[Diagnostics] Use a generic "tuple type not convertible" error message

when there is no contextual type.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -955,6 +955,8 @@ ERROR(types_not_convertible_use_bool_value,none,
 ERROR(tuple_types_not_convertible_nelts,none,
       "%0 is not convertible to %1, "
       "tuples have a different number of elements", (Type, Type))
+ERROR(tuple_types_not_convertible,none,
+       "tuple type %0 is not convertible to tuple type %1", (Type, Type))
 
 ERROR(invalid_force_unwrap,none,
       "cannot force unwrap value of non-optional type %0", (Type))

lib/Sema/CSDiagnostics.cpp

@@ -2804,14 +2804,19 @@ ContextualFailure::getDiagnosticFor(ContextualTypePurpose context,
 }
 
 bool TupleContextualFailure::diagnoseAsError() {
-  auto diagnostic = isNumElementsMismatch()
-                        ? diag::tuple_types_not_convertible_nelts
-                        : getDiagnosticFor(getContextualTypePurpose(),
-                                           /*forProtocol=*/false);
-  if (!diagnostic)
+  Diag<Type, Type> diagnostic;
+  auto purpose = getContextualTypePurpose();
+  auto &cs = getConstraintSystem();
+  if (isNumElementsMismatch())
+    diagnostic = diag::tuple_types_not_convertible_nelts;
+  else if ((purpose == CTP_Initialization) && !cs.getContextualType())
+    diagnostic = diag::tuple_types_not_convertible;
+  else if (auto diag = getDiagnosticFor(purpose, /*forProtocol=*/false))
+    diagnostic = *diag;
+  else
     return false;
 
-  emitDiagnostic(getAnchor()->getLoc(), *diagnostic, getFromType(), getToType());
+  emitDiagnostic(getAnchor()->getLoc(), diagnostic, getFromType(), getToType());
   return true;
 }
 

test/decl/var/variables.swift

@@ -96,7 +96,7 @@ func tuplePatternDestructuring(_ x : Int, y : Int) {
   _ = i+j
 
   // <rdar://problem/20395243> QoI: type variable reconstruction failing for tuple types
-  let (x: g1, a: h1) = (b: x, a: y)  // expected-error {{cannot convert value of type '(b: Int, a: Int)' to specified type '(x: Int, a: Int)'}}
+  let (x: g1, a: h1) = (b: x, a: y)  // expected-error {{tuple type '(b: Int, a: Int)' is not convertible to tuple type '(x: Int, a: Int)'}}
 }
 
 // <rdar://problem/21057425> Crash while compiling attached test-app.