[go] types2, go/types: fix conversion error for pointer to type param

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Chencheng Jiang (Gerrit)

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Jun 6, 2026, 7:19:45 PM (6 hours ago) Jun 6
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types2, go/types: fix conversion error for pointer to type param

During conversion checking, assignableTo probes interface targets to
produce the helpful pointer-to-interface diagnostic. Because type
parameters have interface underlying types, that probe also matched
targets such as *A where A is a type parameter, leaking an assignability
error into generic conversion diagnostics.

Limit the pointer-to-interface probe to pointers whose base is a
non-type-parameter interface, and use interfacePtrError for consistency.
Add a fixedbugs test for conversions to *A.

Fixes #79883
Change-Id: Ie2c5860b6a13a782442716eac6276a97310f6ee3

Change diff

diff --git a/src/cmd/compile/internal/types2/instantiate.go b/src/cmd/compile/internal/types2/instantiate.go
index 3c4044e..fb7c553 100644
--- a/src/cmd/compile/internal/types2/instantiate.go
+++ b/src/cmd/compile/internal/types2/instantiate.go
@@ -245,7 +245,7 @@
 		if cause != nil {
 			var detail string
 			if isInterfacePtr(Tu) {
-				detail = check.sprintf("type %s is pointer to interface, not interface", T)
+				detail = check.interfacePtrError(T)
 			} else {
 				detail = check.sprintf("%s is not an interface", T)
 			}
diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index 107101f..b9744b4 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -564,6 +564,11 @@
 	return p != nil && IsInterface(p.base)
 }
 
+func isNonTypeParamInterfacePtr(T Type) bool {
+	p, _ := T.Underlying().(*Pointer)
+	return p != nil && isNonTypeParamInterface(p.base)
+}
+
 // check may be nil.
 func (check *Checker) interfacePtrError(T Type) string {
 	assert(isInterfacePtr(T))
diff --git a/src/cmd/compile/internal/types2/operand.go b/src/cmd/compile/internal/types2/operand.go
index 0e8aa24..26598c0 100644
--- a/src/cmd/compile/internal/types2/operand.go
+++ b/src/cmd/compile/internal/types2/operand.go
@@ -370,9 +370,9 @@
 	}
 
 	// T is an interface type, but not a type parameter, and V implements T.
-	// Also handle the case where T is a pointer to an interface so that we get
-	// the Checker.implements error cause.
-	if _, ok := Tu.(*Interface); ok && Tp == nil || isInterfacePtr(Tu) {
+	// Also handle the case where T is a pointer to a non-type-parameter
+	// interface so that we get the Checker.implements error cause.
+	if _, ok := Tu.(*Interface); ok && Tp == nil || isNonTypeParamInterfacePtr(Tu) {
 		if check.implements(V, T, false, cause) {
 			return true, 0
 		}
diff --git a/src/go/types/instantiate.go b/src/go/types/instantiate.go
index 6488494..b0df7b9 100644
--- a/src/go/types/instantiate.go
+++ b/src/go/types/instantiate.go
@@ -248,7 +248,7 @@
 		if cause != nil {
 			var detail string
 			if isInterfacePtr(Tu) {
-				detail = check.sprintf("type %s is pointer to interface, not interface", T)
+				detail = check.interfacePtrError(T)
 			} else {
 				detail = check.sprintf("%s is not an interface", T)
 			}
diff --git a/src/go/types/lookup.go b/src/go/types/lookup.go
index 0d056e7..5f9b54b 100644
--- a/src/go/types/lookup.go
+++ b/src/go/types/lookup.go
@@ -567,6 +567,11 @@
 	return p != nil && IsInterface(p.base)
 }
 
+func isNonTypeParamInterfacePtr(T Type) bool {
+	p, _ := T.Underlying().(*Pointer)
+	return p != nil && isNonTypeParamInterface(p.base)
+}
+
 // check may be nil.
 func (check *Checker) interfacePtrError(T Type) string {
 	assert(isInterfacePtr(T))
diff --git a/src/go/types/operand.go b/src/go/types/operand.go
index 860611a..7386b64 100644
--- a/src/go/types/operand.go
+++ b/src/go/types/operand.go
@@ -374,9 +374,9 @@
 	}
 
 	// T is an interface type, but not a type parameter, and V implements T.
-	// Also handle the case where T is a pointer to an interface so that we get
-	// the Checker.implements error cause.
-	if _, ok := Tu.(*Interface); ok && Tp == nil || isInterfacePtr(Tu) {
+	// Also handle the case where T is a pointer to a non-type-parameter
+	// interface so that we get the Checker.implements error cause.
+	if _, ok := Tu.(*Interface); ok && Tp == nil || isNonTypeParamInterfacePtr(Tu) {
 		if check.implements(V, T, false, cause) {
 			return true, 0
 		}
diff --git a/src/internal/types/testdata/fixedbugs/issue79883.go b/src/internal/types/testdata/fixedbugs/issue79883.go
new file mode 100644
index 0000000..fe97955
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue79883.go
@@ -0,0 +1,14 @@
+// Copyright 2026 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 p
+
+func _[A [1]int]() {
+	var s string
+	_ = (*A)(s /* ERRORx `^cannot convert s \(variable of type string\) to type \*A$` */)
+}
+
+func _[A [1]int](p *[1]int) {
+	_ = (*A)(p /* ERRORx `^cannot convert p \(variable of type \*\[1\]int\) to type \*A$` */)
+}

Change information

Files:
  • M src/cmd/compile/internal/types2/instantiate.go
  • M src/cmd/compile/internal/types2/lookup.go
  • M src/cmd/compile/internal/types2/operand.go
  • M src/go/types/instantiate.go
  • M src/go/types/lookup.go
  • M src/go/types/operand.go
  • A src/internal/types/testdata/fixedbugs/issue79883.go
Change size: S
Delta: 7 files changed, 32 insertions(+), 8 deletions(-)
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qiu laidongfeng (Gerrit)

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12:02 AM (2 hours ago) 12:02 AM
to Chencheng Jiang, goph...@pubsubhelper.golang.org, Robert Findley, Robert Griesemer, Gopher Robot, golang-co...@googlegroups.com
Attention needed from Chencheng Jiang, Robert Findley and Robert Griesemer

qiu laidongfeng voted Commit-Queue+1

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