Improve diagnosis of implicit match expressions

When diagnosing failure to typecheck a binary '~=' expression that was
synthesized when typechecking an expression pattern, offer a message
that describes the failure more helpfully.

<rdar://problem/21995744> QoI: Binary operator '~=' cannot be applied to operands of type 'String' and 'String?'

Swift SVN r31286

include/swift/AST/DiagnosticsSema.def

@@ -107,6 +107,10 @@ ERROR(comparison_with_nil_illegal,sema,none,
       "value of type %0 can never be nil, comparison isn't allowed",
       (Type))
 
+ERROR(cannot_match_expr_pattern_with_value,sema,none,
+      "expression pattern of type %0 cannot match values of type %1",
+      (Type, Type))
+
 ERROR(cannot_apply_binop_to_args,sema,none,
       "binary operator '%0' cannot be applied to operands of type "
       "%1 and %2",

lib/Sema/CSDiag.cpp

@@ -3300,6 +3300,15 @@ bool FailureDiagnosis::visitBinaryExpr(BinaryExpr *binop) {
   auto lhsType = lhsExpr->getType()->getRValueType();
   auto rhsType = rhsExpr->getType()->getRValueType();
 
+  if (binop->isImplicit() && overloadName == "~=") {
+    // This binop was synthesized when typechecking an expression pattern.
+    diagnose(lhsExpr->getLoc(),
+             diag::cannot_match_expr_pattern_with_value, lhsType, rhsType)
+      .highlight(lhsExpr->getSourceRange())
+      .highlight(rhsExpr->getSourceRange());
+    return true;
+  }
+
   // If this is a comparison against nil, then we should produce a specific
   // diagnostic.
   if (isa<NilLiteralExpr>(rhsExpr->getValueProvidingExpr()) &&

test/Constraints/patterns.swift

@@ -138,6 +138,12 @@ case nil?: break
 default: break
 }
 
+// <rdar://problem/21995744> QoI: Binary operator '~=' cannot be applied to operands of type 'String' and 'String?'
+switch ("foo" as String?) {
+case "what": break // expected-error{{expression pattern of type 'String' cannot match values of type 'String?'}}
+default: break
+}
+
 
 // Test some value patterns.
 let x : Int? = nil

test/Misc/misc_diagnostics.swift

@@ -126,6 +126,6 @@ func test17875634() {
 
 // <rdar://problem/20770032> Pattern matching ranges against tuples crashes the compiler
 func test20770032() {
-  if case let 1...10 = (1, 1) { // expected-warning{{'let' pattern has no effect; sub-pattern didn't bind any variables}} {{11-15=}} expected-error{{binary operator '~=' cannot be applied to operands of type 'Range<Int>' and '(Int, Int)'}}
+  if case let 1...10 = (1, 1) { // expected-warning{{'let' pattern has no effect; sub-pattern didn't bind any variables}} {{11-15=}} expected-error{{expression pattern of type 'Range<Int>' cannot match values of type '(Int, Int)'}}
   }
 }