[TypeChecker] Diagnose empty switch statements in function builder bodies

If there are no 'case' statements in the body let's try
to diagnose this situation via limited exhaustiveness check
before failing a builder transform, otherwise type-checker
might end up without any diagnostics which leads to crashes
in SILGen.

Resolves: rdar://problem/65983237

Author: xedin

lib/Sema/BuilderTransform.cpp

@@ -699,6 +699,18 @@ class BuilderClosureVisitor
     if (!cs)
       return nullptr;
 
+    // If there are no 'case' statements in the body let's try
+    // to diagnose this situation via limited exhaustiveness check
+    // before failing a builder transform, otherwise type-checker
+    // might end up without any diagnostics which leads to crashes
+    // in SILGen.
+    if (capturedCaseVars.empty()) {
+      TypeChecker::checkSwitchExhaustiveness(switchStmt, dc,
+                                             /*limitChecking=*/true);
+      hadError = true;
+      return nullptr;
+    }
+
     // Form the expressions that inject the result of each case into the
     // appropriate
     llvm::TinyPtrVector<Expr *> injectedCaseExprs;

test/Constraints/function_builder_diags.swift

@@ -584,4 +584,9 @@ struct MyView {
     // expected-note@-1 {{opaque return type declared here}}
     DoesNotConform()
   }
+
+  @TupleBuilder var emptySwitch: some P {
+    switch Optional.some(1) { // expected-error {{'switch' statement body must have at least one 'case' or 'default' block; do you want to add a default case?}}
+    }
+  }
 }