 'S'}} {{10-10=self.}}
+
+    x = 1 // expected-error {{cannot assign to value: 'x' is a 'let' constant}}
+    // expected-note@-1 {{add explicit 'self.' to refer to mutable property of 'S'}} {{5-5=self.}}
+  }
 }