fix rdar://25271859 QoI: Misleading error message when expression result can't be inferred from closure

We previously produced the error message:

rdar25271859.swift:14:11: error: value of tuple type '(Float, Int)' has no member '0'
  a.map { $0.0 }
          ^~ ~

We now produce:

rdar25271859.swift:15:5: error: generic parameter 'U' could not be inferred
   .andThen { dataResult in
    ^

which is at least is correct, if not yet helpful.

Author: lattner

lib/Sema/CSDiag.cpp

@@ -2211,7 +2211,21 @@ bool FailureDiagnosis::diagnoseGeneralMemberFailure(Constraint *constraint) {
     }
   }
 
-  if (baseObjTy->is<TupleType>()) {
+  // If this is a tuple, then the index needs to be valid.
+  if (auto tuple = baseObjTy->getAs<TupleType>()) {
+    StringRef nameStr = memberName.getBaseName().str();
+    int fieldIdx = -1;
+    // Resolve a number reference into the tuple type.
+    unsigned Value = 0;
+    if (!nameStr.getAsInteger(10, Value) && Value < tuple->getNumElements()) {
+      fieldIdx = Value;
+    } else {
+      fieldIdx = tuple->getNamedElementId(memberName.getBaseName());
+    }
+
+    if (fieldIdx != -1)
+      return false;    // Lookup is valid.
+
     diagnose(anchor->getLoc(), diag::could_not_find_tuple_member,
              baseObjTy, memberName)
       .highlight(anchor->getSourceRange()).highlight(memberRange);

test/Constraints/tuple.swift

@@ -179,4 +179,23 @@ func makeRequest() {
   }
 }
 
+// <rdar://problem/25271859> QoI: Misleading error message when expression result can't be inferred from closure
+struct r25271859<T> {
+}
+
+extension r25271859 {
+  func map<U>(f: (T) -> U) -> r25271859<U> {
+  }
+
+  func andThen<U>(f: (T)->r25271859<U>) {
+  }
+}
+
+func f(a : r25271859<(Float, Int)>) {
+  a.map { $0.0 }
+    .andThen { _ in   // expected-error {{generic parameter 'U' could not be inferred}}
+      print("hello") // comment this out and it runs, leave any form of print in and it doesn't
+      return Task<String>()
+  }
+}