cmd/compile: disallow nointerface method satisfying type constraint

Type constraint satisfaction is interface satisfaction. If a type
does not satisfy a regular interface, it should also not satisfy
the same interface as a type constraint. Otherwise, if the type
satisfies the type constraint, one can use that to construct a
(dynamic) interface value with a type that doesn't actually
satisfy the interface. The go:nointerface directive tells the
compiler that the method does not satisfy an interface. Therefore
it should also not satisfy a type constraint.

Fixes #74626.

Change-Id: I7c64c76044a665755e4e74035085daff42447f9e
Reviewed-on: https://go-review.googlesource.com/c/go/+/772620
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

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

@@ -175,6 +175,10 @@ type pragmas struct {
 	WasmExport *WasmExport
 }
 
+func (p *pragmas) Nointerface() bool {
+	return p.Flag&ir.Nointerface != 0
+}
+
 // WasmImport stores metadata associated with the //go:wasmimport pragma
 type WasmImport struct {
 	Pos    syntax.Pos

src/cmd/compile/internal/types2/decl.go

@@ -684,6 +684,12 @@ func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
 	fdecl := decl.fdecl
 	check.funcType(sig, fdecl.Recv, fdecl.TParamList, fdecl.Type)
 
+	if fdecl.Pragma != nil {
+		if p, ok := fdecl.Pragma.(interface{ Nointerface() bool }); ok && p.Nointerface() {
+			obj.nointerface = true
+		}
+	}
+
 	// Set the scope's extent to the complete "func (...) { ... }"
 	// so that Scope.Innermost works correctly.
 	sig.scope.pos = fdecl.Pos()

src/cmd/compile/internal/types2/lookup.go

@@ -390,6 +390,7 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 		ambigSel
 		ptrRecv
 		field
+		nointerface
 	)
 
 	state := ok
@@ -453,6 +454,11 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 				check.objDecl(f)
 			}
 
+			if f.nointerface {
+				state = nointerface
+				break
+			}
+
 			if !equivalent(f.typ, m.typ) {
 				state = wrongSig
 				break
@@ -512,6 +518,8 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 			*cause = check.sprintf("(method %s has pointer receiver)", m.Name())
 		case field:
 			*cause = check.sprintf("(%s.%s is a field, not a method)", V, m.Name())
+		case nointerface:
+			*cause = check.sprintf("(%s method is marked 'nointerface')", m.Name())
 		default:
 			panic("unreachable")
 		}

src/cmd/compile/internal/types2/object.go

@@ -396,8 +396,9 @@ func (*Var) isDependency() {} // a variable may be a dependency of an initializa
 // An abstract method may belong to many interfaces due to embedding.
 type Func struct {
 	object
-	hasPtrRecv_ bool  // only valid for methods that don't have a type yet; use hasPtrRecv() to read
 	origin      *Func // if non-nil, the Func from which this one was instantiated
+	hasPtrRecv_ bool  // only valid for methods that don't have a type yet; use hasPtrRecv() to read
+	nointerface bool
 }
 
 // NewFunc returns a new function with the given signature, representing
@@ -412,7 +413,7 @@ func NewFunc(pos syntax.Pos, pkg *Package, name string, sig *Signature) *Func {
 		// as this would violate object.{Type,color} invariants.
 		// TODO(adonovan): propose to disallow NewFunc with nil *Signature.
 	}
-	return &Func{object{nil, pos, pkg, name, typ, 0, nopos}, false, nil}
+	return &Func{object{nil, pos, pkg, name, typ, 0, nopos}, nil, false, false}
 }
 
 // Signature returns the signature (type) of the function or method.

src/go/types/decl.go

@@ -771,6 +771,9 @@ func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
 	fdecl := decl.fdecl
 	check.funcType(sig, fdecl.Recv, fdecl.Type)
 
+	// types2 handles go:nointerface pragma here by setting obj.nointerface.
+	// go/types currently doesn't handle pragmas.
+
 	// Set the scope's extent to the complete "func (...) { ... }"
 	// so that Scope.Innermost works correctly.
 	sig.scope.pos = fdecl.Pos()

src/go/types/lookup.go

@@ -393,6 +393,7 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 		ambigSel
 		ptrRecv
 		field
+		nointerface
 	)
 
 	state := ok
@@ -456,6 +457,11 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 				check.objDecl(f)
 			}
 
+			if f.nointerface {
+				state = nointerface
+				break
+			}
+
 			if !equivalent(f.typ, m.typ) {
 				state = wrongSig
 				break
@@ -515,6 +521,8 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 			*cause = check.sprintf("(method %s has pointer receiver)", m.Name())
 		case field:
 			*cause = check.sprintf("(%s.%s is a field, not a method)", V, m.Name())
+		case nointerface:
+			*cause = check.sprintf("(%s method is marked 'nointerface')", m.Name())
 		default:
 			panic("unreachable")
 		}

src/go/types/object.go

@@ -399,8 +399,9 @@ func (*Var) isDependency() {} // a variable may be a dependency of an initializa
 // An abstract method may belong to many interfaces due to embedding.
 type Func struct {
 	object
-	hasPtrRecv_ bool  // only valid for methods that don't have a type yet; use hasPtrRecv() to read
 	origin      *Func // if non-nil, the Func from which this one was instantiated
+	hasPtrRecv_ bool  // only valid for methods that don't have a type yet; use hasPtrRecv() to read
+	nointerface bool
 }
 
 // NewFunc returns a new function with the given signature, representing
@@ -415,7 +416,7 @@ func NewFunc(pos token.Pos, pkg *Package, name string, sig *Signature) *Func {
 		// as this would violate object.{Type,color} invariants.
 		// TODO(adonovan): propose to disallow NewFunc with nil *Signature.
 	}
-	return &Func{object{nil, pos, pkg, name, typ, 0, nopos}, false, nil}
+	return &Func{object{nil, pos, pkg, name, typ, 0, nopos}, nil, false, false}
 }
 
 // Signature returns the signature (type) of the function or method.

test/fixedbugs/issue74626.go

@@ -0,0 +1,25 @@
+// errorcheck -goexperiment fieldtrack
+
+// 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 main
+
+type Fooer interface {
+	Foo() string
+}
+
+type FooImpl struct{}
+
+//go:nointerface
+func (FooImpl) Foo() string { return "foo" }
+
+func toInterface[T Fooer](fooer T) Fooer {
+	return fooer
+}
+
+func main() {
+	var iface Fooer = toInterface(FooImpl{}) // ERROR "does not satisfy Fooer"
+	iface.Foo()
+}