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>

2 files changed, 65 insertions, 0 deletions

diff --git a/test/run.go b/test/run.go
index e17d9729bc..2ff7117ea9 100644
--- a/test/run.go
+++ b/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 {
diff --git a/test/typeparam/issue50002.go b/test/typeparam/issue50002.go
new file mode 100644
index 0000000000..670fc2eae3
--- /dev/null
+++ b/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)
+}