Fix <rdar://problem/24267414> QoI: error: cannot convert value of type 'Int' to specified type 'Int'

When a contextual conversion has a matching type, don't diagnose it as a
conversion error, the problem is due to something else (in this case, an
unresolved archetype somewhere else in the expression).

Before:
t.swift:6:17: error: cannot convert value of type 'Int' to specified type 'Int'
After:
t.swift:6:17: error: generic parameter 'T' could not be inferred

This should still be a bit better to provide information about where the T
archetype came from, but at least now it isn't completely wrong diagnostic.

Author: lattner

lib/Sema/CSDiag.cpp

@@ -2894,7 +2894,8 @@ bool FailureDiagnosis::diagnoseContextualConversionError() {
       return true;
     }
 
-    if (isUnresolvedOrTypeVarType(exprType))
+    if (isUnresolvedOrTypeVarType(exprType) ||
+        exprType->isEqual(contextualType))
       return false;
 
     // The conversion destination of throw is always ErrorType (at the moment)
@@ -2960,9 +2961,10 @@ bool FailureDiagnosis::diagnoseContextualConversionError() {
   }
 
   // If we don't have a type for the expression, then we cannot use it in
-  // conversion constraint diagnostic generation.
-  if (isUnresolvedOrTypeVarType(exprType)) {
-    // We can't do anything smart.
+  // conversion constraint diagnostic generation.  If the types match, then it
+  // must not be the contextual type that is the problem.
+  if (isUnresolvedOrTypeVarType(exprType) ||
+      exprType->isEqual(contextualType)) {
     return false;
   }
 

test/Constraints/generics.swift

@@ -184,3 +184,9 @@ func r22509125<T>(a : T?) { // expected-note {{in call to function 'r22509125'}}
 }
 
 
+// <rdar://problem/24267414> QoI: error: cannot convert value of type 'Int' to specified type 'Int'
+struct R24267414<T> {
+  static func foo() -> Int {}
+}
+var _ : Int = R24267414.foo() // expected-error {{generic parameter 'T' could not be inferred}}
+