cmd/compile: deal with unsatisfiable type assertion in some instantiations

Deal with case where a certain instantiation of a generic
function/method leads to an unsatisfiable type assertion or type case.
In that case, the compiler was causing a fatal error while trying to
create an impossible itab for the dictionary. To deal with that case,
allow ITabLsym() to create a dummy itab even when the concrete type
doesn't implement the interface. This dummy itab is analogous to the
"negative" itabs created on-the-fly by the runtime.

We will use the dummy itab in type asserts and type switches in
instantiations that use that dictionary entry. Since the dummy itab can
never be used for any real value at runtime (since the concrete type
doesn't implement the interface), there will always be a failure for the
corresponding type assertion or a non-match for the corresponding
type-switch case.

Fixes #50002

Change-Id: I1df05b1019533e1fc93dd7ab29f331a74fab9202
Reviewed-on: https://go-review.googlesource.com/c/go/+/369894
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/reflectdata/reflect.go

@@ -846,14 +846,19 @@ func TypePtr(t *types.Type) *ir.AddrExpr {
 	return typecheck.Expr(typecheck.NodAddr(n)).(*ir.AddrExpr)
 }
 
-// ITabLsym returns the LSym representing the itab for concreate type typ
-// implementing interface iface.
+// ITabLsym returns the LSym representing the itab for concrete type typ implementing
+// interface iface. A dummy tab will be created in the unusual case where typ doesn't
+// implement iface. Normally, this wouldn't happen, because the typechecker would
+// have reported a compile-time error. This situation can only happen when the
+// destination type of a type assert or a type in a type switch is parameterized, so
+// it may sometimes, but not always, be a type that can't implement the specified
+// interface.
 func ITabLsym(typ, iface *types.Type) *obj.LSym {
 	s, existed := ir.Pkgs.Itab.LookupOK(typ.LinkString() + "," + iface.LinkString())
 	lsym := s.Linksym()
 
 	if !existed {
-		writeITab(lsym, typ, iface)
+		writeITab(lsym, typ, iface, true)
 	}
 	return lsym
 }
@@ -865,7 +870,7 @@ func ITabAddr(typ, iface *types.Type) *ir.AddrExpr {
 	lsym := s.Linksym()
 
 	if !existed {
-		writeITab(lsym, typ, iface)
+		writeITab(lsym, typ, iface, false)
 	}
 
 	n := ir.NewLinksymExpr(base.Pos, lsym, types.Types[types.TUINT8])
@@ -1277,9 +1282,10 @@ func WriteRuntimeTypes() {
 	}
 }
 
-// writeITab writes the itab for concrete type typ implementing
-// interface iface.
-func writeITab(lsym *obj.LSym, typ, iface *types.Type) {
+// writeITab writes the itab for concrete type typ implementing interface iface. If
+// allowNonImplement is true, allow the case where typ does not implement iface, and just
+// create a dummy itab with zeroed-out method entries.
+func writeITab(lsym *obj.LSym, typ, iface *types.Type, allowNonImplement bool) {
 	// TODO(mdempsky): Fix methodWrapper, geneq, and genhash (and maybe
 	// others) to stop clobbering these.
 	oldpos, oldfn := base.Pos, ir.CurFunc
@@ -1306,7 +1312,8 @@ func writeITab(lsym *obj.LSym, typ, iface *types.Type) {
 			}
 		}
 	}
-	if len(sigs) != 0 {
+	completeItab := len(sigs) == 0
+	if !allowNonImplement && !completeItab {
 		base.Fatalf("incomplete itab")
 	}
 
@@ -1323,7 +1330,12 @@ func writeITab(lsym *obj.LSym, typ, iface *types.Type) {
 	o = objw.Uint32(lsym, o, types.TypeHash(typ)) // copy of type hash
 	o += 4                                        // skip unused field
 	for _, fn := range entries {
-		o = objw.SymPtrWeak(lsym, o, fn, 0) // method pointer for each method
+		if !completeItab {
+			// If typ doesn't implement iface, make method entries be zero.
+			o = objw.Uintptr(lsym, o, 0)
+		} else {
+			o = objw.SymPtrWeak(lsym, o, fn, 0) // method pointer for each method
+		}
 	}
 	// Nothing writes static itabs, so they are read only.
 	objw.Global(lsym, int32(o), int16(obj.DUPOK|obj.RODATA))

test/run.go

@@ -2182,6 +2182,7 @@ var unifiedFailures = setOf(
 	"fixedbugs/issue42058b.go", // unified IR doesn't report channel element too large
 	"fixedbugs/issue49767.go",  // unified IR doesn't report channel element too large
 	"fixedbugs/issue49814.go",  // unified IR doesn't report array type too large
+	"typeparam/issue50002.go",  // pure stenciling leads to a static type assertion error
 )
 
 func setOf(keys ...string) map[string]bool {

test/typeparam/issue50002.go

@@ -0,0 +1,64 @@
+// run -gcflags=-G=3
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test for cases where certain instantiations of a generic function (F in this
+// example) will always fail on a type assertion or mismatch on a type case.
+
+package main
+
+import "fmt"
+
+type S struct{}
+
+func (S) M() byte {
+	return 0
+}
+
+type I[T any] interface {
+	M() T
+}
+
+func F[T, A any](x I[T], shouldMatch bool) {
+	switch x.(type) {
+	case A:
+		if !shouldMatch {
+			fmt.Printf("wanted mis-match, got match")
+		}
+	default:
+		if shouldMatch {
+			fmt.Printf("wanted match, got mismatch")
+		}
+	}
+
+	_, ok := x.(A)
+	if ok != shouldMatch {
+		fmt.Printf("ok: got %v, wanted %v", ok, shouldMatch)
+	}
+
+	if !shouldMatch {
+		defer func() {
+			if shouldMatch {
+				fmt.Printf("Shouldn't have panicked")
+			}
+			recover()
+		}()
+	}
+	_ = x.(A)
+	if !shouldMatch {
+		fmt.Printf("Should have panicked")
+	}
+}
+
+func main() {
+	// Test instantiation where the type switch/type asserts can't possibly succeed
+	// (since string does not implement I[byte]).
+	F[byte, string](S{}, false)
+
+	// Test instantiation where the type switch/type asserts should succeed
+	// (since S does implement I[byte])
+	F[byte, S](S{}, true)
+	F[byte, S](I[byte](S{}), true)
+}