cmd/compile: add early a CONVIFACE normally created in the order phase

Most CONVIFACEs are created in the transform phase (or old typechecker,
in -G=0 mode). But if the main result of a multi-value assignment (map,
channel, or dot-type) must be converted to an interface during the
assignment, that CONVIFACE is not created until (*orderState).as2ok in
the order phase (because the AS2* ops and their sub-ops are so tightly
intertwined). But we need to create the CONVIFACE during the
stenciling/transform phase to enable dictionary lookups. So, in
transformAssign(), if we are doing a special multi-value assignment
involving a type-param-derived type, assign the results first to temps,
so that we can manifest the CONVIFACE during the transform in assigning
the first temp to lhs[0].

Added a test for both AS2RECV (channel receives) and AS2MAPR (maps). I
don't think we can have a type assertion on a type-param-derived type.

Fixes #50642

Change-Id: I4d079fc46c93d8494d7db4ea8234d91522edb02a
Reviewed-on: https://go-review.googlesource.com/c/go/+/379054
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -356,6 +356,37 @@ assignOK:
 		}
 		checkLHS(0, r.Type())
 		checkLHS(1, types.UntypedBool)
+		t := lhs[0].Type()
+		if t != nil && rhs[0].Type().HasShape() && t.IsInterface() && !types.IdenticalStrict(t, rhs[0].Type()) {
+			// This is a multi-value assignment (map, channel, or dot-type)
+			// where the main result is converted to an interface during the
+			// assignment. Normally, the needed CONVIFACE is not created
+			// until (*orderState).as2ok(), because the AS2* ops and their
+			// sub-ops are so tightly intertwined. But we need to create the
+			// CONVIFACE now to enable dictionary lookups. So, assign the
+			// results first to temps, so that we can manifest the CONVIFACE
+			// in assigning the first temp to lhs[0]. If we added the
+			// CONVIFACE into rhs[0] directly, we would break a lot of later
+			// code that depends on the tight coupling between the AS2* ops
+			// and their sub-ops. (Issue #50642).
+			v := typecheck.Temp(rhs[0].Type())
+			ok := typecheck.Temp(types.Types[types.TBOOL])
+			as := ir.NewAssignListStmt(base.Pos, stmt.Op(), []ir.Node{v, ok}, []ir.Node{r})
+			as.Def = true
+			as.PtrInit().Append(ir.NewDecl(base.Pos, ir.ODCL, v))
+			as.PtrInit().Append(ir.NewDecl(base.Pos, ir.ODCL, ok))
+			as.SetTypecheck(1)
+			// Change stmt to be a normal assignment of the temps to the final
+			// left-hand-sides. We re-create the original multi-value assignment
+			// so that it assigns to the temps and add it as an init of stmt.
+			//
+			// TODO: fix the order of evaluation, so that the lval of lhs[0]
+			// is evaluated before rhs[0] (similar to problem in #50672).
+			stmt.SetOp(ir.OAS2)
+			stmt.PtrInit().Append(as)
+			// assignconvfn inserts the CONVIFACE.
+			stmt.Rhs = []ir.Node{assignconvfn(v, t), ok}
+		}
 		return
 	}
 

test/typeparam/issue50642.go

@@ -0,0 +1,63 @@
+// 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.
+
+package main
+
+import "fmt"
+
+type Temp[T any] struct {
+}
+
+var temp, temp1 any
+var ch any
+
+func (it Temp[T]) HasNext() bool {
+	var ok bool
+	temp1 = <-ch.(chan T)
+	// test conversion of T to interface{} during an OAS2RECV
+	temp, ok = <-ch.(chan T)
+	return ok
+}
+
+type MyInt int
+
+func (i MyInt) String() string {
+	return "a"
+}
+
+type Stringer interface {
+	String() string
+}
+
+type Temp2[T Stringer] struct {
+}
+
+var temp2 Stringer
+
+func (it Temp2[T]) HasNext() string {
+	var x map[int]T
+
+	var ok bool
+	// test conversion of T to Stringer during an OAS2MAPR
+	temp2, ok = x[43]
+	_ = ok
+	return temp2.String()
+}
+
+func main() {
+	ch1 := make(chan int, 2)
+	ch1 <- 5
+	ch1 <- 6
+	ch = ch1
+	iter := Temp[int]{}
+	iter.HasNext()
+
+	iter2 := Temp2[MyInt]{}
+	if got, want := iter2.HasNext(), "a"; got != want {
+		panic(fmt.Sprintf("got %v, want %v", got, want))
+	}
+
+}