fix <rdar://problem/27384685> QoI: Poor diagnostic when assigning a value to a method

We previously said:
  x.method = 1 // error: cannot assign to property: 'x' is immutable

we now say:
error: cannot assign to property: 'method' is a method

Author: lattner

lib/Sema/CSDiag.cpp

@@ -538,12 +538,17 @@ resolveImmutableBase(Expr *expr, ConstraintSystem &CS) {
   if (auto *UDE = dyn_cast<UnresolvedDotExpr>(expr)) {
     // If we found a decl for the UDE, check it.
     auto loc = CS.getConstraintLocator(UDE, ConstraintLocator::Member);
-    auto *member = dyn_cast_or_null<VarDecl>(findResolvedMemberRef(loc, CS));
-
-    // If the member isn't settable, then it is the problem: return it.
-    if (member) {
-      if (!member->isSettable(nullptr) ||
-          !member->isSetterAccessibleFrom(CS.DC))
+    
+    // If we can resolve a member, we can determine whether it is settable in
+    // this context.
+    if (auto *member = findResolvedMemberRef(loc, CS)) {
+      auto *memberVD = dyn_cast<VarDecl>(member);
+      
+      // If the member isn't a vardecl (e.g. its a funcdecl), or it isn't
+      // settable, then it is the problem: return it.
+      if (!memberVD ||
+          !member->isSettable(nullptr) ||
+          !memberVD->isSetterAccessibleFrom(CS.DC))
         return { expr, member };
     }
 
@@ -641,6 +646,22 @@ static void diagnoseSubElementFailure(Expr *destExpr,
       .highlight(immInfo.first->getSourceRange());
     return;
   }
+  
+  // If we're trying to set an unapplied method, say that.
+  if (auto *VD = dyn_cast_or_null<ValueDecl>(immInfo.second)) {
+    std::string message = "'";
+    message += VD->getName().str().str();
+    message += "'";
+    
+    if (auto *AFD = dyn_cast<AbstractFunctionDecl>(VD))
+      message += AFD->getImplicitSelfDecl() ? " is a method" : " is a function";
+    else
+      message += " is not settable";
+    
+    TC.diagnose(loc, diagID, message)
+      .highlight(immInfo.first->getSourceRange());
+    return;
+  }
 
   // If the expression is the result of a call, it is an rvalue, not a mutable
   // lvalue.

test/ClangModules/objc_parse.swift

@@ -119,7 +119,7 @@ func properties(_ b: B) {
   // An informal property cannot be made formal in a subclass. The
   // formal property is simply ignored.
   b.informalMadeFormal()
-  b.informalMadeFormal = i // expected-error{{cannot assign to property: 'b' is a 'let' constant}}
+  b.informalMadeFormal = i // expected-error{{cannot assign to property: 'informalMadeFormal' is a method}}
   b.setInformalMadeFormal(5)
 
   b.overriddenProp = 17

test/Sema/immutability.swift

@@ -563,3 +563,11 @@ func testConditional(b : Bool) {
 
   (b ? x : z.t).mutatingfunc() // expected-error {{cannot use mutating member on immutable value: result of conditional operator '? :' is never mutable}}
 }
+
+
+
+// <rdar://problem/27384685> QoI: Poor diagnostic when assigning a value to a method
+func f(a : FooClass, b : LetStructMembers) {
+  a.baz = 1 // expected-error {{cannot assign to property: 'baz' is a method}}
+  b.f = 42     // expected-error {{cannot assign to property: 'f' is a method}}
+}