[dev.typeparams] cmd/compile: handle derived types that are converted to interfaces

Up to this point, we were only handling typeparams that were converted
to empty or non-empty interfaces. But we have a dictionary entry for
each derived type (i.e. type derived from typeparams) as well. So, when
doing a conversion, look for the source type in both the type params and
derived types of the generic info, and then use the appropriate
dictionary entry.

Added some cases to ifaceconv.go (e.g. converting []T to an empty
interface).

Change-Id: I7bbad0128bec20ccccd93ae1d65c1ffd44ca79a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/333011
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/noder/stencil.go

@@ -979,21 +979,14 @@ func getDictionaryEntry(pos src.XPos, dict *ir.Name, i int, size int) ir.Node {
 	return r
 }
 
-// getDictionaryType returns a *runtime._type from the dictionary corresponding to the input type.
-// The input type must be a type parameter (TODO: or a local derived type).
-func (subst *subster) getDictionaryType(pos src.XPos, t *types.Type) ir.Node {
-	tparams := subst.ts.Tparams
-	var i = 0
-	for i = range tparams {
-		if t == tparams[i] {
-			break
-		}
-	}
-	if i == len(tparams) {
-		base.Fatalf(fmt.Sprintf("couldn't find type param %+v", t))
+// getDictionaryType returns a *runtime._type from the dictionary entry i
+// (which refers to a type param or a derived type that uses type params).
+func (subst *subster) getDictionaryType(pos src.XPos, i int) ir.Node {
+	if i < 0 || i >= subst.info.startSubDict {
+		base.Fatalf(fmt.Sprintf("bad dict index %d", i))
 	}
 
-	r := getDictionaryEntry(pos, subst.info.dictParam, i, len(tparams))
+	r := getDictionaryEntry(pos, subst.info.dictParam, i, subst.info.startSubDict)
 	// change type of retrieved dictionary entry to *byte, which is the
 	// standard typing of a *runtime._type in the compiler
 	typed(types.Types[types.TUINT8].PtrTo(), r)
@@ -1134,11 +1127,12 @@ func (subst *subster) node(n ir.Node) ir.Node {
 				// type argument.
 				m = transformConvCall(call)
 				if m.Op() == ir.OCONVIFACE {
-					if srcType := x.(*ir.CallExpr).Args[0].Type(); srcType.IsTypeParam() { // TODO: or derived type
 					// Note: srcType uses x.Args[0], not m.X or call.Args[0], because
 					// we need the type before the type parameter -> type argument substitution.
+					srcType := x.(*ir.CallExpr).Args[0].Type()
+					if ix := subst.findDictType(srcType); ix >= 0 {
 						c := m.(*ir.ConvExpr)
-						m = subst.convertUsingDictionary(c.Pos(), c.X, c.Type(), srcType)
+						m = subst.convertUsingDictionary(c.Pos(), c.X, c.Type(), srcType, ix)
 					}
 				}
 
@@ -1240,8 +1234,9 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			x := x.(*ir.ConvExpr)
 			// Note: x's argument is still typed as a type parameter.
 			// m's argument now has an instantiated type.
-			if t := x.X.Type(); t.IsTypeParam() {
-				m = subst.convertUsingDictionary(x.Pos(), m.(*ir.ConvExpr).X, m.Type(), t)
+			t := x.X.Type()
+			if ix := subst.findDictType(t); ix >= 0 {
+				m = subst.convertUsingDictionary(x.Pos(), m.(*ir.ConvExpr).X, m.Type(), t, ix)
 			}
 		}
 		return m
@@ -1250,18 +1245,31 @@ func (subst *subster) node(n ir.Node) ir.Node {
 	return edit(n)
 }
 
-// convertUsingDictionary converts value v from generic type src to an interface type dst.
-func (subst *subster) convertUsingDictionary(pos src.XPos, v ir.Node, dst, src *types.Type) ir.Node {
-	// TODO: handle converting from derived types. For now, just from naked
-	// type parameters.
-	if !src.IsTypeParam() {
-		base.Fatalf("source must be a type parameter %+v", src)
+// findDictType looks for type t in the typeparams or derived types in the generic
+// function info subst.info.gfInfo. This will indicate the dictionary entry with the
+// correct concrete type for the associated instantiated function.
+func (subst *subster) findDictType(t *types.Type) int {
+	for i, dt := range subst.info.gfInfo.tparams {
+		if dt == t {
+			return i
 		}
+	}
+	for i, dt := range subst.info.gfInfo.derivedTypes {
+		if types.Identical(dt, t) {
+			return i + len(subst.info.gfInfo.tparams)
+		}
+	}
+	return -1
+}
+
+// convertUsingDictionary converts value v from instantiated type src (which is index
+// 'ix' in the instantiation's dictionary) to an interface type dst.
+func (subst *subster) convertUsingDictionary(pos src.XPos, v ir.Node, dst, src *types.Type, ix int) ir.Node {
 	if !dst.IsInterface() {
 		base.Fatalf("can only convert type parameters to interfaces %+v -> %+v", src, dst)
 	}
 	// Load the actual runtime._type of the type parameter from the dictionary.
-	rt := subst.getDictionaryType(pos, src)
+	rt := subst.getDictionaryType(pos, ix)
 
 	// Convert value to an interface type, so the data field is what we want.
 	if !v.Type().IsInterface() {

test/typeparam/ifaceconv.go

@@ -18,6 +18,13 @@ func f[T any](x T) interface{} {
 	var i interface{} = x
 	return i
 }
+
+func fs[T any](x T) interface{} {
+	y := []T{x}
+	var i interface{} = y
+	return i
+}
+
 func g[T any](x T) E {
 	var i E = x
 	return i
@@ -46,10 +53,18 @@ func j[T C](t T) C {
 	return C(t) // explicit conversion
 }
 
+func js[T any](x T) interface{} {
+	y := []T{x}
+	return interface{}(y)
+}
+
 func main() {
 	if got, want := f[int](7), 7; got != want {
 		panic(fmt.Sprintf("got %d want %d", got, want))
 	}
+	if got, want := fs[int](7), []int{7}; got.([]int)[0] != want[0] {
+		panic(fmt.Sprintf("got %d want %d", got, want))
+	}
 	if got, want := g[int](7), 7; got != want {
 		panic(fmt.Sprintf("got %d want %d", got, want))
 	}
@@ -62,4 +77,7 @@ func main() {
 	if got, want := j[myInt](7).foo(), 8; got != want {
 		panic(fmt.Sprintf("got %d want %d", got, want))
 	}
+	if got, want := js[int](7), []int{7}; got.([]int)[0] != want[0] {
+		panic(fmt.Sprintf("got %d want %d", got, want))
+	}
 }