[Type checker] Fix assertion/spin with ill-formed property delegate initialization

Fixes rdar://problem/50822051.

Author: DougGregor

lib/Sema/TypeCheckConstraints.cpp

@@ -2486,7 +2486,8 @@ bool TypeChecker::typeCheckBinding(Pattern *&pattern, Expr *&initializer,
 
     /// Retrieve the type to which the pattern should be coerced.
     Type getPatternInitType() const {
-      if (!appliedPropertyDelegate || initType->hasError())
+      if (!appliedPropertyDelegate || initType->hasError() ||
+          initType->is<TypeVariableType>())
         return initType;
 
       // We applied a property delegate, so dig the pattern initialization

test/decl/var/property_delegates.swift

@@ -690,3 +690,22 @@ struct UsesExplicitClosures {
   @WrapperAcceptingAutoclosure(body: { return 42 })
   var y: Int
 }
+
+// ---------------------------------------------------------------------------
+// Miscellaneous bugs
+// ---------------------------------------------------------------------------
+
+// rdar://problem/50822051 - compiler assertion / hang
+@_propertyDelegate
+struct PD<Value> {
+  var value: Value
+
+  init<A>(initialValue: Value, a: A) { // expected-note{{'init(initialValue:a:)' declared here}}
+    self.value = initialValue
+  }
+}
+
+struct TestPD {
+  @PD(a: "foo") var foo: Int = 42 // expected-error{{property 'foo' with attached delegate cannot initialize both the delegate type and the property}}
+  // expected-error@-1{{missing argument for parameter 'initialValue' in call}}
+}