[Diagnostics] Improve diagnostics for mutating callAsFunction used on r-value

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -121,6 +121,12 @@ Expr *FailureDiagnostic::getBaseExprFor(Expr *anchor) const {
     return SE->getBase();
   else if (auto *MRE = dyn_cast<MemberRefExpr>(anchor))
     return MRE->getBase();
+  else if (auto *call = dyn_cast<CallExpr>(anchor)) {
+    auto fnType = getType(call->getFn());
+    if (fnType->isCallableNominalType(getDC())) {
+      return call->getFn();
+    }
+  }
 
   return nullptr;
 }

test/Sema/call_as_function_simple.swift

@@ -102,9 +102,7 @@ struct Mutating {
   }
 }
 func testMutating(_ x: Mutating, _ y: inout Mutating) {
-  // TODO(SR-11378): Improve this error to match the error using a direct `callAsFunction` member reference.
-  // expected-error @+2 {{cannot call value of non-function type 'Mutating'}}
-  // expected-error @+1 {{cannot invoke 'x' with no arguments}}
+  // expected-error @+1 {{cannot use mutating member on immutable value: 'x' is a 'let' constant}}
   _ = x()
   // expected-error @+1 {{cannot use mutating member on immutable value: 'x' is a 'let' constant}}
   _ = x.callAsFunction()