[ConstraintSystem] Specialize diagnostic for ~= to talk about expression pattern use

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -2814,6 +2814,9 @@ static void diagnoseOperatorAmbiguity(ConstraintSystem &cs,
                     operatorName.str());
         return;
       }
+    } else if (operatorName.is("~=")) {
+      DE.diagnose(anchor->getLoc(), diag::cannot_match_expr_pattern_with_value,
+                  lhsType, rhsType);
     } else {
       DE.diagnose(anchor->getLoc(), diag::cannot_apply_binop_to_args,
                   operatorName.str(), lhsType, rhsType)

test/Parse/switch.swift

@@ -336,6 +336,7 @@ func f1(x: String, y: Whichever) {
     case Whichever.buzz: // expected-error {{type 'Whichever' has no member 'buzz'}}
         break
     case Whichever.alias: // expected-error {{expression pattern of type 'Whichever' cannot match values of type 'String'}}
+    // expected-note@-1 {{overloads for '~=' exist with these partially matching parameter lists: (Substring, String)}}
         break
     default:
       break