[Type checker] If any argument to an application failed to type-check, don’t check the application.

Eliminates an assertion that came up while investigating rdar://problem/27940842.

Author: DougGregor

lib/Sema/TypeCheckError.cpp

@@ -406,6 +406,12 @@ class ApplyClassifier {
     SmallVector<Expr*, 4> args;
     auto fnRef = AbstractFunction::decomposeApply(E, args);
 
+    // If any of the arguments didn't type check, fail.
+    for (auto arg : args) {
+      if (!arg->getType() || arg->getType()->is<ErrorType>())
+        return Classification::forInvalidCode();
+    }
+
     // If we're applying more arguments than the natural argument
     // count, then this is a call to the opaque value returned from
     // the function.

test/decl/func/operator.swift

@@ -345,3 +345,11 @@ class C5 {
     _ = x == x
   }
 }
+
+class C6 {
+  static func == (lhs: C6, rhs: C6) -> Bool { return false }
+
+  func test1(x: C6) {
+    if x == x && x = x { } // expected-error{{cannot assign to value: '&&' returns immutable value}}
+  }
+}