go/ssa: Origin is only available after building

Origin of an instantiation is only available after the origin
function is done being built. Build the origin on demand if
not yet built.

Fixes golang/go#59427

Change-Id: I930ae496ce0d814890ea2d947100cdfe6ede059f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/491755
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tim King <taking@google.com>

go/ssa/ssa.go

@@ -1535,12 +1535,25 @@ func (fn *Function) TypeParams() *typeparams.TypeParamList {
 // from fn.Origin().
 func (fn *Function) TypeArgs() []types.Type { return fn.typeargs }
 
-// Origin is the function fn is an instantiation of. Returns nil if fn is not
-// an instantiation.
+// Origin returns the generic function from which fn was instantiated,
+// or nil if fn is not an instantiation.
 func (fn *Function) Origin() *Function {
 	if fn.parent != nil && len(fn.typeargs) > 0 {
-		// Nested functions are BUILT at a different time than there instances.
-		return fn.parent.Origin().AnonFuncs[fn.anonIdx]
+		// Nested functions are BUILT at a different time than their instances.
+		// Build declared package if not yet BUILT. This is not an expected use
+		// case, but is simple and robust.
+		fn.declaredPackage().Build()
+	}
+	return origin(fn)
+}
+
+// origin is the function that fn is an instantiation of. Returns nil if fn is
+// not an instantiation.
+//
+// Precondition: fn and the origin function are done building.
+func origin(fn *Function) *Function {
+	if fn.parent != nil && len(fn.typeargs) > 0 {
+		return origin(fn.parent).AnonFuncs[fn.anonIdx]
 	}
 	return fn.topLevelOrigin
 }