cmd/gc: fix spurious type errors in walkselect.

The lowering to runtime calls introduces hidden pointers to the
arguments of select clauses. When implicit conversions were
involved it could end up with incompatible pointers. Since the
pointed-to types have the same representation, we can introduce a
forced conversion.

Fixes #6847.

LGTM=rsc
R=rsc, iant, khr
CC=golang-codereviews
https://golang.org/cl/72380043

src/cmd/gc/select.c

@@ -214,6 +214,11 @@ walkselect(Node *sel)
 				n->left = nod(OADDR, n->left, N);
 				n->left->etype = 1;  // pointer does not escape
 				typecheck(&n->left, Erv);
+				if(!eqtype(ch->type->type, n->left->type->type)) {
+					n->left = nod(OCONVNOP, n->left, N);
+					n->left->type = ptrto(ch->type->type);
+					n->left->typecheck = 1;
+				}
 			} else {
 				tmp = temp(ch->type->type);
 				a = nod(OADDR, tmp, N);
@@ -330,6 +335,13 @@ walkselect(Node *sel)
 				n->right = nod(OADDR, n->right, N);
 				n->right->etype = 1;  // pointer does not escape
 				typecheck(&n->right, Erv);
+				// cast to appropriate type if necessary.
+				if(!eqtype(n->right->type->type, n->left->type->type) &&
+					assignop(n->right->type->type, n->left->type->type, nil) == OCONVNOP) {
+					n->right = nod(OCONVNOP, n->right, N);
+					n->right->type = ptrto(n->left->type->type);
+					n->right->typecheck = 1;
+				}
 				r->ntest = mkcall1(chanfn("selectsend", 2, n->left->type), types[TBOOL],
 					&r->ninit, var, n->left, n->right);
 				break;

test/fixedbugs/issue6847.go

+// compile
+
+// Copyright 2014 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.
+
+// Issue 6847: select clauses involving implicit conversion
+// of channels trigger a spurious typechecking error during walk.
+
+package p
+
+type I1 interface {
+	String()
+}
+type I2 interface {
+	String()
+}
+
+func F() {
+	var (
+		cr <-chan int
+		cs chan<- int
+		c  chan int
+
+		ccr chan (<-chan int)
+		ccs chan chan<- int
+		cc  chan chan int
+
+		ok bool
+	)
+	// Send cases.
+	select {
+	case ccr <- cr:
+	case ccr <- c:
+	}
+	select {
+	case ccs <- cs:
+	case ccs <- c:
+	}
+	select {
+	case ccr <- c:
+	default:
+	}
+	// Receive cases.
+	select {
+	case cr = <-cc:
+	case cs = <-cc:
+	case c = <-cc:
+	}
+	select {
+	case cr = <-cc:
+	default:
+	}
+	select {
+	case cr, ok = <-cc:
+	case cs, ok = <-cc:
+	case c = <-cc:
+	}
+      // Interfaces.
+	var (
+		c1 chan I1
+		c2 chan I2
+		x1 I1
+		x2 I2
+	)
+	select {
+	case c1 <- x1:
+	case c1 <- x2:
+	case c2 <- x1:
+	case c2 <- x2:
+	}
+	select {
+	case x1 = <-c1:
+	case x1 = <-c2:
+	case x2 = <-c1:
+	case x2 = <-c2:
+	}
+	select {
+	case x1, ok = <-c1:
+	case x1, ok = <-c2:
+	case x2, ok = <-c1:
+	case x2, ok = <-c2:
+	}
+	_ = ok
+}