I am new to Eigen. Can someone explain why line 3 below does not compile (line 2 is fine)?

VectorXcd v(3);
complex<double> t1 = v.transpose()*v;
t1 = v.transpose()*v;

I am running this on MacOS 10.8.3 using Xcode 4.6.2.

The error message is
No viable conversion from 'const typename ProductReturnType<CwiseUnaryOp<scalar_conjugate_op<complex<double> >, const Transpose<const Matrix<complex<double>, -1, 1, 0, -1, 1> > >, Matrix<complex<double>, -1, 1, 0, -1, 1> >::Type' (aka 'const GeneralProduct<Eigen::CwiseUnaryOp<Eigen::internal::scalar_conjugate_op<std::__1::complex<double> >, const Eigen::Transpose<const Eigen::Matrix<std::__1::complex<double>, -1, 1, 0, -1, 1> > >, Eigen::Matrix<std::__1::complex<double>, -1, 1, 0, -1, 1>, 3>') to 'std::__1::complex<double>'

Thanks...

Works fine for me with gcc and clang. Also, the error message does not seem to match the line your are pointing out because it presents a CwiseUnaryOp<scalar_conjugate_op<complex<double> > (produced by a .conjugate() or .adjoint()).

Thanks,
Based on this I changed the compiler option from Apple LLVM compiler 4.2 (default) to LLVM GCC 4.2, and it compiles fine.
You are correct about the mismatch, that was from a line using adjoint() instead of transpose().
I am beginning to learn my lesson: never use the Apple compiler. Here is the code and the error if I do (with 2 lines producing essentially the same error:

#include <iostream>
#include <complex>
#include <Eigen/Dense>


int main(int argc, const char * argv[])
{
Eigen::VectorXcd vc2(3);
std::complex<double> t1 = vc2.transpose()*vc2;//Okay

std::complex<double> t2 (vc2.transpose()*vc2); //error
t1 = vc2.transpose()*vc2; //error
return 0;
}
/* Error message:

.../main.cpp:10:10: No viable conversion from 'const typename ProductReturnType<Transpose<Matrix<complex<double>, -1, 1, 0, -1, 1> >, Matrix<complex<double>, -1, 1, 0, -1, 1> >::Type' (aka 'const GeneralProduct<Eigen::Transpose<Eigen::Matrix<std::__1::complex<double>, -1, 1, 0, -1, 1> >, Eigen::Matrix<std::__1::complex<double>, -1, 1, 0, -1, 1>, 3>') to 'std::__1::complex<double>'

*/

I'm still surprised. Your example does compile fine with "Apple LLVM version 4.2 (clang-425.0.24) (based on LLVM 3.2svn)". What are the compilation flags?

Not sure I grasp all the compiler options, but it still is not working for me. Here is what I get by copying the compiler options from the Xcode target, if that helps you understand what is going on:

//:configuration = Debug
GCC_VERSION = com.apple.compilers.llvm.clang.1_0

//:configuration = Release
GCC_VERSION = com.apple.compilers.llvm.clang.1_0

//:completeSettings = some
BUILD_VARIANTS
GCC_VERSION
DEBUG_INFORMATION_FORMAT
GENERATE_PROFILING_CODE
PRECOMPS_INCLUDE_HEADERS_FROM_BUILT_PRODUCTS_DIR
RUN_CLANG_STATIC_ANALYZER
SCAN_ALL_SOURCE_FILES_FOR_INCLUDES
VALIDATE_PRODUCT
GCC_FAST_OBJC_DISPATCH
CLANG_X86_VECTOR_INSTRUCTIONS
GCC_STRICT_ALIASING
GCC_GENERATE_DEBUGGING_SYMBOLS
GCC_DYNAMIC_NO_PIC
GCC_GENERATE_TEST_COVERAGE_FILES
GCC_INLINES_ARE_PRIVATE_EXTERN
GCC_INSTRUMENT_PROGRAM_FLOW_ARCS
GCC_ENABLE_KERNEL_DEVELOPMENT
LLVM_LTO
GCC_REUSE_STRINGS
GCC_NO_COMMON_BLOCKS
GCC_OPTIMIZATION_LEVEL
GCC_FAST_MATH
GCC_THREADSAFE_STATICS
GCC_SYMBOLS_PRIVATE_EXTERN
GCC_UNROLL_LOOPS
GCC_CHAR_IS_UNSIGNED_CHAR
GCC_ENABLE_ASM_KEYWORD
GCC_C_LANGUAGE_STANDARD
CLANG_CXX_LANGUAGE_STANDARD
CLANG_CXX_LIBRARY
GCC_CW_ASM_SYNTAX
GCC_INPUT_FILETYPE
GCC_ENABLE_CPP_EXCEPTIONS
GCC_ENABLE_CPP_RTTI
GCC_LINK_WITH_DYNAMIC_LIBRARIES
GCC_ENABLE_OBJC_EXCEPTIONS
GCC_ENABLE_TRIGRAPHS
GCC_ENABLE_FLOATING_POINT_LIBRARY_CALLS
GCC_USE_INDIRECT_FUNCTION_CALLS
GCC_USE_REGISTER_FUNCTION_CALLS
CLANG_LINK_OBJC_RUNTIME
GCC_INCREASE_PRECOMPILED_HEADER_SHARING
CLANG_ENABLE_OBJC_ARC
OTHER_CFLAGS
OTHER_CPLUSPLUSFLAGS
GCC_PRECOMPILE_PREFIX_HEADER
GCC_PREFIX_HEADER
GCC_ENABLE_BUILTIN_FUNCTIONS
GCC_ENABLE_PASCAL_STRINGS
GCC_SHORT_ENUMS
GCC_USE_STANDARD_INCLUDE_SEARCHING
GCC_PREPROCESSOR_DEFINITIONS
GCC_PREPROCESSOR_DEFINITIONS_NOT_USED_IN_PRECOMPS
GCC_WARN_INHIBIT_ALL_WARNINGS
GCC_WARN_PEDANTIC
GCC_TREAT_WARNINGS_AS_ERRORS
GCC_WARN_CHECK_SWITCH_STATEMENTS
GCC_WARN_ABOUT_DEPRECATED_FUNCTIONS
CLANG_WARN_EMPTY_BODY
GCC_WARN_FOUR_CHARACTER_CONSTANTS
GCC_WARN_SHADOW
CLANG_WARN_CONSTANT_CONVERSION
GCC_WARN_64_TO_32_BIT_CONVERSION
CLANG_WARN_ENUM_CONVERSION
CLANG_WARN_INT_CONVERSION
CLANG_WARN_IMPLICIT_SIGN_CONVERSION
GCC_WARN_INITIALIZER_NOT_FULLY_BRACKETED
GCC_WARN_ABOUT_RETURN_TYPE
GCC_WARN_MISSING_PARENTHESES
GCC_WARN_ABOUT_MISSING_FIELD_INITIALIZERS
GCC_WARN_ABOUT_MISSING_PROTOTYPES
GCC_WARN_ABOUT_MISSING_NEWLINE
WARNING_CFLAGS
GCC_WARN_ABOUT_POINTER_SIGNEDNESS
GCC_WARN_SIGN_COMPARE
CLANG_WARN_SUSPICIOUS_IMPLICIT_CONVERSION
GCC_TREAT_INCOMPATIBLE_POINTER_TYPE_WARNINGS_AS_ERRORS
GCC_TREAT_IMPLICIT_FUNCTION_DECLARATIONS_AS_ERRORS
GCC_WARN_TYPECHECK_CALLS_TO_PRINTF
GCC_WARN_UNINITIALIZED_AUTOS
GCC_WARN_UNKNOWN_PRAGMAS
GCC_WARN_UNUSED_FUNCTION
GCC_WARN_UNUSED_LABEL
GCC_WARN_UNUSED_PARAMETER
GCC_WARN_UNUSED_VALUE
GCC_WARN_UNUSED_VARIABLE
CLANG_WARN__EXIT_TIME_DESTRUCTORS
GCC_WARN_NON_VIRTUAL_DESTRUCTOR
GCC_WARN_HIDDEN_VIRTUAL_FUNCTIONS
GCC_WARN_ABOUT_INVALID_OFFSETOF_MACRO
CLANG_WARN_CXX0X_EXTENSIONS

It seems the issue shows up with -std=c++11 only. I reported to clang's team: http://llvm.org/bugs/show_bug.cgi?id=16042.

Thanks for looking into this. I guess in the meantime I will just have to define lots of local variables since that seems to be the only way I can get the assignment to work.

You have many options:
- disable c++11
- use v1.dot(v2)
- use (v1.transpose()*v2).value()
- use (v1.transpose()*v2)[0]