cmd/go: cgo export header to be compatible with MSVC complex types

After CL 379474 has landed, the only remaining cgo export header
incompatibility with MSVC is the use of the _Complex macro,
which is not supported in MSVC even when it is part of the ISO C99
standard (1).

Since MSVC 2015 (2), complex math are supported via _Fcomplex and
_Dcomplex, which are equivalent to float _Complex and double _Complex.

As MSVC and C complex types have the same memory layout, we should
be able to typedef GoComplex64 and GoComplex128 to the appropriate
type in MSVC.

It is important to note that this CL is not adding MSVC support to cgo.
C compilers should still be GCC-compatible.

This CL is about allowing to include, without further modifications,
a DLL export header generated by cgo, normally using Mingw-W64 compiler,
into a MSVC project. This was already possible if the export header
changes introduced in this CL were done outside cgo, either manually or
in a post-build script.

Fixes #36233

1: https://docs.microsoft.com/en-us/cpp/c-runtime-library/complex-math-support
2: https://docs.microsoft.com/en-us/cpp/overview/visual-cpp-language-conformance?c-standard-library-features-1
Change-Id: Iad8f26984b115c728e3b73f3a8334ade7a11cfa1
Reviewed-on: https://go-review.googlesource.com/c/go/+/397134
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Auto-Submit: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

misc/cgo/testcshared/cshared_test.go

@@ -5,6 +5,7 @@
 package cshared_test
 
 import (
+	"bufio"
 	"bytes"
 	"debug/elf"
 	"debug/pe"
@@ -838,3 +839,51 @@ func TestGo2C2Go(t *testing.T) {
 	run(t, goenv, "go", "build", "-o", bin, "./go2c2go/m2")
 	runExe(t, runenv, bin)
 }
+
+func TestIssue36233(t *testing.T) {
+	t.Parallel()
+
+	// Test that the export header uses GoComplex64 and GoComplex128
+	// for complex types.
+
+	tmpdir, err := os.MkdirTemp("", "cshared-TestIssue36233")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	const exportHeader = "issue36233.h"
+
+	run(t, nil, "go", "tool", "cgo", "-exportheader", exportHeader, "-objdir", tmpdir, "./issue36233/issue36233.go")
+	data, err := os.ReadFile(exportHeader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	funcs := []struct{ name, signature string }{
+		{"exportComplex64", "GoComplex64 exportComplex64(GoComplex64 v)"},
+		{"exportComplex128", "GoComplex128 exportComplex128(GoComplex128 v)"},
+		{"exportComplexfloat", "GoComplex64 exportComplexfloat(GoComplex64 v)"},
+		{"exportComplexdouble", "GoComplex128 exportComplexdouble(GoComplex128 v)"},
+	}
+
+	scanner := bufio.NewScanner(bytes.NewReader(data))
+	var found int
+	for scanner.Scan() {
+		b := scanner.Bytes()
+		for _, fn := range funcs {
+			if bytes.Contains(b, []byte(fn.name)) {
+				found++
+				if !bytes.Contains(b, []byte(fn.signature)) {
+					t.Errorf("function signature mismatch; got %q, want %q", b, fn.signature)
+				}
+			}
+		}
+	}
+	if err = scanner.Err(); err != nil {
+		t.Errorf("scanner encountered error: %v", err)
+	}
+	if found != len(funcs) {
+		t.Error("missing functions")
+	}
+}

misc/cgo/testcshared/testdata/issue36233/issue36233.go

@@ -0,0 +1,29 @@
+// Copyright 2022 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
+
+// #include <complex.h>
+import "C"
+
+//export exportComplex64
+func exportComplex64(v complex64) complex64 {
+	return v
+}
+
+//export exportComplex128
+func exportComplex128(v complex128) complex128 {
+	return v
+}
+
+//export exportComplexfloat
+func exportComplexfloat(v C.complexfloat) C.complexfloat {
+	return v
+}
+
+//export exportComplexdouble
+func exportComplexdouble(v C.complexdouble) C.complexdouble {
+	return v
+}
+
+func main() {}

src/cmd/cgo/out.go

@@ -1399,6 +1399,19 @@ func (p *Package) cgoType(e ast.Expr) *Type {
 	case *ast.ChanType:
 		return &Type{Size: p.PtrSize, Align: p.PtrSize, C: c("GoChan")}
 	case *ast.Ident:
+		goTypesFixup := func(r *Type) *Type {
+			if r.Size == 0 { // int or uint
+				rr := new(Type)
+				*rr = *r
+				rr.Size = p.IntSize
+				rr.Align = p.IntSize
+				r = rr
+			}
+			if r.Align > p.PtrSize {
+				r.Align = p.PtrSize
+			}
+			return r
+		}
 		// Look up the type in the top level declarations.
 		// TODO: Handle types defined within a function.
 		for _, d := range p.Decl {
@@ -1417,6 +1430,17 @@ func (p *Package) cgoType(e ast.Expr) *Type {
 			}
 		}
 		if def := typedef[t.Name]; def != nil {
+			if defgo, ok := def.Go.(*ast.Ident); ok {
+				switch defgo.Name {
+				case "complex64", "complex128":
+					// MSVC does not support the _Complex keyword
+					// nor the complex macro.
+					// Use GoComplex64 and GoComplex128 instead,
+					// which are typedef-ed to a compatible type.
+					// See go.dev/issues/36233.
+					return goTypesFixup(goTypes[defgo.Name])
+				}
+			}
 			return def
 		}
 		if t.Name == "uintptr" {
@@ -1430,17 +1454,7 @@ func (p *Package) cgoType(e ast.Expr) *Type {
 			return &Type{Size: 2 * p.PtrSize, Align: p.PtrSize, C: c("GoInterface")}
 		}
 		if r, ok := goTypes[t.Name]; ok {
-			if r.Size == 0 { // int or uint
-				rr := new(Type)
-				*rr = *r
-				rr.Size = p.IntSize
-				rr.Align = p.IntSize
-				r = rr
-			}
-			if r.Align > p.PtrSize {
-				r.Align = p.PtrSize
-			}
-			return r
+			return goTypesFixup(r)
 		}
 		error_(e.Pos(), "unrecognized Go type %s", t.Name)
 		return &Type{Size: 4, Align: 4, C: c("int")}
@@ -1895,8 +1909,14 @@ typedef GoUintGOINTBITS GoUint;
 typedef size_t GoUintptr;
 typedef float GoFloat32;
 typedef double GoFloat64;
+#ifdef _MSC_VER
+#include <complex.h>
+typedef _Fcomplex GoComplex64;
+typedef _Dcomplex GoComplex128;
+#else
 typedef float _Complex GoComplex64;
 typedef double _Complex GoComplex128;
+#endif
 
 /*
   static assertion to make sure the file is being used on architecture