SILGen: Don't call _diagnoseUnavailableCodeReached() from already unreachable functions.

We must avoid emitting applies of `_diagnoseUnavailableCodeReached()` in
function bodies that are already marked unreachable since there isn't a valid
insertion point once an `unreachable` instruction has been emitted. A function
body may be marked unreachable if, for example, the parameters of the function
are uninhabited.

Resolves rdar://116246677

Author: tshortli

lib/SILGen/SILGenApply.cpp

@@ -5974,6 +5974,13 @@ RValue SILGenFunction::emitApplyOfLibraryIntrinsic(SILLocation loc,
 }
 
 void SILGenFunction::emitApplyOfUnavailableCodeReached() {
+  // Avoid attempting to insert a call if we don't have a valid insertion point.
+  // The insertion point could be invalid in, for instance, a function that
+  // takes uninhabited parameters and has therefore already has an unreachable
+  // instruction.
+  if (!B.hasValidInsertionPoint())
+    return;
+
   auto loc = RegularLocation::getAutoGeneratedLocation(F.getLocation());
   FuncDecl *fd = getASTContext().getDiagnoseUnavailableCodeReached();
 

test/SILGen/unavailable_decl_optimization_stub.swift

@@ -39,3 +39,12 @@ public func unavailableThrowingFunc() throws {
 // CHECK:       } // end sil function '$s4Test9globalVarAA1SVvau'
 @available(*, unavailable)
 public var globalVar = S()
+
+public enum Uninhabited {}
+
+//
+// CHECK-LABEL: sil{{.*}}@$s4Test28unavailableTakingUninhabitedyyAA0D0OF : $@convention(thin) (Uninhabited) -> () {
+// CHECK:         unreachable
+// CHECK:       } // end sil function '$s4Test28unavailableTakingUninhabitedyyAA0D0OF'
+@available(*, unavailable)
+public func unavailableTakingUninhabited(_ u: Uninhabited) {}