[interop] avoid importing math functions from the C++ standard library

They cause ambiguities with math functions from platform's libc

Author: hyp

lib/ClangImporter/ImportDecl.cpp

@@ -3121,6 +3121,36 @@ namespace {
       if (decl->isDeleted())
         return nullptr;
 
+      if (Impl.SwiftContext.LangOpts.EnableCXXInterop &&
+          !isa<clang::CXXMethodDecl>(decl)) {
+        // Do not import math functions from the C++ standard library, as
+        // they're also imported from the Darwin/Glibc module, and their
+        // presence in the C++ standard library will cause overloading
+        // ambiguities or other type checking errors in Swift.
+        auto isAlternativeCStdlibFunctionFromTextualHeader =
+            [](const clang::FunctionDecl *d) -> bool {
+          // stdlib.h might be a textual header in libc++'s module map.
+          // in this case, check for known ambiguous functions by their name
+          // instead of checking if they come from the `std` module.
+          if (!d->getDeclName().isIdentifier())
+            return false;
+          return d->getName() == "abs" || d->getName() == "div";
+        };
+        if (decl->getOwningModule() &&
+            (decl->getOwningModule()
+                     ->getTopLevelModule()
+                     ->getFullModuleName() == "std" ||
+             isAlternativeCStdlibFunctionFromTextualHeader(decl))) {
+          auto filename =
+              Impl.getClangPreprocessor().getSourceManager().getFilename(
+                  decl->getLocation());
+          if (filename.endswith("cmath") || filename.endswith("math.h") ||
+              filename.endswith("stdlib.h") || filename.endswith("cstdlib")) {
+            return nullptr;
+          }
+        }
+      }
+
       auto dc =
           Impl.importDeclContextOf(decl, importedName.getEffectiveContext());
       if (!dc)

test/Interop/Cxx/stdlib/avoid-import-cxx-math.swift

@@ -0,0 +1,23 @@
+// RUN: %target-swift-frontend %s -typecheck -verify -enable-experimental-cxx-interop -Xcc -std=c++17
+
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+import CxxStdlib
+
+func test() {
+    let x: Float = 1.0
+    let y: Double = 2.0
+
+    // Note: we dispatch `pow(Float,Double)`
+    // to ensure we don't pick up the
+    // C++ stdlib `pow` function template.
+    // The `pow` function is still reexported
+    // from Darwin via CxxStdlib, so there are
+    // matching overloads that can be found still.
+    // Note: the error is different on Glibc instead
+    // of Darwin, so do not check the exact error.
+    let _ =  CxxStdlib.pow(x, y) // expected-error {{}}
+
+    let _ = CxxStdlib.abs(x) // expected-error {{module 'CxxStdlib' has no member named 'abs'}}
+    let _ = CxxStdlib.div(x) // expected-error {{module 'CxxStdlib' has no member named 'div'}}
+}

test/Interop/Cxx/stdlib/import-cxx-math-ambiguities.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-frontend %s -typecheck -enable-experimental-cxx-interop -Xcc -std=c++17
+
+#if canImport(Foundation)
+// Foundation depends on C++ standard library
+// on some platforms already, and as such
+// may bring math functions from it.
+import Foundation
+
+func test() -> Float {
+  let x: Float = 1.0
+  // Note: we mix 'Float' and 'Double' (literal)
+  // intentionally, as this might trigger Swift
+  // to instantiate a function template from
+  // the C++ standard library.
+  let z = pow(x, 2.0)
+  let _ = abs(x)
+  let m = z + 1.0
+  return m
+}
+#endif