[QoI] Check before trying to emit fix-it to convert from array to dictionary

Before trying to replace every other "," with ":" to help convert
from array to dictionary, let's first check if it's appropriate e.g.
the number of commas matches the number of elements and number of
elements in the array is even which constitutes key/value pairs.

Resolves: SR-4952.

Author: xedin

lib/Sema/CSDiag.cpp

@@ -7254,11 +7254,12 @@ bool FailureDiagnosis::visitArrayExpr(ArrayExpr *E) {
         }
       }
     }
-    
+
+    auto numElements = E->getNumElements();
     if (!Conformance) {
       // If the contextual type conforms to ExpressibleByDictionaryLiteral and
       // this is an empty array, then they meant "[:]".
-      if (E->getNumElements() == 0 &&
+      if (numElements == 0 &&
           isDictionaryLiteralCompatible(contextualType, CS, E->getLoc())) {
         diagnose(E->getStartLoc(), diag::should_use_empty_dictionary_literal)
           .fixItInsert(E->getEndLoc(), ":");
@@ -7271,13 +7272,19 @@ bool FailureDiagnosis::visitArrayExpr(ArrayExpr *E) {
 
       // If the contextual type conforms to ExpressibleByDictionaryLiteral, then
       // they wrote "x = [1,2]" but probably meant "x = [1:2]".
-      if ((E->getElements().size() & 1) == 0 && !E->getElements().empty() &&
+      if ((numElements & 1) == 0 && numElements > 0 &&
           isDictionaryLiteralCompatible(contextualType, CS, E->getLoc())) {
         auto diag = diagnose(E->getStartLoc(), diag::meant_dictionary_lit);
 
-        // Change every other comma into a colon.
-        for (unsigned i = 0, e = E->getElements().size()/2; i != e; ++i)
-          diag.fixItReplace(E->getCommaLocs()[i*2], ":");
+        // Change every other comma into a colon, only if the number
+        // of commas present matches the number of elements, because
+        // otherwise it might a structural problem with the expression
+        // e.g. ["a""b": 1].
+        const auto commaLocs = E->getCommaLocs();
+        if (commaLocs.size() == numElements - 1) {
+          for (unsigned i = 0, e = numElements / 2; i != e; ++i)
+            diag.fixItReplace(commaLocs[i*2], ":");
+        }
       }
 
       return true;

test/Constraints/dictionary_literal.swift

@@ -97,3 +97,17 @@ func testDefaultExistentials() {
   let _ = ["a" : "hello", 17 : "string"]
   // expected-error@-1{{heterogeneous collection literal could only be inferred to 'Dictionary<AnyHashable, String>'}}
 }
+
+// SR-4952, rdar://problem/32330004 - Assertion failure during swift::ASTVisitor<::FailureDiagnosis,...>::visit
+func rdar32330004_1() -> [String: Any] {
+  return ["a""one": 1, "two": 2, "three": 3] // expected-note {{did you mean to use a dictionary literal instead?}}
+  // expected-error@-1 2 {{expected ',' separator}}
+  // expected-error@-2 {{expected expression in container literal}}
+  // expected-error@-3 {{contextual type '[String : Any]' cannot be used with array literal}}
+}
+
+func rdar32330004_2() -> [String: Any] {
+  return ["a", 0, "one", 1, "two", 2, "three", 3]
+  // expected-error@-1 {{contextual type '[String : Any]' cannot be used with array literal}}
+  // expected-note@-2 {{did you mean to use a dictionary literal instead?}} {{14-15=:}} {{24-25=:}} {{34-35=:}} {{46-47=:}}
+}