[Diagnostics] For array element conversion failures, emit an error message

for each element whose type variable was merged in addJoinConstraint

Author: hborla

lib/Sema/CSDiagnostics.cpp

@@ -4942,6 +4942,9 @@ bool CollectionElementContextualFailure::diagnoseAsError() {
 
   Optional<InFlightDiagnostic> diagnostic;
   if (isExpr<ArrayExpr>(anchor)) {
+    if (diagnoseMergedLiteralElements())
+      return true;
+
     diagnostic.emplace(emitDiagnostic(diag::cannot_convert_array_element,
                                       eltType, contextualType));
   }
@@ -4992,6 +4995,30 @@ bool CollectionElementContextualFailure::diagnoseAsError() {
   return true;
 }
 
+bool CollectionElementContextualFailure::diagnoseMergedLiteralElements() {
+  auto elementAnchor = simplifyLocatorToAnchor(getLocator());
+  if (!elementAnchor)
+    return false;
+
+  auto *typeVar = getRawType(elementAnchor)->getAs<TypeVariableType>();
+  if (!typeVar || !typeVar->getImpl().getAtomicLiteralKind())
+    return false;
+
+  // This element is a literal whose type variable could have been merged with others,
+  // but the conversion constraint to the array element type was only placed on one
+  // of them. So, we want to emit the error for each element whose type variable is in
+  // this equivalence class.
+  auto &cs = getConstraintSystem();
+  auto node = cs.getRepresentative(typeVar)->getImpl().getGraphNode();
+  for (const auto *typeVar : node->getEquivalenceClass()) {
+    auto anchor = typeVar->getImpl().getLocator()->getAnchor();
+    emitDiagnosticAt(constraints::getLoc(anchor), diag::cannot_convert_array_element,
+                     getFromType(), getToType());
+  }
+
+  return true;
+}
+
 bool MissingContextualConformanceFailure::diagnoseAsError() {
   auto anchor = getAnchor();
   auto path = getLocator()->getPath();

lib/Sema/CSDiagnostics.h

@@ -1637,6 +1637,8 @@ class CollectionElementContextualFailure final : public ContextualFailure {
       : ContextualFailure(solution, eltType, contextualType, locator) {}
 
   bool diagnoseAsError() override;
+
+  bool diagnoseMergedLiteralElements();
 };
 
 class MissingContextualConformanceFailure final : public ContextualFailure {

test/Constraints/diagnostics.swift

@@ -338,7 +338,7 @@ variadic(arrayWithOtherEltType) // expected-error {{cannot convert value of type
 variadic(1, arrayWithOtherEltType) // expected-error {{cannot convert value of type '[String]' to expected argument type 'Int'}}
 
 // FIXME: SR-11104
-variadic(["hello", "world"]) // expected-error {{cannot convert value of type 'String' to expected element type 'Int'}}
+variadic(["hello", "world"]) // expected-error 2 {{cannot convert value of type 'String' to expected element type 'Int'}}
 // expected-error@-1 {{cannot pass array of type '[Int]' as variadic arguments of type 'Int'}}
 // expected-note@-2 {{remove brackets to pass array elements directly}}
 

test/Parse/pointer_conversion.swift.gyb

@@ -164,7 +164,7 @@ func constPointerArguments(_ p: UnsafeMutablePointer<Int>,
   takesConstPointer([0, 1, 2])
   // <rdar://problem/22308330> QoI: CSDiags doesn't handle array -> pointer impl conversions well
   takesConstPointer([0.0, 1.0, 2.0])
-  // expected-error@-1 {{cannot convert value of type 'Double' to expected element type 'Int'}}
+  // expected-error@-1 3 {{cannot convert value of type 'Double' to expected element type 'Int'}}
 
   // We don't allow these conversions outside of function arguments.
   var x: UnsafePointer<Int> = &i // expected-error {{use of extraneous '&'}}

test/expr/cast/as_coerce.swift

@@ -84,13 +84,13 @@ c3 as C4 // expected-error {{'C3' is not convertible to 'C4'; did you mean to us
 Double(1) as Double as String // expected-error{{cannot convert value of type 'Double' to type 'String' in coercion}}
 ["awd"] as [Int] // expected-error{{cannot convert value of type 'String' to expected element type 'Int'}}
 ([1, 2, 1.0], 1) as ([String], Int)
-// expected-error@-1 {{cannot convert value of type 'Int' to expected element type 'String'}}
+// expected-error@-1 2 {{cannot convert value of type 'Int' to expected element type 'String'}}
 // expected-error@-2   {{cannot convert value of type 'Double' to expected element type 'String'}}
 [[1]] as [[String]] // expected-error{{cannot convert value of type 'Int' to expected element type 'String'}}
 (1, 1.0) as (Int, Int) // expected-error{{cannot convert value of type '(Int, Double)' to type '(Int, Int)' in coercion}}
 (1.0, 1, "asd") as (String, Int, Float) // expected-error{{cannot convert value of type '(Double, Int, String)' to type '(String, Int, Float)' in coercion}}
 (1, 1.0, "a", [1, 23]) as (Int, Double, String, [String])
-// expected-error@-1 {{cannot convert value of type 'Int' to expected element type 'String'}}
+// expected-error@-1 2 {{cannot convert value of type 'Int' to expected element type 'String'}}
 
 _ = [1] as! [String] // OK
 _ = [(1, (1, 1))] as! [(Int, (String, Int))] // OK